WAEC SSCE - Chemistry - 1996 (Essay)

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(a) Oxidation number of manganese in the tetraoxomanganate(VII) ion, \(MnO_4^-\).

Let the oxidation number of Mn be \(x\). Each oxygen is \(-2\) and the overall charge on the ion is \(-1\):

\[x + 4(-2) = -1\]

\[x - 8 = -1\]

\[x = +7\]

So manganese has an oxidation number of +7.

(b)(i) Two metals extracted from their ores by electrolysis: sodium and aluminium. (Potassium, calcium and magnesium are also acceptable, as they are all high in the electrochemical series.)

(ii) Difference between a conductor and an electrolyte:

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(a)(i) An effective collision is a collision between reacting particles in which the particles possess energy equal to or greater than the activation energy and collide with the correct orientation, so that a chemical reaction actually takes place.

(ii) In \(\Delta G = \Delta H - T\Delta S\):

• \(\Delta G\) = the change in Gibbs free energy of the system.
• \(\Delta H\) = the change in enthalpy (heat content) of the system.
• \(T\) = the absolute temperature (in kelvin).
• \(\Delta S\) = the change in entropy (degree of disorder) of the system.

(iii) If \(\Delta G = 0\), the deduction is that the system is at equilibrium (the reaction is neither spontaneous in the forward nor in the reverse direction).

(b)(i) Le Chatelier's principle: when a system in dynamic equilibrium is subjected to a change in temperature, pressure or concentration, the equilibrium shifts in the direction that tends to oppose (reduce) the effect of that change.

(ii) For \(2A_{2(g)} + B_{2(g)} \rightleftharpoons 2A_2B_{(g)}\), \(\Delta H = x\,kJ\) (taking the forward reaction to be exothermic), three conditions for maximum yield of \(A_2B\):

• Low temperature (favours the exothermic forward reaction).
• High pressure (3 moles of gas on the left change to 2 moles on the right, so high pressure shifts the equilibrium to the side of fewer gas molecules).
• Continuous removal of \(A_2B\) as it is formed (and/or use of a catalyst to attain equilibrium faster).

(iii) Equilibrium constant expression:

\[K_c = \frac{[A_2B]^2}{[A_2]^2\,[B_2]}\]

(c)(i) A log of wood burns more slowly than the same mass of sawdust because the sawdust is finely divided and therefore has a much larger surface area exposed to air (oxygen); the rate of reaction increases with surface area, so the sawdust burns out faster.

(iii) Two reactions catalysed by light: the photosynthesis of glucose in green plants, and the reaction of hydrogen with chlorine (or the substitution reaction of methane with chlorine).

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(a) Three postulates of the kinetic theory

• All matter is made up of tiny particles (atoms or molecules) which are in constant, continuous random motion.
• The particles possess kinetic energy which increases as the temperature rises (and decreases as it falls).
• There are spaces (intermolecular spaces) between the particles, and forces of attraction exist between them; these spaces are largest in gases and smallest in solids.

(Also acceptable: collisions between the particles are perfectly elastic, so no kinetic energy is lost.)

(b) Processes involved

• (i) Spread of the smell of a perfume across a room: diffusion.
• (ii) Decrease in the volume of a liquid after exposure in an open vessel: evaporation.

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(a)(i) Two physical properties of hydrogen sulphide

• It is a colourless gas with an offensive smell (like that of rotten eggs).
• It is denser than air and is slightly soluble in water.

(It is also poisonous; any two physical properties are acceptable.)

(a)(ii) Laboratory equipment for intermittent production of hydrogen sulphide

Kipp's apparatus.

(b) Property illustrated

In the reaction H₂S_(g) + 2NaOH_(aq) → Na₂S_(aq) + 2H₂O_(l), hydrogen sulphide reacts with a base (sodium hydroxide) to form a salt and water. This illustrates the acidic property of hydrogen sulphide (it behaves as an acid).

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Reading the diagram. The vertical axis is Energy in \(kJ\,mol^{-1}\), gridded at \(0, 100, 200, 300, 400\). From the plotted levels:

• energy of the reactants (A + B) \(\approx 100\ kJ\,mol^{-1}\);
• energy at the top of the curve (the peak / transition state) \(\approx 330\ kJ\,mol^{-1}\);
• energy of the products (C + D) \(\approx 50\ kJ\,mol^{-1}\).

(a)(i) Enthalpy of the reaction. The enthalpy change is the difference between the energy of the products and that of the reactants:

\[\Delta H = H_{products} - H_{reactants} = 50 - 100 = -50\ kJ\,mol^{-1}\]

The negative sign shows that energy is given out.

(a)(ii) Activation energy of the reaction. The activation energy is measured from the reactant level up to the top of the energy barrier:

\[E_a = E_{peak} - H_{reactants} = 330 - 100 = 230\ kJ\,mol^{-1}\]

(b) Type of reaction. The products lie at a lower energy than the reactants and \(\Delta H\) is negative, so the profile is for an exothermic reaction (energy is released to the surroundings).

(c) Substances providing an alternative pathway. Catalysts. A catalyst provides an alternative reaction pathway of lower activation energy, so the reaction proceeds faster without the catalyst itself being used up.

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(a) Two monatomic gases

Helium and neon (argon is also acceptable). The noble gases exist as single atoms, so they are monatomic.

(b) Gases

• (i) Dense white fumes with ammonia vapour: hydrogen chloride (HCl). It combines with ammonia to form white fumes of ammonium chloride: NH₃ + HCl → NH₄Cl.
• (ii) Yellow precipitate with ammoniacal silver trioxonitrate (V) solution: ethyne (C₂H₂). It reacts to form a precipitate of silver ethynide (silver acetylide).

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(a)(i) One major use of alkenes: they are used in the manufacture of plastics/polymers (for example ethene is polymerized to polyethene). Ethene is also used to ripen fruits and to make ethanol.

(ii) The simplest alkane that exhibits structural isomerism is butane, \(C_4H_{10}\) (it exists as n-butane and 2-methylpropane).

(iii) For an alkane \(C_xH_{22}\), the general formula is \(C_nH_{2n+2}\):

\[2n + 2 = 22 \;\Rightarrow\; n = 10\]

So the alkane is \(C_{10}H_{22}\) (decane). Molar mass:

\[= (10 \times 12) + (22 \times 1) = 120 + 22 = 142\,g\,mol^{-1}\]

(iv) Two types of reaction that alkenes and alkynes undergo: addition reactions and polymerization (oxidation is also acceptable).

(b)(i) The conversion of palm oil to margarine is an addition reaction (specifically hydrogenation, the addition of hydrogen across the C=C double bonds).

(ii) The palm oil is first treated with activated charcoal in order to adsorb and remove the coloured impurities, i.e. to bleach/decolourize the oil.

(iii) The catalyst commonly used to convert vegetable oils to margarine is finely divided nickel.

(c)(i) Oils belong to the class of organic compounds called esters (specifically esters of glycerol and long-chain alkanoic (fatty) acids, i.e. lipids).

(ii) Manufacture of soap (saponification): the vegetable oil is boiled with concentrated sodium hydroxide solution; the oil is hydrolysed into soap (sodium salt of the fatty acid) and glycerol. Common salt (sodium chloride) is then added to \"salt out\" the soap, which rises to the surface as a solid, is skimmed off, washed and moulded.

(iii) Calcium ions in domestic water waste soap because they react with the soap (the sodium salt of the fatty acid) to form an insoluble scum (the calcium salt of the fatty acid). This scum forms before the soap can lather, so extra soap must be used before a lather is obtained.

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(a) Anhydride of each acid

An acid anhydride is the oxide that combines with water to give the acid.

• (i) H₂SO₄: sulphur (VI) oxide, SO₃ (SO₃ + H₂O → H₂SO₄).
• (ii) HNO₃: nitrogen (V) oxide, N₂O₅ (N₂O₅ + H₂O → 2HNO₃).

(b) Classification of salts

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(a)(i) Copper(II) oxide, \(CuO\), is a basic oxide (a metallic oxide that reacts with acids to give salt and water only).

(ii) Copper(II) oxide increases in mass on exposure to the atmosphere because, being a basic oxide, it slowly absorbs moisture (water vapour) and carbon(IV) oxide from the air, forming a basic copper carbonate/hydroxide. The added water and carbon(IV) oxide account for the gain in mass.

(iii) Action of dilute hydrochloric acid on copper(II) oxide:

\[CuO_{(s)} + 2HCl_{(aq)} \rightarrow CuCl_{2(aq)} + H_2O_{(l)}\]

(b) Two methods of removing total hardness (both temporary and permanent) from water:

• Addition of washing soda (sodium trioxocarbonate(IV), \(Na_2CO_3\)), which precipitates the calcium and magnesium ions as insoluble carbonates.
• Ion-exchange method (passing the water through a permutit/zeolite or ion-exchange resin), which replaces the \(Ca^{2+}\) and \(Mg^{2+}\) ions with sodium ions.

(Distillation of the water is also acceptable, as it leaves the dissolved salts behind.)

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(a) The elements \(_9\text{F}\), \(_{17}\text{Cl}\), \(_{35}\text{Br}\) and \(_{53}\text{I}\) all belong to Group VII of the Periodic Table.

(i) The first member is fluorine (atomic number 9). Its electronic structure is:

\[ \text{F}: 1s^2\,2s^2\,2p^5 \quad \text{(i.e. } 2,7). \]

(ii) The family name of the elements is the halogens.

(iii) Fluorine (F) has the strongest oxidizing ability. Oxidizing power decreases down the group as atomic size increases and the ease of gaining an electron falls; fluorine, being the smallest, attracts an incoming electron most strongly.

(b)(i) The chloride that exists as discrete molecules is BCl₄ (a covalent chloride, existing as separate molecules with low melting and boiling points).

(b)(ii) The bonding that holds atoms of A and chlorine together in ACl is ionic (electrovalent) bonding, formed by the transfer of an electron from the metal A to chlorine.

(b)(iii) The chloride that is a good conductor of electricity (when molten or in aqueous solution) is ACl, because it is ionic and contains free-moving ions.

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(a) Monomer units:

• (i) Polyethene is formed from the monomer ethene (ethylene), \(CH_2=CH_2\).
• (ii) Cellulose is a natural polymer whose monomer unit is glucose (\(\beta\)-glucose), \(C_6H_{12}O_6\).

(b) When steam and air are passed over red-hot coke, the two fuel gases obtained are:

• Water gas (a mixture of carbon(II) oxide and hydrogen, \(CO + H_2\)), formed by the action of steam on red-hot coke: \[C_{(s)} + H_2O_{(g)} \rightarrow CO_{(g)} + H_{2(g)}\]
• Producer gas (a mixture of carbon(II) oxide and nitrogen, \(CO + N_2\)), formed by the action of air on red-hot coke: \[2C_{(s)} + O_{2(g)} \rightarrow 2CO_{(g)}\] the nitrogen of the air passing through unreacted.

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(a)(i) The process for obtaining ethanol from sugars is fermentation.

(ii) When ethanol undergoes bacterial oxidation, the compound produced is ethanoic acid (acetic acid), \(CH_3COOH\).

(iii) The enzyme in yeast which catalyses the conversion of maltose to glucose is maltase.

(b)(i) Structural formula of ethanol:

\[CH_3-CH_2-OH\]

showing the hydroxyl \(-OH\) group attached to the terminal carbon.

(ii) When propanoic acid reacts with ethanol (esterification), the organic product is the ester ethyl propanoate, \(CH_3CH_2COOC_2H_5\).

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(a)(i) Three factors that affect the discharge of ions during electrolysis

• The position of the ion in the electrochemical (discharge) series; the ion lower in the series is discharged in preference.
• The concentration of the ion in the electrolyte; a high concentration can favour the discharge of an ion that would otherwise not be discharged.
• The nature of the electrode used (whether it is inert, e.g. platinum/carbon, or attackable, e.g. copper).

(a)(ii) Purification of copper by electrolysis

• A thick block of impure copper is made the anode and a thin strip of pure copper is made the cathode.
• The electrolyte is acidified copper(II) tetraoxosulphate(VI) solution, \( CuSO_4 \).
• When current flows, the impure anode dissolves: \( Cu \rightarrow Cu^{2+} + 2e^- \).
• The copper ions migrate to the cathode and are deposited there as pure copper: \( Cu^{2+} + 2e^- \rightarrow Cu \).
• Soluble impurities such as iron and zinc remain in solution, while insoluble impurities such as silver and gold fall below the anode as anode mud.

(b)(i) Completed table

The quantity of electricity is \( Q = I \times t \), with \( I = 0.20\ A \) in each case.

Sample calculation: \( Q = 0.20 \times 900 = 180\ C \), and so on for the other rows.

(b)(ii) Graph of mass of M deposited against quantity of electricity passed

The points lie on a straight line passing through the origin, showing that the mass deposited is directly proportional to the quantity of electricity passed.

(b)(iii) Mass of M deposited by the same current for 20 minutes

Time \( t = 20 \times 60 = 1200\ s \), so the quantity of electricity is \[ Q = I \times t = 0.20 \times 1200 = 240\ C. \]Reading up from \( Q = 240\ C \) on the horizontal axis to the line of best fit, and across to the vertical axis, the mass deposited is 0.08 g. (Check: the gradient is \( \dfrac{0.24}{720} = 3.33\times10^{-4}\ \text{g C}^{-1} \), so \( 3.33\times10^{-4} \times 240 = 0.08\ g \).)

(b)(iv) Law verified

Because the graph is a straight line through the origin, the mass deposited is proportional to the quantity of electricity passed. The experiment therefore verifies Faraday's first law of electrolysis.

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(a)(i) An acid is a substance that ionizes in water (or other suitable solvent) to produce hydrogen ions, \(H^+\), as the only positive ions. (More generally, a proton, \(H^+\), donor.)

(ii) Three chemical properties of tetraoxosulphate(VI) acid, \(H_2SO_4\):

• It reacts with bases and metallic oxides (neutralization) to form a salt (tetraoxosulphate(VI)) and water only, e.g. \(H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O\).
• It reacts with reactive metals to liberate hydrogen gas, e.g. \(H_2SO_4 + Zn \rightarrow ZnSO_4 + H_2\).
• It reacts with trioxocarbonate(IV) salts to give carbon(IV) oxide, e.g. \(H_2SO_4 + Na_2CO_3 \rightarrow Na_2SO_4 + H_2O + CO_2\). (Hot concentrated \(H_2SO_4\) also acts as an oxidizing and a dehydrating agent.)

(iii) Two uses of \(H_2SO_4\): in the manufacture of fertilizers (e.g. ammonium tetraoxosulphate(VI)); as the electrolyte in lead-acid accumulators (car batteries). (Also used to make detergents and as a drying/dehydrating agent.)

(b)(i) Laboratory preparation of zinc tetraoxosulphate(VI) crystals from zinc oxide (insoluble base + acid method):

1. Warm some dilute tetraoxosulphate(VI) acid in a beaker.
2. Add zinc oxide (the insoluble base) a little at a time, stirring, until some remains undissolved (i.e. add the oxide in excess to ensure all the acid reacts).
3. Filter to remove the excess zinc oxide; the filtrate is zinc tetraoxosulphate(VI) solution.
4. Evaporate the filtrate to the point of crystallization (until a hot saturated solution is obtained).
5. Allow it to cool slowly so that crystals form; filter off the crystals and dry them between filter papers.

\[ZnO + H_2SO_4 \rightarrow ZnSO_4 + H_2O\]

(ii) Zinc oxide is amphoteric because it reacts with both acids and bases:

As a base (with acid): \[ZnO + H_2SO_4 \rightarrow ZnSO_4 + H_2O\]

As an acid (with alkali): \[ZnO + 2NaOH \rightarrow Na_2ZnO_2 + H_2O\]

(c)(i) The reaction \(Zn + CuSO_4 \rightarrow ZnSO_4 + Cu\) is a displacement (redox) reaction (zinc, being more reactive, displaces copper from its salt).

(ii) Mass of zinc that reacts with 8.0 g of \(CuSO_4\):

\[\text{moles of } CuSO_4 = \frac{8.0}{160} = 0.05\,mol\]

From the equation, 1 mole \(CuSO_4\) reacts with 1 mole \(Zn\), so moles of \(Zn = 0.05\,mol\).

\[\text{mass of } Zn = 0.05 \times 65 = 3.25\,g\]