WAEC SSCE - Chemistry - 2018 (Essay)
Pregunta 1 Informe
(a) Explain the statement, the standard electrode potential of zinc is -0.76 v.What is meant by the term periodic property of elements?
(b) Consider the following standard electrode potentials:
| \( \mathrm{Zn}^{2+}_{(aq)} + 2e^- \) | \( \mathrm{Zn(s)} \) | Eᶿ = - 0.76 V |
| \( \mathrm{Cu}^{2+}_{(aq)} + 2e^- \) | \( \mathrm{Cu(s)} \) | Eᶿ = + 0.34 V |
When the two half cells are connected:
(i) write the reaction equation at each electrode;
(ii) write the overall cell reaction equation;
(iii) state the type of reaction occurring at each electrode;
(iv) calculate the e.m.f. of the cell.
(c) (i) Name two chemical industries.
(ii) State two factors that should be considered when siting a chemical industry.
(iii) List two effects of a chemical industry on the community in which it is sited.
(d) Using chemical equations, explain briefly what would happen when hydrogen peroxide is added to:
(i) silver oxide;
(ii) chlorine gas.
(e) List three physical properties of nitrogen.
(a) The statement means that when a zinc electrode dipped in a 1 mol dm-3 solution of its ions at 25 °C is connected to a standard hydrogen electrode, the measured potential difference is 0.76 V, with zinc as the negative electrode. Zinc therefore loses electrons more readily than hydrogen (it is a stronger reducing agent).
A periodic property is a property (e.g. atomic radius, ionization energy, electronegativity) that varies regularly and recurs at intervals with increasing atomic number of the elements.
(b)(i) Anode (Zn): \( \text{Zn} \to \text{Zn}^{2+} + 2e^- \)
Cathode (Cu): \( \text{Cu}^{2+} + 2e^- \to \text{Cu} \)
(b)(ii) \[ \text{Zn} + \text{Cu}^{2+} \to \text{Zn}^{2+} + \text{Cu} \]
(b)(iii) Oxidation at the zinc electrode; reduction at the copper electrode.
(b)(iv) \[ E_{cell} = E_{cathode} - E_{anode} = (+0.34) - (-0.76) = +1.10\ \text{V} \]
(c)(i) The petrochemical industry and the fertilizer industry.
(c)(ii) Nearness to raw materials and availability of water and power.
(c)(iii) Provision of employment/development (beneficial) and environmental pollution of air and water (harmful).
(d)(i) Hydrogen peroxide reduces silver oxide, liberating oxygen:
\[ \text{Ag}_2\text{O} + \text{H}_2\text{O}_2 \to 2\text{Ag} + \text{H}_2\text{O} + \text{O}_2 \]
(d)(ii) Hydrogen peroxide reduces chlorine:
\[ \text{H}_2\text{O}_2 + \text{Cl}_2 \to 2\text{HCl} + \text{O}_2 \]
(e) Nitrogen is a colourless, odourless, tasteless gas; it is slightly (sparingly) soluble in water; and it is slightly less dense than air (and neutral to litmus).
Detalles de la respuesta
(a) The statement means that when a zinc electrode dipped in a 1 mol dm-3 solution of its ions at 25 °C is connected to a standard hydrogen electrode, the measured potential difference is 0.76 V, with zinc as the negative electrode. Zinc therefore loses electrons more readily than hydrogen (it is a stronger reducing agent).
A periodic property is a property (e.g. atomic radius, ionization energy, electronegativity) that varies regularly and recurs at intervals with increasing atomic number of the elements.
(b)(i) Anode (Zn): \( \text{Zn} \to \text{Zn}^{2+} + 2e^- \)
Cathode (Cu): \( \text{Cu}^{2+} + 2e^- \to \text{Cu} \)
(b)(ii) \[ \text{Zn} + \text{Cu}^{2+} \to \text{Zn}^{2+} + \text{Cu} \]
(b)(iii) Oxidation at the zinc electrode; reduction at the copper electrode.
(b)(iv) \[ E_{cell} = E_{cathode} - E_{anode} = (+0.34) - (-0.76) = +1.10\ \text{V} \]
(c)(i) The petrochemical industry and the fertilizer industry.
(c)(ii) Nearness to raw materials and availability of water and power.
(c)(iii) Provision of employment/development (beneficial) and environmental pollution of air and water (harmful).
(d)(i) Hydrogen peroxide reduces silver oxide, liberating oxygen:
\[ \text{Ag}_2\text{O} + \text{H}_2\text{O}_2 \to 2\text{Ag} + \text{H}_2\text{O} + \text{O}_2 \]
(d)(ii) Hydrogen peroxide reduces chlorine:
\[ \text{H}_2\text{O}_2 + \text{Cl}_2 \to 2\text{HCl} + \text{O}_2 \]
(e) Nitrogen is a colourless, odourless, tasteless gas; it is slightly (sparingly) soluble in water; and it is slightly less dense than air (and neutral to litmus).
Pregunta 2 Informe
(a)
(i) List two gaseous pollutants that can be generated by burning coal.
(ii) Explain briefly why coal burns more easily when it is broken into pieces than when it is in lumps.
(iii) What gas is responsible for most of the explosions in coal mines?
(iv) Name the non-volatile residue left behind after the destructive distillation of coal.
(b) State one oxide in each case which:
(i) is used in bleaching;
(ii) oxidizes hot concentrated HCl to chlorine;
(iii) dissolves in water to give a solution with pH greater than 7;
(iv) reacts with NaOH and also with HCl;
(v) is a reddish-brown gas.
(c)
(i) Write a balanced chemical equation for the reaction between chlorine gas and iron(II) chloride solution.
(ii) State the type of reaction in (c)(i).
(iii) Give a reason for your answer in (c)(ii).
(d) Consider the following set-up: 
(i) Identify A and B.
(ii) Write a balanced chemical equation for the reaction.
(iii) Name the gas produced.
(iv) Why was the flask tilted downwards?
(v) What is the:
(I) function of B in the experiment;
(II) method of collection of the gas?
(e) Give one product obtained from refining petroleum that is solid.
(a) Coal
(i) Two gaseous pollutants produced by burning coal are sulphur(IV) oxide, \(SO_2\) and carbon(II) oxide (carbon monoxide), \(CO\). (Oxides of nitrogen, \(NO_x\), are also acceptable.)
(ii) Breaking coal into pieces greatly increases the total surface area of solid exposed to the oxygen of the air. Combustion is a surface reaction, so a larger area of contact means more collisions per second between the coal and oxygen molecules. The rate of burning therefore rises, and the broken coal ignites and burns more easily than the same mass held in one large lump.
(iii) Methane, \(CH_4\) (called firedamp) is responsible for most explosions in coal mines.
(iv) The non-volatile residue left after the destructive distillation of coal is coke.
(b) Oxides
| Property | Oxide |
|---|---|
| (i) used in bleaching | Sulphur(IV) oxide, \(SO_2\) |
| (ii) oxidises hot concentrated HCl to chlorine | Manganese(IV) oxide, \(MnO_2\) |
| (iii) dissolves in water to give a solution of pH > 7 | Sodium oxide, \(Na_2O\) (basic oxide) |
| (iv) reacts with both NaOH and HCl | Aluminium oxide, \(Al_2O_3\) (amphoteric) |
| (v) is a reddish-brown gas | Nitrogen(IV) oxide, \(NO_2\) |
The reaction in (ii) is:
\[MnO_2 + 4HCl \rightarrow MnCl_2 + Cl_2 + 2H_2O\]
(c) Chlorine and iron(II) chloride
(i) \[2FeCl_2 + Cl_2 \rightarrow 2FeCl_3\]
(ii) It is a redox (oxidation-reduction) reaction.
(iii) The iron is oxidised while the chlorine is reduced in the same reaction. \(Fe^{2+}\) loses one electron to become \(Fe^{3+}\) (oxidation), and each atom of the added chlorine gains an electron, changing from oxidation state \(0\) in \(Cl_2\) to \(-1\) in the chloride (reduction). Since oxidation and reduction occur together, the reaction is redox.
(d) The set-up shown
The diagram shows a round-bottom flask containing a mixture of calcium hydroxide, \(Ca(OH)_2\), and ammonium chloride, \(NH_4Cl\), mounted on a stand and heated with a burner. The flask is clamped so that its mouth points slightly downwards. A delivery tube A carries the gas evolved into a vertical tower B which is packed with granular solid, and the gas is finally collected in an inverted gas jar at the top.
(i) A is the delivery tube (which conveys the gas from the flask). B is a drying tower (drying column) packed with lumps of calcium oxide (quicklime), \(CaO\).
(ii) \[2NH_4Cl + Ca(OH)_2 \rightarrow CaCl_2 + 2NH_3 + 2H_2O\]
(iii) The gas produced is ammonia, \(NH_3\).
(iv) The flask was tilted with its mouth downwards so that the water (steam) formed in the reaction condenses near the cooler mouth and does not run back onto the hot base of the flask. If cold water ran onto the strongly heated glass, the sudden contraction would crack the flask.
(v)(I) The function of B is to dry the ammonia gas. The calcium oxide absorbs the water vapour carried over from the flask; ordinary drying agents such as concentrated \(H_2SO_4\) or anhydrous \(CaCl_2\) cannot be used because they react with the alkaline ammonia, so the basic drying agent \(CaO\) is used.
(v)(II) The gas is collected by downward displacement of air (upward delivery into an inverted gas jar). Ammonia is used this way because it is less dense than air and is very soluble in water, so it cannot be collected over water.
(e) Solid product from refining petroleum
A solid product obtained from refining petroleum is bitumen (asphalt). (Paraffin wax is also acceptable.)
Detalles de la respuesta
(a) Coal
(i) Two gaseous pollutants produced by burning coal are sulphur(IV) oxide, \(SO_2\) and carbon(II) oxide (carbon monoxide), \(CO\). (Oxides of nitrogen, \(NO_x\), are also acceptable.)
(ii) Breaking coal into pieces greatly increases the total surface area of solid exposed to the oxygen of the air. Combustion is a surface reaction, so a larger area of contact means more collisions per second between the coal and oxygen molecules. The rate of burning therefore rises, and the broken coal ignites and burns more easily than the same mass held in one large lump.
(iii) Methane, \(CH_4\) (called firedamp) is responsible for most explosions in coal mines.
(iv) The non-volatile residue left after the destructive distillation of coal is coke.
(b) Oxides
| Property | Oxide |
|---|---|
| (i) used in bleaching | Sulphur(IV) oxide, \(SO_2\) |
| (ii) oxidises hot concentrated HCl to chlorine | Manganese(IV) oxide, \(MnO_2\) |
| (iii) dissolves in water to give a solution of pH > 7 | Sodium oxide, \(Na_2O\) (basic oxide) |
| (iv) reacts with both NaOH and HCl | Aluminium oxide, \(Al_2O_3\) (amphoteric) |
| (v) is a reddish-brown gas | Nitrogen(IV) oxide, \(NO_2\) |
The reaction in (ii) is:
\[MnO_2 + 4HCl \rightarrow MnCl_2 + Cl_2 + 2H_2O\]
(c) Chlorine and iron(II) chloride
(i) \[2FeCl_2 + Cl_2 \rightarrow 2FeCl_3\]
(ii) It is a redox (oxidation-reduction) reaction.
(iii) The iron is oxidised while the chlorine is reduced in the same reaction. \(Fe^{2+}\) loses one electron to become \(Fe^{3+}\) (oxidation), and each atom of the added chlorine gains an electron, changing from oxidation state \(0\) in \(Cl_2\) to \(-1\) in the chloride (reduction). Since oxidation and reduction occur together, the reaction is redox.
(d) The set-up shown
The diagram shows a round-bottom flask containing a mixture of calcium hydroxide, \(Ca(OH)_2\), and ammonium chloride, \(NH_4Cl\), mounted on a stand and heated with a burner. The flask is clamped so that its mouth points slightly downwards. A delivery tube A carries the gas evolved into a vertical tower B which is packed with granular solid, and the gas is finally collected in an inverted gas jar at the top.
(i) A is the delivery tube (which conveys the gas from the flask). B is a drying tower (drying column) packed with lumps of calcium oxide (quicklime), \(CaO\).
(ii) \[2NH_4Cl + Ca(OH)_2 \rightarrow CaCl_2 + 2NH_3 + 2H_2O\]
(iii) The gas produced is ammonia, \(NH_3\).
(iv) The flask was tilted with its mouth downwards so that the water (steam) formed in the reaction condenses near the cooler mouth and does not run back onto the hot base of the flask. If cold water ran onto the strongly heated glass, the sudden contraction would crack the flask.
(v)(I) The function of B is to dry the ammonia gas. The calcium oxide absorbs the water vapour carried over from the flask; ordinary drying agents such as concentrated \(H_2SO_4\) or anhydrous \(CaCl_2\) cannot be used because they react with the alkaline ammonia, so the basic drying agent \(CaO\) is used.
(v)(II) The gas is collected by downward displacement of air (upward delivery into an inverted gas jar). Ammonia is used this way because it is less dense than air and is very soluble in water, so it cannot be collected over water.
(e) Solid product from refining petroleum
A solid product obtained from refining petroleum is bitumen (asphalt). (Paraffin wax is also acceptable.)
Pregunta 3 Informe
(a)(i) Sketch a graphical representation of Charles' law.
(ii) Calculate the volume of oxygen that would be required for the complete combustion of 2.5 moles of ethanol at s.t.p. [molar volume at s.t.p. = 22.4 dm\(^3\)]
(b)(i) State the collision theory of reaction rates.
(ii) Using the collision theory, explain briefly how temperature can affect the rate of a chemical reaction.
(c)(i) Define esterification.
(ii) Give two uses of alkanoates.
(iii) Give the products of the alkaline hydrolysis of ethyl ethanoate.
(d) A tin coated plate and a galvanized plate were exposed for the same length of time.
(i) Which of the two plates corrodes faster
(ii) Explain briefly your answer in 2(d)(i)
(a)(i) Graphical representation of Charles’ law
At constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute temperature. A plot of volume against temperature in degrees Celsius is a straight line which, when extrapolated, cuts the temperature axis at 173 0C.
Thus, \(V\propto T\) when \(T\) is measured in kelvin. A graph of \(V\) against temperature in kelvin passes through the origin.
(a)(ii)
The balanced equation for the combustion of ethanol is:
\[\mathrm{C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O}\]
\(1\) mole of ethanol requires \(3\) moles of \(\mathrm{O_2}\).
\[n(\mathrm{O_2})=2.5\times3=7.5\text{ mol}\]
\[V(\mathrm{O_2})=7.5\times22.4=168.0\text{ dm}^3\]
Volume of oxygen required = \(168.0\text{ dm}^3\) at s.t.p.
(b)(i) Collision theory states that a reaction occurs only when reacting particles collide effectively, that is, with energy equal to or greater than the activation energy and in the appropriate orientation.
(b)(ii) On raising the temperature, reacting particles gain kinetic energy and move faster. Collisions become more frequent and a greater proportion of the collisions possess at least the activation energy. The number of effective collisions per second therefore increases, so the reaction rate increases.
(c)(i) Esterification is the reaction of an alkanol with an alkanoic acid, usually in the presence of a concentrated mineral acid catalyst, to form an alkanoate and water.
(c)(ii) Alkanoates are used:
- in perfumes and cosmetics;
- as artificial food flavourings.
(c)(iii) Alkaline hydrolysis of ethyl ethanoate produces ethanol and a salt of ethanoic acid. For example, with sodium hydroxide:
\[\mathrm{CH_3COOC_2H_5+NaOH\rightarrow CH_3COONa+C_2H_5OH}\]
The products are sodium ethanoate and ethanol.
(d)(i) The tin-coated plate corrodes faster.
(d)(ii) Tin is less reactive than iron, whereas zinc is more reactive than iron. If the coating is damaged, iron in contact with tin becomes the anode and corrodes rapidly. In a galvanized plate, zinc acts as a sacrificial metal and corrodes in preference to the iron, thereby protecting the iron plate.
Detalles de la respuesta
(a)(i) Graphical representation of Charles’ law
At constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute temperature. A plot of volume against temperature in degrees Celsius is a straight line which, when extrapolated, cuts the temperature axis at 173 0C.
Thus, \(V\propto T\) when \(T\) is measured in kelvin. A graph of \(V\) against temperature in kelvin passes through the origin.
(a)(ii)
The balanced equation for the combustion of ethanol is:
\[\mathrm{C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O}\]
\(1\) mole of ethanol requires \(3\) moles of \(\mathrm{O_2}\).
\[n(\mathrm{O_2})=2.5\times3=7.5\text{ mol}\]
\[V(\mathrm{O_2})=7.5\times22.4=168.0\text{ dm}^3\]
Volume of oxygen required = \(168.0\text{ dm}^3\) at s.t.p.
(b)(i) Collision theory states that a reaction occurs only when reacting particles collide effectively, that is, with energy equal to or greater than the activation energy and in the appropriate orientation.
(b)(ii) On raising the temperature, reacting particles gain kinetic energy and move faster. Collisions become more frequent and a greater proportion of the collisions possess at least the activation energy. The number of effective collisions per second therefore increases, so the reaction rate increases.
(c)(i) Esterification is the reaction of an alkanol with an alkanoic acid, usually in the presence of a concentrated mineral acid catalyst, to form an alkanoate and water.
(c)(ii) Alkanoates are used:
- in perfumes and cosmetics;
- as artificial food flavourings.
(c)(iii) Alkaline hydrolysis of ethyl ethanoate produces ethanol and a salt of ethanoic acid. For example, with sodium hydroxide:
\[\mathrm{CH_3COOC_2H_5+NaOH\rightarrow CH_3COONa+C_2H_5OH}\]
The products are sodium ethanoate and ethanol.
(d)(i) The tin-coated plate corrodes faster.
(d)(ii) Tin is less reactive than iron, whereas zinc is more reactive than iron. If the coating is damaged, iron in contact with tin becomes the anode and corrodes rapidly. In a galvanized plate, zinc acts as a sacrificial metal and corrodes in preference to the iron, thereby protecting the iron plate.
Pregunta 4 Informe
(a) Outline the procedures for the treatment of water for town supply.
(b) (i) State two main impurities present in bauxite.
(ii) Give one reason why bauxite is usually preferred as the ore for the extraction of aluminium.
(iii) Outline the manufacture of aluminium form purified bauxite.
(c) (i) Explain briefly the term fine chemical industry.
(ii) State two differences between a fine chemical and a heavy chemical.
(d) (i) Write a balanced chemical equation for the combustion of coal.
(ii) If 5.4 g of coal is burnt completely, calculate the amount of oxygen measured at s.t.p. that would be required for the combustion.
[C = 12.0, O = 16.0, Molar volume of a gas at s.t.p. = \(22.4\ \text{dm}^3\)]
(e) Name two substances which can be used as electrodes during the electrolysis of acidified water.
(a) Treatment of water for town supply
- Screening/sedimentation: the water is screened and allowed to stand so that suspended particles settle, often aided by a coagulant such as alum.
- Filtration: the water passes through beds of sand and gravel to remove fine suspended matter.
- Aeration to improve taste and oxidise dissolved impurities.
- Chlorination (sterilization): chlorine is added to kill germs.
- Storage and distribution to consumers.
(b)(i) Iron(III) oxide (Fe2O3) and silica (SiO2).
(b)(ii) Bauxite is preferred because it is rich in aluminium oxide and is the most abundant and economical source of the metal.
(b)(iii) The purified alumina (Al2O3) is dissolved in molten cryolite (to lower the melting point) and the mixture is electrolysed using carbon (graphite) anodes and a carbon-lined cathode. Aluminium is deposited at the cathode and oxygen is liberated at the anode:
Cathode: \( \text{Al}^{3+} + 3e^- \to \text{Al} \); \quad Anode: \( 2\text{O}^{2-} \to \text{O}_2 + 4e^- \).
(c)(i) A fine chemical industry manufactures chemicals in small quantities but of high purity and high value (e.g. drugs, dyes, cosmetics).
(c)(ii) (1) Fine chemicals are made in small quantities, heavy chemicals in large/bulk quantities. (2) Fine chemicals are of high purity and high cost per unit, whereas heavy chemicals are of lower purity and cheaper per unit.
(d)(i) \[ \text{C} + \text{O}_2 \to \text{CO}_2 \]
(d)(ii) \( n(\text{C}) = \dfrac{5.4}{12} = 0.45\ \text{mol} \); \( n(\text{O}_2) = 0.45\ \text{mol} \).
\[ V = 0.45 \times 22.4 = 10.08\ \text{dm}^3 \]
(e) Platinum and carbon (graphite) (inert electrodes).
Detalles de la respuesta
(a) Treatment of water for town supply
- Screening/sedimentation: the water is screened and allowed to stand so that suspended particles settle, often aided by a coagulant such as alum.
- Filtration: the water passes through beds of sand and gravel to remove fine suspended matter.
- Aeration to improve taste and oxidise dissolved impurities.
- Chlorination (sterilization): chlorine is added to kill germs.
- Storage and distribution to consumers.
(b)(i) Iron(III) oxide (Fe2O3) and silica (SiO2).
(b)(ii) Bauxite is preferred because it is rich in aluminium oxide and is the most abundant and economical source of the metal.
(b)(iii) The purified alumina (Al2O3) is dissolved in molten cryolite (to lower the melting point) and the mixture is electrolysed using carbon (graphite) anodes and a carbon-lined cathode. Aluminium is deposited at the cathode and oxygen is liberated at the anode:
Cathode: \( \text{Al}^{3+} + 3e^- \to \text{Al} \); \quad Anode: \( 2\text{O}^{2-} \to \text{O}_2 + 4e^- \).
(c)(i) A fine chemical industry manufactures chemicals in small quantities but of high purity and high value (e.g. drugs, dyes, cosmetics).
(c)(ii) (1) Fine chemicals are made in small quantities, heavy chemicals in large/bulk quantities. (2) Fine chemicals are of high purity and high cost per unit, whereas heavy chemicals are of lower purity and cheaper per unit.
(d)(i) \[ \text{C} + \text{O}_2 \to \text{CO}_2 \]
(d)(ii) \( n(\text{C}) = \dfrac{5.4}{12} = 0.45\ \text{mol} \); \( n(\text{O}_2) = 0.45\ \text{mol} \).
\[ V = 0.45 \times 22.4 = 10.08\ \text{dm}^3 \]
(e) Platinum and carbon (graphite) (inert electrodes).
Pregunta 5 Informe
(a)(i) Draw and label a diagram for the laboratory preparation of a dry sample of sulphur(IV)oxide.
(ii) Write a balanced chemical equation for the reaction in (a)(i).
(iii) State the precaution that must be taken in the preparation of the gas stated in (a)(i).
(iv) Give a reason why the precaution stated in (a)(ii) must be taken.
(b)(i) State Dalton's law of partial pressures.
(ii) The volume of a sample of methane collected over-water at a temperature of 12°C and a pressure of 700 mmHg was 30cm\(^3\). Calculate the volume of the dry gas at s.t.p. [Saturated vapour pressure of water at 12°C is 10 mmHg] •
(c)(i) Write an equation for the reaction between chlorine and water.
(ii) Why does litmus paper turn red when put in the resulting solution in (c)(i)?
(d)(i) State the trend in the boiling points of chlorine, bromine and iodine.
(ii) Explain briefly why water has a higher boiling point than ammonia.
(a)(i) The apparatus for preparing and drying sulphur(IV) oxide is shown below.
(ii)
\[\mathrm{Na_2SO_3(aq)+2HCl(aq)\rightarrow 2NaCl(aq)+H_2O(l)+SO_2(g)}\]
(iii) The preparation should be carried out in a fume cupboard.
(iv) Sulphur(IV) oxide is poisonous and irritates the eyes and respiratory tract; the fume cupboard prevents inhalation of the gas.
(b)(i) Dalton's law of partial pressures states that the total pressure of a mixture of non-reacting gases is equal to the sum of the partial pressures of the individual gases.
(ii)
The methane was collected over water, so its pressure is:
\[P_1=700-10=690\ \mathrm{mmHg}\]
\[T_1=12+273=285\ \mathrm{K}\]
At s.t.p., \(P_2=760\ \mathrm{mmHg}\) and \(T_2=273\ \mathrm{K}\).
Using \(\dfrac{P_1V_1}{T_1}=\dfrac{P_2V_2}{T_2}\):
\[V_2=\frac{P_1V_1T_2}{P_2T_1}=\frac{690\times30\times273}{760\times285}=26.1\ \mathrm{cm^3}\]
Therefore, the volume of dry methane at s.t.p. is \(26.1\ \mathrm{cm^3}\).
(c)(i)
\[\mathrm{Cl_2(g)+H_2O(l)\rightleftharpoons HCl(aq)+HClO(aq)}\]
(ii) Hydrogen chloride, \(\mathrm{HCl}\), ionises in water to produce \(\mathrm{H^+}\) ions. The resulting solution is acidic and turns blue litmus paper red.
(d)(i) The boiling points increase from chlorine to iodine:
\[\mathrm{Cl_2<Br_2<I_2}\]
(d)(ii) Water has a higher boiling point because its intermolecular hydrogen bonding is stronger and more extensive than that in ammonia. Oxygen is more electronegative than nitrogen, and each water molecule can form more hydrogen bonds; hence more energy is required to separate water molecules.
Detalles de la respuesta
(a)(i) The apparatus for preparing and drying sulphur(IV) oxide is shown below.
(ii)
\[\mathrm{Na_2SO_3(aq)+2HCl(aq)\rightarrow 2NaCl(aq)+H_2O(l)+SO_2(g)}\]
(iii) The preparation should be carried out in a fume cupboard.
(iv) Sulphur(IV) oxide is poisonous and irritates the eyes and respiratory tract; the fume cupboard prevents inhalation of the gas.
(b)(i) Dalton's law of partial pressures states that the total pressure of a mixture of non-reacting gases is equal to the sum of the partial pressures of the individual gases.
(ii)
The methane was collected over water, so its pressure is:
\[P_1=700-10=690\ \mathrm{mmHg}\]
\[T_1=12+273=285\ \mathrm{K}\]
At s.t.p., \(P_2=760\ \mathrm{mmHg}\) and \(T_2=273\ \mathrm{K}\).
Using \(\dfrac{P_1V_1}{T_1}=\dfrac{P_2V_2}{T_2}\):
\[V_2=\frac{P_1V_1T_2}{P_2T_1}=\frac{690\times30\times273}{760\times285}=26.1\ \mathrm{cm^3}\]
Therefore, the volume of dry methane at s.t.p. is \(26.1\ \mathrm{cm^3}\).
(c)(i)
\[\mathrm{Cl_2(g)+H_2O(l)\rightleftharpoons HCl(aq)+HClO(aq)}\]
(ii) Hydrogen chloride, \(\mathrm{HCl}\), ionises in water to produce \(\mathrm{H^+}\) ions. The resulting solution is acidic and turns blue litmus paper red.
(d)(i) The boiling points increase from chlorine to iodine:
\[\mathrm{Cl_2<Br_2<I_2}\]
(d)(ii) Water has a higher boiling point because its intermolecular hydrogen bonding is stronger and more extensive than that in ammonia. Oxygen is more electronegative than nitrogen, and each water molecule can form more hydrogen bonds; hence more energy is required to separate water molecules.
Pregunta 6 Informe
(a)(i) Draw the structure of the sixth member of the alkenes.
(ii) Calculate the relative molecular mass of the sixth member of the alkene.
(iii) State one difference between cracking and reforming in the petroleum industry. [H = 1, C = 12]
(b)(i) Define the term enthalpy of neutralization.
(ii) Describe briefly how the enthalpy of neutralization of the reaction of dilute hydrochloric acid and aqueous potassium hydroxide could be determined.
(c) An electrochemical cell is constructed with copper and silver electrodes.
(i) State which of the electrodes will be the: 1. anode; II. cathode.
(ii) Give the reason for your answer in 3(c)(i).
(iii) State the type of reaction occurring at each electrode.
(iv) Write a balanced equation for the overall cell reaction.
(d)(i) Name the compound formed when iron is exposed to moist air for a long time.
(ii) Write a balanced chemical equation for the reaction in 3(d)(i).
(iii) Name one ore of iron.
(a)(i) Alkenes have the general formula \(\mathrm{C_nH_{2n}}\). Starting from ethene as the first member, the sixth member is heptene, \(\mathrm{C_7H_{14}}\). One correct structure is hept-1-ene:
(ii)
(iii) Cracking breaks large hydrocarbon molecules into smaller molecules, whereas reforming rearranges straight-chain hydrocarbons into branched-chain or cyclic hydrocarbons to improve petrol quality.
(b)(i) The enthalpy of neutralization is the enthalpy change when one mole of water is formed by the reaction of an acid with a base in dilute aqueous solution.
(ii) Prepare equimolar dilute hydrochloric acid and aqueous potassium hydroxide. Measure a known volume of the hydrochloric acid into a polystyrene cup calorimeter and record its initial temperature. Measure an equal volume of potassium hydroxide at the same initial temperature, add it quickly to the acid, stir, and record the maximum temperature reached.
Calculate the heat gained by the solution using \(q=mc\Delta T\), where \(m\) is the mass of the mixed solution and \(c=4.18\ \mathrm{J\,g^{-1}\,K^{-1}}\). Since neutralization is exothermic, the heat of reaction is \(-q\). Divide this value by the number of moles of water formed to obtain the enthalpy of neutralization:
(c)(i)
- Anode: copper electrode.
- Cathode: silver electrode.
(ii) Copper is more electropositive, or higher in the electrochemical series, than silver. Therefore copper loses electrons more readily, while \(\mathrm{Ag^+}\) ions gain electrons more readily.
(iii) Oxidation occurs at the copper anode and reduction occurs at the silver cathode.
(iv)
(d)(i) The compound formed is rust, hydrated iron(III) oxide, \(\mathrm{Fe_2O_3\cdot xH_2O}\).
(ii)
(iii) Haematite, \(\mathrm{Fe_2O_3}\), is an ore of iron.
Detalles de la respuesta
(a)(i) Alkenes have the general formula \(\mathrm{C_nH_{2n}}\). Starting from ethene as the first member, the sixth member is heptene, \(\mathrm{C_7H_{14}}\). One correct structure is hept-1-ene:
(ii)
(iii) Cracking breaks large hydrocarbon molecules into smaller molecules, whereas reforming rearranges straight-chain hydrocarbons into branched-chain or cyclic hydrocarbons to improve petrol quality.
(b)(i) The enthalpy of neutralization is the enthalpy change when one mole of water is formed by the reaction of an acid with a base in dilute aqueous solution.
(ii) Prepare equimolar dilute hydrochloric acid and aqueous potassium hydroxide. Measure a known volume of the hydrochloric acid into a polystyrene cup calorimeter and record its initial temperature. Measure an equal volume of potassium hydroxide at the same initial temperature, add it quickly to the acid, stir, and record the maximum temperature reached.
Calculate the heat gained by the solution using \(q=mc\Delta T\), where \(m\) is the mass of the mixed solution and \(c=4.18\ \mathrm{J\,g^{-1}\,K^{-1}}\). Since neutralization is exothermic, the heat of reaction is \(-q\). Divide this value by the number of moles of water formed to obtain the enthalpy of neutralization:
(c)(i)
- Anode: copper electrode.
- Cathode: silver electrode.
(ii) Copper is more electropositive, or higher in the electrochemical series, than silver. Therefore copper loses electrons more readily, while \(\mathrm{Ag^+}\) ions gain electrons more readily.
(iii) Oxidation occurs at the copper anode and reduction occurs at the silver cathode.
(iv)
(d)(i) The compound formed is rust, hydrated iron(III) oxide, \(\mathrm{Fe_2O_3\cdot xH_2O}\).
(ii)
(iii) Haematite, \(\mathrm{Fe_2O_3}\), is an ore of iron.
Pregunta 7 Informe
(a) A compound of carbon, hydrogen and chlorine contains 0.48 g of carbon, 0.08 g of hydrogen and 1.42 g of chlorine.
(i) Determine the empirical formula of the compound.
(ii) If the molar mass of the compound is 99, calculate the molecular formula of the compound.
[H = 1.0, C = 12.0, Cl = 35.5]
(b) State three properties of NaCl (s) which shows that it is ionic.
(c) Consider the following reaction equation:
(i) On the same diagram, sketch and label a reaction profile for a catalysed and uncatalysed reaction between \(H_2\) and \(O_2\).
(ii) Indicate the possible positions of the activated complexes for the reaction profiles in (c)(i).
(d) The petrochemical industry produces addition polymers using one of the fractions obtained from crude oil.
(i) Name the fraction used as a raw material for the process.
(ii) What process is used to obtain the fraction from crude oil?
(iii) Name two gaseous hydrocarbons that can be used in making polymers.
(iv) Describe briefly how these hydrocarbons can be obtained.
(v) Name the polymer produced from one of the hydrocarbons named in (d)(iii).
(a) (i) Empirical formula
| Element | Mass/g | Moles | Simplest ratio |
|---|---|---|---|
| C | 0.48 | \(\frac{0.48}{12}=0.04\) | \(\frac{0.04}{0.04}=1\) |
| H | 0.08 | \(\frac{0.08}{1}=0.08\) | \(\frac{0.08}{0.04}=2\) |
| Cl | 1.42 | \(\frac{1.42}{35.5}=0.04\) | \(\frac{0.04}{0.04}=1\) |
Ratio of atoms \(=1:2:1\).
Empirical formula \(=\boxed{\mathrm{CH_2Cl}}\).
(a) (ii) Molecular formula
Empirical formula mass \(=12+2(1)+35.5=49.5\).
\[n=\frac{99}{49.5}=2\]
Molecular formula \(=(\mathrm{CH_2Cl})_2=\boxed{\mathrm{C_2H_4Cl_2}}\).
(b) Properties of sodium chloride showing that it is ionic
- It has high melting and boiling points because of strong electrostatic forces between oppositely charged ions.
- Solid sodium chloride does not conduct electricity, but molten sodium chloride and aqueous sodium chloride conduct electricity because their ions are mobile.
- It is readily soluble in water, a polar solvent.
(c) (i) and (ii) Reaction profiles for the reaction between hydrogen and oxygen
The activated complexes are at the tops of the respective curves, as labelled. The catalysed route has a lower activation energy than the uncatalysed route.
(d)
(i) The fraction used as raw material is naphtha.
(ii) It is obtained from crude oil by fractional distillation.
(iii) Two gaseous hydrocarbons used in making polymers are ethene and propene.
(iv) They are obtained by cracking long-chain hydrocarbons in fractions such as naphtha. In catalytic cracking, the hydrocarbon vapour is heated to about \(600^\circ\mathrm{C}\) over a silica/alumina catalyst, producing smaller molecules including alkenes.
(v) Ethene produces poly(ethene) or polythene. Propene produces poly(propene).
Detalles de la respuesta
(a) (i) Empirical formula
| Element | Mass/g | Moles | Simplest ratio |
|---|---|---|---|
| C | 0.48 | \(\frac{0.48}{12}=0.04\) | \(\frac{0.04}{0.04}=1\) |
| H | 0.08 | \(\frac{0.08}{1}=0.08\) | \(\frac{0.08}{0.04}=2\) |
| Cl | 1.42 | \(\frac{1.42}{35.5}=0.04\) | \(\frac{0.04}{0.04}=1\) |
Ratio of atoms \(=1:2:1\).
Empirical formula \(=\boxed{\mathrm{CH_2Cl}}\).
(a) (ii) Molecular formula
Empirical formula mass \(=12+2(1)+35.5=49.5\).
\[n=\frac{99}{49.5}=2\]
Molecular formula \(=(\mathrm{CH_2Cl})_2=\boxed{\mathrm{C_2H_4Cl_2}}\).
(b) Properties of sodium chloride showing that it is ionic
- It has high melting and boiling points because of strong electrostatic forces between oppositely charged ions.
- Solid sodium chloride does not conduct electricity, but molten sodium chloride and aqueous sodium chloride conduct electricity because their ions are mobile.
- It is readily soluble in water, a polar solvent.
(c) (i) and (ii) Reaction profiles for the reaction between hydrogen and oxygen
The activated complexes are at the tops of the respective curves, as labelled. The catalysed route has a lower activation energy than the uncatalysed route.
(d)
(i) The fraction used as raw material is naphtha.
(ii) It is obtained from crude oil by fractional distillation.
(iii) Two gaseous hydrocarbons used in making polymers are ethene and propene.
(iv) They are obtained by cracking long-chain hydrocarbons in fractions such as naphtha. In catalytic cracking, the hydrocarbon vapour is heated to about \(600^\circ\mathrm{C}\) over a silica/alumina catalyst, producing smaller molecules including alkenes.
(v) Ethene produces poly(ethene) or polythene. Propene produces poly(propene).
Pregunta 8 Informe
(a)(i) State two industrial uses of hdrogen.
(ii) Consider the equation below. Mg(HCO\(_3\))\(_{2(aq)}\) \(\to\) MgCO\(_{3(g)}\) + H\(_2\)0\(_{(l)}\) + CO\(_{2(g)}\)
1. State the type of hardness of water being removed as shown by the above equation.
2. Give two disadvantages of hardness of water.
(b)(i) In the extraction of aluminium by electrolysis, graphite electrodes are used. State the disadvantages of using this type of electrode.
(ii) Calcuim oxide reacts with water to form slaked line: I. Write a balanced equation for this reaction; II. State one use of slaked line.
(c)(i) What is meant by saponification?
(ii) List the raw materials needed for the manufacture of soap.
(iii) Name the main by-product obtained from the manufacture of soap.
(d) With the aid of chemical equations explain briefly how iron is extracted in the blast furnace using iron ore, coke and limestone as raw materials at the:
(i) bottom of the furnace; (ii) middle of the furnace (iii) top of the furnace.
(a)(i) Manufacture of ammonia (Haber process); hydrogenation of vegetable oils to make margarine. (Also manufacture of methanol, and use as a fuel.)
(a)(ii) 1. The hardness removed is temporary hardness. 2. Disadvantages of hard water: it wastes soap (forms an insoluble scum) and it forms scale/fur in kettles, boilers and pipes, reducing their efficiency.
(b)(i) The graphite (carbon) anodes react with the oxygen liberated during electrolysis, burning away as CO2, so they are used up and must be replaced frequently (adding to cost).
(b)(ii) I. \[ \text{CaO} + \text{H}_2\text{O} \to \text{Ca(OH)}_2 \] II. Slaked lime is used to neutralize soil acidity (also for making mortar and softening water).
(c)(i) Saponification is the alkaline hydrolysis of fats or oils (esters) by a hot alkali to produce soap and glycerol.
(c)(ii) Raw materials: a fat or oil (animal fat/vegetable oil) and a concentrated alkali (sodium hydroxide).
(c)(iii) The main by-product is glycerol (propane-1,2,3-triol).
(d) Extraction of iron in the blast furnace
- (i) Bottom (hottest zone): coke burns in the blast of hot air, supplying heat, and slag forms:
\( \text{C} + \text{O}_2 \to \text{CO}_2 \); \quad \( \text{CaO} + \text{SiO}_2 \to \text{CaSiO}_3 \) (slag). Molten iron and slag collect here. - (ii) Middle zone: the CO2 is reduced by more coke, and the carbon(II) oxide formed reduces the ore:
\( \text{CO}_2 + \text{C} \to 2\text{CO} \); \quad \( \text{Fe}_2\text{O}_3 + 3\text{CO} \to 2\text{Fe} + 3\text{CO}_2 \). - (iii) Top (coolest zone): the limestone decomposes:
\( \text{CaCO}_3 \to \text{CaO} + \text{CO}_2 \).
Detalles de la respuesta
(a)(i) Manufacture of ammonia (Haber process); hydrogenation of vegetable oils to make margarine. (Also manufacture of methanol, and use as a fuel.)
(a)(ii) 1. The hardness removed is temporary hardness. 2. Disadvantages of hard water: it wastes soap (forms an insoluble scum) and it forms scale/fur in kettles, boilers and pipes, reducing their efficiency.
(b)(i) The graphite (carbon) anodes react with the oxygen liberated during electrolysis, burning away as CO2, so they are used up and must be replaced frequently (adding to cost).
(b)(ii) I. \[ \text{CaO} + \text{H}_2\text{O} \to \text{Ca(OH)}_2 \] II. Slaked lime is used to neutralize soil acidity (also for making mortar and softening water).
(c)(i) Saponification is the alkaline hydrolysis of fats or oils (esters) by a hot alkali to produce soap and glycerol.
(c)(ii) Raw materials: a fat or oil (animal fat/vegetable oil) and a concentrated alkali (sodium hydroxide).
(c)(iii) The main by-product is glycerol (propane-1,2,3-triol).
(d) Extraction of iron in the blast furnace
- (i) Bottom (hottest zone): coke burns in the blast of hot air, supplying heat, and slag forms:
\( \text{C} + \text{O}_2 \to \text{CO}_2 \); \quad \( \text{CaO} + \text{SiO}_2 \to \text{CaSiO}_3 \) (slag). Molten iron and slag collect here. - (ii) Middle zone: the CO2 is reduced by more coke, and the carbon(II) oxide formed reduces the ore:
\( \text{CO}_2 + \text{C} \to 2\text{CO} \); \quad \( \text{Fe}_2\text{O}_3 + 3\text{CO} \to 2\text{Fe} + 3\text{CO}_2 \). - (iii) Top (coolest zone): the limestone decomposes:
\( \text{CaCO}_3 \to \text{CaO} + \text{CO}_2 \).
Pregunta 9 Informe
(a) Arrange the three states of matter in order of decreasing:
(i) kinetic energy;
(ii) force of cohesion.
(b) Consider the redox reaction equation:
(i) State the change in oxidation number of:
I. magnesium;
II. hydrogen.
(ii) Which of the species is being:
I. oxidized;
II. reduced?
(iii) Identify the oxidizing agent.
(c) (i) State two differences between boiling and evaporation.
(ii) What will be the effect of reduction of atmospheric pressure on the boiling point of water?
(d) For a given chemical equilibrium system, what is the significance of the equilibrium constant \(K\)?
(e) Consider the following organic compounds:
\(\mathrm{C_3H_7COOH}\); \(\mathrm{(CH_3)_3COH}\).
Give the IUPAC name of each compound.
(f) Why are organic compounds classified on the basis of functional groups?
(g) State three differences between the solubility of solids in liquids and gases in liquids.
(h) Write a balanced chemical equation for the reaction between fluorine and water.
(i) Define the term basicity of an acid.
(a)
(i) Decreasing kinetic energy: gas > liquid > solid.
(ii) Decreasing force of cohesion: solid > liquid > gas.
(b) For the reaction \( \text{Mg} + 2\text{HCl} \to \text{MgCl}_2 + \text{H}_2 \):
- (i) I. Magnesium: oxidation number changes from 0 to +2. II. Hydrogen: from +1 to 0.
- (ii) I. Oxidized: magnesium (Mg). II. Reduced: hydrogen (H+).
- (iii) Oxidizing agent: hydrochloric acid (HCl / H+).
(c)(i) Two differences between boiling and evaporation:
- Boiling occurs at a fixed temperature (the boiling point) and throughout the whole liquid with bubbles forming, whereas evaporation occurs at any temperature and only at the surface of the liquid.
- Boiling requires an external supply of heat and produces no cooling, whereas evaporation uses the internal energy of the liquid and produces cooling.
(c)(ii) Reducing the atmospheric pressure lowers the boiling point of water, so it boils at a temperature below 100 °C.
(d) The equilibrium constant K indicates the extent to which a reaction proceeds and where the position of equilibrium lies: a large K means the products are favoured (reaction nearly complete), while a small K means the reactants are favoured.
(e) C3H7COOH is butanoic acid; (CH3)3COH is 2-methylpropan-2-ol.
(f) Because a functional group is the atom or group of atoms responsible for the characteristic chemical reactions of a compound. Compounds sharing the same functional group have similar chemical properties, so classifying by functional group makes their study systematic and their reactions predictable.
(g) Three differences between solubility of solids and of gases in liquids:
- The solubility of most solids increases with a rise in temperature, whereas the solubility of gases decreases with a rise in temperature.
- The solubility of gases increases markedly with pressure, whereas that of solids is virtually unaffected by pressure.
- Dissolving a solid is generally accompanied by absorption of heat (endothermic), whereas dissolving a gas is generally accompanied by release of heat (exothermic).
(h) \[ 2\text{F}_2 + 2\text{H}_2\text{O} \to 4\text{HF} + \text{O}_2 \]
(i) The basicity of an acid is the number of replaceable (ionizable) hydrogen ions produced by one molecule of the acid in aqueous solution.
Detalles de la respuesta
(a)
(i) Decreasing kinetic energy: gas > liquid > solid.
(ii) Decreasing force of cohesion: solid > liquid > gas.
(b) For the reaction \( \text{Mg} + 2\text{HCl} \to \text{MgCl}_2 + \text{H}_2 \):
- (i) I. Magnesium: oxidation number changes from 0 to +2. II. Hydrogen: from +1 to 0.
- (ii) I. Oxidized: magnesium (Mg). II. Reduced: hydrogen (H+).
- (iii) Oxidizing agent: hydrochloric acid (HCl / H+).
(c)(i) Two differences between boiling and evaporation:
- Boiling occurs at a fixed temperature (the boiling point) and throughout the whole liquid with bubbles forming, whereas evaporation occurs at any temperature and only at the surface of the liquid.
- Boiling requires an external supply of heat and produces no cooling, whereas evaporation uses the internal energy of the liquid and produces cooling.
(c)(ii) Reducing the atmospheric pressure lowers the boiling point of water, so it boils at a temperature below 100 °C.
(d) The equilibrium constant K indicates the extent to which a reaction proceeds and where the position of equilibrium lies: a large K means the products are favoured (reaction nearly complete), while a small K means the reactants are favoured.
(e) C3H7COOH is butanoic acid; (CH3)3COH is 2-methylpropan-2-ol.
(f) Because a functional group is the atom or group of atoms responsible for the characteristic chemical reactions of a compound. Compounds sharing the same functional group have similar chemical properties, so classifying by functional group makes their study systematic and their reactions predictable.
(g) Three differences between solubility of solids and of gases in liquids:
- The solubility of most solids increases with a rise in temperature, whereas the solubility of gases decreases with a rise in temperature.
- The solubility of gases increases markedly with pressure, whereas that of solids is virtually unaffected by pressure.
- Dissolving a solid is generally accompanied by absorption of heat (endothermic), whereas dissolving a gas is generally accompanied by release of heat (exothermic).
(h) \[ 2\text{F}_2 + 2\text{H}_2\text{O} \to 4\text{HF} + \text{O}_2 \]
(i) The basicity of an acid is the number of replaceable (ionizable) hydrogen ions produced by one molecule of the acid in aqueous solution.
Pregunta 10 Informe
(a)(i) Describe briefly the laboratory preparation of hydrogen gas from the action of steam on iron.
(ii) Write the equation for the reaction in (a)(i).
(iii) List three methods for the industrial preparation of hydrogen gas.
(b) Consider the following reaction equation:
\(\mathrm{CO(g)} + 2\mathrm{H}_{2(g)} \qquad \mathrm{CH}_3\mathrm{OH}_{(g)} + 201\) kJmol-J
Predict the effect of each of the following factors on the position of equilibrium:
(i) decrease in temperature;
(ii) increase in pressure;
(iii) increase in concentration of \(\mathrm{CO}_{(g)}\).
(c) (i) Define condensation polymerization.
(ii) Name two condensation polymers.
(iii) Write the formula of ethylethanoate.
(d) (i) Define the term allotropy.
(ii) Name two crystalline allotropes of carbon.
(iii) State one use of each of the allotropes named in (d)(ii).
(e) State two differences between nitrogen (I) oxide and oxygen.
(a)(i) Place iron filings in a hard-glass (combustion) tube and heat strongly. Pass steam (from heated water) over the red-hot iron; the hydrogen produced is led away by a delivery tube and collected over water.
(a)(ii) \[ 3\text{Fe} + 4\text{H}_2\text{O} \rightleftharpoons \text{Fe}_3\text{O}_4 + 4\text{H}_2 \]
(a)(iii) Industrial preparation of hydrogen: (1) electrolysis of acidified water or brine; (2) steam reforming of natural gas/hydrocarbons; (3) passing steam over red-hot coke (water gas, then the water-gas shift).
(b) The forward reaction is exothermic and reduces the number of gas molecules (3 \(\to\) 1):
- (i) Decrease in temperature: equilibrium shifts forward (right), giving more methanol.
- (ii) Increase in pressure: equilibrium shifts forward (right), giving more methanol.
- (iii) Increase in [CO]: equilibrium shifts forward (right), giving more methanol.
(c)(i) Condensation polymerization is a polymerization in which monomers (each with two functional groups) join together with the elimination of a small molecule such as water.
(c)(ii) Nylon and terylene (polyester).
(c)(iii) \( \text{CH}_3\text{COOC}_2\text{H}_5 \)
(d)(i) Allotropy is the existence of an element in two or more different forms in the same physical state.
(d)(ii) Diamond and graphite.
(d)(iii) Diamond is used in cutting/drilling tools and as an abrasive; graphite is used as a lubricant and as electrodes (pencil lead).
(e) (1) Nitrogen(I) oxide has a faint sweet smell whereas oxygen is odourless. (2) Burning sulphur continues to burn in oxygen but is extinguished in nitrogen(I) oxide (N2O does not support the combustion of a feebly burning substance).
Detalles de la respuesta
(a)(i) Place iron filings in a hard-glass (combustion) tube and heat strongly. Pass steam (from heated water) over the red-hot iron; the hydrogen produced is led away by a delivery tube and collected over water.
(a)(ii) \[ 3\text{Fe} + 4\text{H}_2\text{O} \rightleftharpoons \text{Fe}_3\text{O}_4 + 4\text{H}_2 \]
(a)(iii) Industrial preparation of hydrogen: (1) electrolysis of acidified water or brine; (2) steam reforming of natural gas/hydrocarbons; (3) passing steam over red-hot coke (water gas, then the water-gas shift).
(b) The forward reaction is exothermic and reduces the number of gas molecules (3 \(\to\) 1):
- (i) Decrease in temperature: equilibrium shifts forward (right), giving more methanol.
- (ii) Increase in pressure: equilibrium shifts forward (right), giving more methanol.
- (iii) Increase in [CO]: equilibrium shifts forward (right), giving more methanol.
(c)(i) Condensation polymerization is a polymerization in which monomers (each with two functional groups) join together with the elimination of a small molecule such as water.
(c)(ii) Nylon and terylene (polyester).
(c)(iii) \( \text{CH}_3\text{COOC}_2\text{H}_5 \)
(d)(i) Allotropy is the existence of an element in two or more different forms in the same physical state.
(d)(ii) Diamond and graphite.
(d)(iii) Diamond is used in cutting/drilling tools and as an abrasive; graphite is used as a lubricant and as electrodes (pencil lead).
(e) (1) Nitrogen(I) oxide has a faint sweet smell whereas oxygen is odourless. (2) Burning sulphur continues to burn in oxygen but is extinguished in nitrogen(I) oxide (N2O does not support the combustion of a feebly burning substance).
Pregunta 11 Informe
(a) (i) Define the term fermentation
(ii) Name the catalyst that can be used for this process
(b) Name two factors which determine the choice of an indicator for an acid-base titration
(c) Consider the following reaction equation: Fe + H\(_2\)SO\(_4\) \(\to\) FeSO\(_4\) + H\(_2\). Calculate the mass of unreacted iron when 5.0g of iron reacts with 10cm\(^3\) of 1.0 moldm\(^3\) H\(_2\)SO\(_4\), [Fe = 56.0]
(d) Name one:
(i) Heavy chemical used in electrolytic cells
(ii) Fine chemical used in textile industries
(e) Explain briefly how a catalyst increases the rate of a chemical reaction.
(f) (i) Write the chemical formula for the product formed when ethanoic acid reacts with ammonia
(ii) Give the name of the product formed in (f)(i)
(g) List three properties of aluminum that makes it suitable for the manufacture of drinks cans
(h) State two industrial uses of alkylalkanoates
(i) Name two steps involved in the crystallization of a salt from its solution
(j) List two effects of global warming
(a)(i) Fermentation is the slow breakdown of complex organic substances (especially carbohydrates/sugars) into simpler substances such as ethanol and carbon(IV) oxide by enzymes produced by micro-organisms.
(a)(ii) The enzyme zymase (from yeast).
(b) The choice of indicator depends on: (1) the strengths of the acid and base being titrated (i.e. the type of titration), and (2) the pH range at which the indicator changes colour, which must match the pH at the equivalence point.
(c) Mass of unreacted iron
\( n(\text{H}_2\text{SO}_4) = 1.0 \times \dfrac{10}{1000} = 0.01\ \text{mol} \)
Fe + H2SO4 \(\to\) FeSO4 + H2 (1 : 1), so Fe reacting = 0.01 mol.
Mass of Fe reacted = \(0.01 \times 56 = 0.56\ \text{g}\).
\[ \text{Unreacted Fe} = 5.0 - 0.56 = 4.44\ \text{g} \]
(d)(i) A heavy chemical: sodium chloride (brine) (or sulphuric acid / sodium hydroxide).
(d)(ii) A fine chemical: a dye.
(e) A catalyst provides an alternative reaction pathway with a lower activation energy, so a greater fraction of the colliding particles have enough energy to react, and the rate increases.
(f)(i) \( \text{CH}_3\text{COONH}_4 \)
(f)(ii) Ammonium ethanoate.
(g) Aluminium is light (low density), resistant to corrosion (protective oxide layer), and malleable/easily shaped and non-toxic.
(h) Alkylalkanoates (esters) are used as flavourings/perfumes and as solvents.
(i) (1) Evaporate the solution to the point of saturation; (2) cool the hot saturated solution to allow crystals to separate (then filter and dry).
(j) Melting of polar ice caps and rise in sea level, and climate change/flooding (or increased desertification).
Detalles de la respuesta
(a)(i) Fermentation is the slow breakdown of complex organic substances (especially carbohydrates/sugars) into simpler substances such as ethanol and carbon(IV) oxide by enzymes produced by micro-organisms.
(a)(ii) The enzyme zymase (from yeast).
(b) The choice of indicator depends on: (1) the strengths of the acid and base being titrated (i.e. the type of titration), and (2) the pH range at which the indicator changes colour, which must match the pH at the equivalence point.
(c) Mass of unreacted iron
\( n(\text{H}_2\text{SO}_4) = 1.0 \times \dfrac{10}{1000} = 0.01\ \text{mol} \)
Fe + H2SO4 \(\to\) FeSO4 + H2 (1 : 1), so Fe reacting = 0.01 mol.
Mass of Fe reacted = \(0.01 \times 56 = 0.56\ \text{g}\).
\[ \text{Unreacted Fe} = 5.0 - 0.56 = 4.44\ \text{g} \]
(d)(i) A heavy chemical: sodium chloride (brine) (or sulphuric acid / sodium hydroxide).
(d)(ii) A fine chemical: a dye.
(e) A catalyst provides an alternative reaction pathway with a lower activation energy, so a greater fraction of the colliding particles have enough energy to react, and the rate increases.
(f)(i) \( \text{CH}_3\text{COONH}_4 \)
(f)(ii) Ammonium ethanoate.
(g) Aluminium is light (low density), resistant to corrosion (protective oxide layer), and malleable/easily shaped and non-toxic.
(h) Alkylalkanoates (esters) are used as flavourings/perfumes and as solvents.
(i) (1) Evaporate the solution to the point of saturation; (2) cool the hot saturated solution to allow crystals to separate (then filter and dry).
(j) Melting of polar ice caps and rise in sea level, and climate change/flooding (or increased desertification).
Pregunta 12 Informe
1. (a ) Define the term compound.
(b) State two conditions necessary for the cracking of petroleum fractions.
(c) Name two transition elements that are used as catalyst.
(d) (i) Write an equation for the reaction between zinc dust and trioxonitrate (V) solution.
(ii) Which of the reactants in 1(d)(i) is:
I. reduced;
II. oxidized.
(e) Two isotopes of oxygen \(16O\) and \(18O\) have relative abundance of 90 % and 10 % respectively. Calculate the relative atomic mass of oxygen.
(f) List two ores of iron.
(g) (i) What is biotechnology?
(ii) Name one product that can be obtained using biotechnology.
(h) Define the term element.
(i) State two sources of methane in the atmosphere.
(j) Explain briefly why the trend of the boiling points for group VII elements is in the order \(I_2 > Brl_2 > Cl_2\).
(a) A compound is a pure substance formed when two or more elements combine chemically in a fixed proportion by mass.
(b) A high temperature and the presence of a catalyst (e.g. silica-alumina/zeolite). (High pressure is used in thermal cracking.)
(c) Iron and vanadium (also platinum or nickel).
(d)(i) (Taking the salt as silver trioxonitrate(V), AgNO3.)
\[ \text{Zn} + 2\text{AgNO}_3 \to \text{Zn(NO}_3)_2 + 2\text{Ag} \]
(d)(ii) I. Reduced: the silver ion, Ag+ (gains electrons). II. Oxidized: zinc (loses electrons).
(e) Relative atomic mass of oxygen
\[ A_r = \frac{(16 \times 90) + (18 \times 10)}{100} = \frac{1440 + 180}{100} = \frac{1620}{100} = 16.2 \]
(f) Haematite (Fe2O3) and magnetite (Fe3O4).
(g)(i) Biotechnology is the use of living organisms (or their enzymes/systems) to make or modify products useful to man.
(g)(ii) Ethanol (or antibiotics, insulin, yoghurt).
(h) An element is a substance made of only one kind of atom that cannot be split into simpler substances by ordinary chemical means.
(i) Decomposition of organic matter in swamps/marshes (biogas) and the digestion of food by ruminant animals (also natural gas and rice paddies).
(j) Boiling point increases in the order Cl2 < Br2 < I2 because, down the group, the molecular size and number of electrons increase, so the van der Waals (dispersion) forces between molecules become stronger and more energy is needed to separate them.
Detalles de la respuesta
(a) A compound is a pure substance formed when two or more elements combine chemically in a fixed proportion by mass.
(b) A high temperature and the presence of a catalyst (e.g. silica-alumina/zeolite). (High pressure is used in thermal cracking.)
(c) Iron and vanadium (also platinum or nickel).
(d)(i) (Taking the salt as silver trioxonitrate(V), AgNO3.)
\[ \text{Zn} + 2\text{AgNO}_3 \to \text{Zn(NO}_3)_2 + 2\text{Ag} \]
(d)(ii) I. Reduced: the silver ion, Ag+ (gains electrons). II. Oxidized: zinc (loses electrons).
(e) Relative atomic mass of oxygen
\[ A_r = \frac{(16 \times 90) + (18 \times 10)}{100} = \frac{1440 + 180}{100} = \frac{1620}{100} = 16.2 \]
(f) Haematite (Fe2O3) and magnetite (Fe3O4).
(g)(i) Biotechnology is the use of living organisms (or their enzymes/systems) to make or modify products useful to man.
(g)(ii) Ethanol (or antibiotics, insulin, yoghurt).
(h) An element is a substance made of only one kind of atom that cannot be split into simpler substances by ordinary chemical means.
(i) Decomposition of organic matter in swamps/marshes (biogas) and the digestion of food by ruminant animals (also natural gas and rice paddies).
(j) Boiling point increases in the order Cl2 < Br2 < I2 because, down the group, the molecular size and number of electrons increase, so the van der Waals (dispersion) forces between molecules become stronger and more energy is needed to separate them.
Pregunta 13 Informe
a) (i) Describe, using the kinetic theory of matter, what happens when potassium chloride dissolves in water.
(ii) Give a reason why the process in (a) (i) is endothermic.
(b) (i) An underground iron pipe is less likely to corrode if it is bonded at intervals with magnesium rods. Give reasons for this observation.
(ii) State the stages involved in the rusting of iron.
(iii) State the condition for the rusting of iron in water.
(c) (i) What is a spontaneous reaction?
(ii) State two conditions that could make a reaction spontaneous.
(iii) Explain briefly why one gramme of sodium reacts more rapidly with water at 250C than one gramme of calcium at the same temperature.
(iv) Write equations for the reactions in (c)(iii).
(d) What mass of lead (II) trioxocarbonate (IV) would contain 35.0 g of lead?
[C=12.0, O = 16.0, Pb = 207.0]
(e) Name the type of intermolecular force present in:
(i) fluorine;
(ii) hydrogen fluoride.
(a)(i) The moving, polar water molecules collide with and surround the K+ and Cl- ions at the surface of the crystal, overcoming the electrostatic forces of the lattice. The ions are pulled away (hydrated) and, because of their kinetic motion, diffuse and become evenly spread throughout the water.
(a)(ii) The process is endothermic because the energy absorbed to break up the ionic lattice (lattice energy) is greater than the energy released when the ions are hydrated (hydration energy); the net heat is taken in from the surroundings.
(b)(i) Magnesium is more reactive (more electropositive) than iron, so it acts as a sacrificial anode: it corrodes in preference to the iron, protecting the pipe (cathodic protection).
(b)(ii) Stages of rusting: iron is oxidised at anodic areas, \( \text{Fe} \to \text{Fe}^{2+} + 2e^- \); oxygen and water are reduced at cathodic areas, \( \text{O}_2 + 2\text{H}_2\text{O} + 4e^- \to 4\text{OH}^- \); the Fe2+ is further oxidised by oxygen to hydrated iron(III) oxide (rust).
(b)(iii) Both water and oxygen (air) must be present.
(c)(i) A spontaneous reaction is one that, once started, proceeds on its own without a continuous supply of external energy.
(c)(ii) A negative enthalpy change (exothermic) and an increase in entropy/disorder (giving a negative free-energy change, \(\Delta G < 0\)).
(c)(iii) Sodium is more electropositive (more reactive) than calcium and loses its outer electron more readily; also 1 g of sodium contains more atoms than 1 g of calcium (Na = 23, Ca = 40), so it reacts faster.
(c)(iv) \[ 2\text{Na} + 2\text{H}_2\text{O} \to 2\text{NaOH} + \text{H}_2 \] \[ \text{Ca} + 2\text{H}_2\text{O} \to \text{Ca(OH)}_2 + \text{H}_2 \]
(d) Mass of PbCO3
\( M(\text{PbCO}_3) = 207 + 12 + 48 = 267 \). In 267 g there is 207 g Pb, so:
\[ \text{mass} = \frac{267}{207} \times 35.0 = 45.1\ \text{g} \]
(e) (i) Fluorine: van der Waals (dispersion) forces. (ii) Hydrogen fluoride: hydrogen bonding.
Detalles de la respuesta
(a)(i) The moving, polar water molecules collide with and surround the K+ and Cl- ions at the surface of the crystal, overcoming the electrostatic forces of the lattice. The ions are pulled away (hydrated) and, because of their kinetic motion, diffuse and become evenly spread throughout the water.
(a)(ii) The process is endothermic because the energy absorbed to break up the ionic lattice (lattice energy) is greater than the energy released when the ions are hydrated (hydration energy); the net heat is taken in from the surroundings.
(b)(i) Magnesium is more reactive (more electropositive) than iron, so it acts as a sacrificial anode: it corrodes in preference to the iron, protecting the pipe (cathodic protection).
(b)(ii) Stages of rusting: iron is oxidised at anodic areas, \( \text{Fe} \to \text{Fe}^{2+} + 2e^- \); oxygen and water are reduced at cathodic areas, \( \text{O}_2 + 2\text{H}_2\text{O} + 4e^- \to 4\text{OH}^- \); the Fe2+ is further oxidised by oxygen to hydrated iron(III) oxide (rust).
(b)(iii) Both water and oxygen (air) must be present.
(c)(i) A spontaneous reaction is one that, once started, proceeds on its own without a continuous supply of external energy.
(c)(ii) A negative enthalpy change (exothermic) and an increase in entropy/disorder (giving a negative free-energy change, \(\Delta G < 0\)).
(c)(iii) Sodium is more electropositive (more reactive) than calcium and loses its outer electron more readily; also 1 g of sodium contains more atoms than 1 g of calcium (Na = 23, Ca = 40), so it reacts faster.
(c)(iv) \[ 2\text{Na} + 2\text{H}_2\text{O} \to 2\text{NaOH} + \text{H}_2 \] \[ \text{Ca} + 2\text{H}_2\text{O} \to \text{Ca(OH)}_2 + \text{H}_2 \]
(d) Mass of PbCO3
\( M(\text{PbCO}_3) = 207 + 12 + 48 = 267 \). In 267 g there is 207 g Pb, so:
\[ \text{mass} = \frac{267}{207} \times 35.0 = 45.1\ \text{g} \]
(e) (i) Fluorine: van der Waals (dispersion) forces. (ii) Hydrogen fluoride: hydrogen bonding.
Pregunta 14 Informe
(a) (i) List the two gaseous fuels produced from coke.
(ii) Which of the two fuels listed in 5(a)(i) is a better fuel?
(iii) Give reasons for your answer in 5(a)(ii)
(iv) Write a balanced equation for the production of each of the fuels. [9 marks]
(b)(i) Differentiate between thermosets and thermoplastics.
(ii) Give one example of:
I. thermosets;
II. thermoplastics.
(iii) State three properties of plastics.
(c)(i) State the method of collecting gases which are denser than air.
(ii) Name two gases that could be used to perform the fountain experiment in the laboratory.
(iii) State the physical properties of the gases named in 5(c)(ii) which makes them suitable for the experiment. [4 marks]
(d) (i) State two compounds that could be used to test for water.
(ii) Give three disadvantages of hard water.
(a)(i) Water gas (CO + H2) and producer gas (CO + N2).
(a)(ii) Water gas is the better fuel.
(a)(iii) Water gas has a higher heating (calorific) value because it is a mixture of two combustible gases (CO and H2) and is not diluted by large amounts of non-combustible nitrogen, whereas producer gas contains much inert N2.
(a)(iv) Water gas: \[ \text{C} + \text{H}_2\text{O} \to \text{CO} + \text{H}_2 \] Producer gas: \[ 2\text{C} + \text{O}_2 \to 2\text{CO} \quad (\text{air over hot coke}) \]
(b)(i) Thermosets harden permanently once moulded and cannot be softened or re-shaped on further heating (they are cross-linked); thermoplastics soften on heating and can be re-moulded repeatedly.
(b)(ii) I. Thermoset: bakelite (or melamine). II. Thermoplastic: polythene (polyethene) (or PVC).
(b)(iii) Plastics are light, resistant to corrosion/chemicals, and poor conductors of heat and electricity (also easily moulded).
(c)(i) By downward delivery (upward displacement of air), collecting the gas in an upright jar.
(c)(ii) Ammonia and hydrogen chloride.
(c)(iii) They are extremely soluble in water, so the water rushes up to fill the partial vacuum created when the gas dissolves, producing the fountain.
(d)(i) Anhydrous copper(II) sulphate (white to blue) and anhydrous cobalt(II) chloride (blue to pink).
(d)(ii) Hard water wastes soap (forms scum), forms scale/fur in kettles, boilers and pipes, and the scale reduces heat efficiency and can block pipes.
Detalles de la respuesta
(a)(i) Water gas (CO + H2) and producer gas (CO + N2).
(a)(ii) Water gas is the better fuel.
(a)(iii) Water gas has a higher heating (calorific) value because it is a mixture of two combustible gases (CO and H2) and is not diluted by large amounts of non-combustible nitrogen, whereas producer gas contains much inert N2.
(a)(iv) Water gas: \[ \text{C} + \text{H}_2\text{O} \to \text{CO} + \text{H}_2 \] Producer gas: \[ 2\text{C} + \text{O}_2 \to 2\text{CO} \quad (\text{air over hot coke}) \]
(b)(i) Thermosets harden permanently once moulded and cannot be softened or re-shaped on further heating (they are cross-linked); thermoplastics soften on heating and can be re-moulded repeatedly.
(b)(ii) I. Thermoset: bakelite (or melamine). II. Thermoplastic: polythene (polyethene) (or PVC).
(b)(iii) Plastics are light, resistant to corrosion/chemicals, and poor conductors of heat and electricity (also easily moulded).
(c)(i) By downward delivery (upward displacement of air), collecting the gas in an upright jar.
(c)(ii) Ammonia and hydrogen chloride.
(c)(iii) They are extremely soluble in water, so the water rushes up to fill the partial vacuum created when the gas dissolves, producing the fountain.
(d)(i) Anhydrous copper(II) sulphate (white to blue) and anhydrous cobalt(II) chloride (blue to pink).
(d)(ii) Hard water wastes soap (forms scum), forms scale/fur in kettles, boilers and pipes, and the scale reduces heat efficiency and can block pipes.
Pregunta 15 Informe
(a) (i) State the collision theory of reaction rates.
(ii) Using the collision theory, explain briefly how temperature can affect the rate of a chemical reaction.
(b) (i) Sketch a graphical representation of Charles’ law.
(ii) Calculate the volume of oxygen that would be required for the complete combustion of 2.5 moles of ethanol at s.t.p.
[molar volume at s.t.p = \(22.4\ \text{dm}^3\)]
(c) (i) Define esterification.
(ii) Give two uses of alkanoates.
(iii) Give the products of the alkaline hydrolysis of ethyl ethanoate.
(d) A tin coated plate and a galvanized plate were exposed for the same length of time.
(i) Which of the two plates corrodes faster?
(ii) Explain briefly your answer in 2 (d) (i).
(a) (i) The collision theory states that reactant particles must collide before a reaction can occur. Only collisions with sufficient energy to overcome the activation energy, and with the correct orientation where necessary, result in reaction.
(a) (ii) When the temperature is increased, the reactant particles gain kinetic energy and move faster. Collisions occur more frequently, and a greater proportion of the collisions have energy equal to or greater than the activation energy. Therefore, the number of effective collisions per second increases and the reaction rate increases.
(b) (i) At constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute temperature in kelvin. Hence, a graph of volume against temperature in kelvin is a straight line passing through the origin.
(b) (ii)
The balanced equation for the complete combustion of ethanol is:
\[\mathrm{C_2H_5OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(l)}\]
From the equation, 1 mole of ethanol requires 3 moles of oxygen.
\[\text{Moles of }O_2 = 2.5 \times 3 = 7.5\text{ mol}\]
\[\text{Volume of }O_2 = 7.5 \times 22.4 = 168.0\text{ dm}^3\]
Therefore, the volume of oxygen required is 168.0 dm3 at s.t.p.
(c) (i) Esterification is the reaction between an alkanol and an alkanoic acid, usually in the presence of concentrated sulfuric acid, to form an ester and water.
(c) (ii) Uses of alkanoates include:
- as solvents, for example in paints, varnishes and nail polish removers;
- in perfumes and artificial food flavourings.
(c) (iii) Alkaline hydrolysis of ethyl ethanoate produces ethanol and an ethanoate salt.
\[\mathrm{CH_3COOC_2H_5 + NaOH \rightarrow CH_3COONa + C_2H_5OH}\]
The products are ethanol and sodium ethanoate.
(d) (i) The tin-coated plate corrodes faster.
(d) (ii) Zinc is more reactive than iron and acts as a sacrificial metal in a galvanized plate. Zinc corrodes preferentially and protects the iron plate. Tin is less reactive than iron; therefore, when the tin coating is damaged or exposed, the iron corrodes readily in contact with tin.
Detalles de la respuesta
(a) (i) The collision theory states that reactant particles must collide before a reaction can occur. Only collisions with sufficient energy to overcome the activation energy, and with the correct orientation where necessary, result in reaction.
(a) (ii) When the temperature is increased, the reactant particles gain kinetic energy and move faster. Collisions occur more frequently, and a greater proportion of the collisions have energy equal to or greater than the activation energy. Therefore, the number of effective collisions per second increases and the reaction rate increases.
(b) (i) At constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute temperature in kelvin. Hence, a graph of volume against temperature in kelvin is a straight line passing through the origin.
(b) (ii)
The balanced equation for the complete combustion of ethanol is:
\[\mathrm{C_2H_5OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(l)}\]
From the equation, 1 mole of ethanol requires 3 moles of oxygen.
\[\text{Moles of }O_2 = 2.5 \times 3 = 7.5\text{ mol}\]
\[\text{Volume of }O_2 = 7.5 \times 22.4 = 168.0\text{ dm}^3\]
Therefore, the volume of oxygen required is 168.0 dm3 at s.t.p.
(c) (i) Esterification is the reaction between an alkanol and an alkanoic acid, usually in the presence of concentrated sulfuric acid, to form an ester and water.
(c) (ii) Uses of alkanoates include:
- as solvents, for example in paints, varnishes and nail polish removers;
- in perfumes and artificial food flavourings.
(c) (iii) Alkaline hydrolysis of ethyl ethanoate produces ethanol and an ethanoate salt.
\[\mathrm{CH_3COOC_2H_5 + NaOH \rightarrow CH_3COONa + C_2H_5OH}\]
The products are ethanol and sodium ethanoate.
(d) (i) The tin-coated plate corrodes faster.
(d) (ii) Zinc is more reactive than iron and acts as a sacrificial metal in a galvanized plate. Zinc corrodes preferentially and protects the iron plate. Tin is less reactive than iron; therefore, when the tin coating is damaged or exposed, the iron corrodes readily in contact with tin.
Pregunta 16 Informe
(a) (i) Name the ore mostly used in the extraction of aluminium.
(ii) Name two major impurities in the ore named in (a)(i).
(iii) Name the material used in making the electrodes in the extraction of aluminium.
(iv) Give two reasons why aluminium is commonly recycled.
(v) Explain briefly why the anode has to be replaced at regular intervals during the extraction of aluminium.
(b) A current of 0.75 amperes was passed through an electrolysis containing chromium ions for one hour and four minutes. If the mass of chromium deposited was 0.52 g, calculate the:
(i) quantity of electricity passed;
(ii) moles of chromium deposited;
(iii) quantity of electricity required to deposit one mole of chromium;
(iv) charge on the chromium ion.
[Cr = 52.0, 1 F = 96500 C]
(c) In the contact process for the manufacture of tetraoxosulphate(VI) acid, the following reaction occurs:
\[2\mathrm{SO}_{2(g)} + \mathrm{O}_{2(g)} \qquad 2\mathrm{SO}_{3(g)} \qquad H = -197\ \mathrm{kJ\ mol}^{-1}\]
(i) Name the catalyst used in the reaction;
(ii) State the optimum temperature for this reaction;
(iii) What would be the effect on the yield of \(\mathrm{SO}_3\) if a temperature higher than the optimum is used?
(d)(i) State two chemical methods by which temporary hardness of water can be removed.
(ii) Write a balanced chemical equation for each of the methods stated in (d)(i).
(a)(i) Bauxite (Al2O3·2H2O).
(a)(ii) Iron(III) oxide (Fe2O3) and silica (SiO2).
(a)(iii) Graphite (carbon).
(a)(iv) Recycling saves the large amount of electrical energy needed for extraction and conserves the limited bauxite reserves (also reduces waste/pollution).
(a)(v) The carbon anode reacts with the oxygen liberated at it, burning away as CO2, so it is gradually consumed and must be replaced.
(b) \(t = 1\,\text{h}\,4\,\text{min} = 3840\ \text{s}\).
(i) \( Q = It = 0.75 \times 3840 = 2880\ \text{C} \)
(ii) \( n(\text{Cr}) = \dfrac{0.52}{52} = 0.01\ \text{mol} \)
(iii) Quantity to deposit 1 mol = \( \dfrac{2880}{0.01} = 288000\ \text{C} \)
(iv) Number of Faradays per mole = \( \dfrac{288000}{96500} \approx 3 \), so the charge on the chromium ion is +3 (Cr3+).
(c)(i) Vanadium(V) oxide, V2O5.
(c)(ii) About 450 °C (400-450 °C).
(c)(iii) Since the forward reaction is exothermic, a temperature higher than the optimum decreases the yield of SO3 (equilibrium shifts backward).
(d)(i) & (ii) Removal of temporary hardness
- Boiling: \( \text{Ca(HCO}_3)_2 \to \text{CaCO}_3 + \text{H}_2\text{O} + \text{CO}_2 \)
- Adding slaked lime (Clark's method): \( \text{Ca(HCO}_3)_2 + \text{Ca(OH)}_2 \to 2\text{CaCO}_3 + 2\text{H}_2\text{O} \)
Detalles de la respuesta
(a)(i) Bauxite (Al2O3·2H2O).
(a)(ii) Iron(III) oxide (Fe2O3) and silica (SiO2).
(a)(iii) Graphite (carbon).
(a)(iv) Recycling saves the large amount of electrical energy needed for extraction and conserves the limited bauxite reserves (also reduces waste/pollution).
(a)(v) The carbon anode reacts with the oxygen liberated at it, burning away as CO2, so it is gradually consumed and must be replaced.
(b) \(t = 1\,\text{h}\,4\,\text{min} = 3840\ \text{s}\).
(i) \( Q = It = 0.75 \times 3840 = 2880\ \text{C} \)
(ii) \( n(\text{Cr}) = \dfrac{0.52}{52} = 0.01\ \text{mol} \)
(iii) Quantity to deposit 1 mol = \( \dfrac{2880}{0.01} = 288000\ \text{C} \)
(iv) Number of Faradays per mole = \( \dfrac{288000}{96500} \approx 3 \), so the charge on the chromium ion is +3 (Cr3+).
(c)(i) Vanadium(V) oxide, V2O5.
(c)(ii) About 450 °C (400-450 °C).
(c)(iii) Since the forward reaction is exothermic, a temperature higher than the optimum decreases the yield of SO3 (equilibrium shifts backward).
(d)(i) & (ii) Removal of temporary hardness
- Boiling: \( \text{Ca(HCO}_3)_2 \to \text{CaCO}_3 + \text{H}_2\text{O} + \text{CO}_2 \)
- Adding slaked lime (Clark's method): \( \text{Ca(HCO}_3)_2 + \text{Ca(OH)}_2 \to 2\text{CaCO}_3 + 2\text{H}_2\text{O} \)
