WAEC SSCE - Chemistry - 2017 (Essay)
Question 1 Report
(a) Write the molecular formula of X.
(i) What type of reaction is represented by the equation?
(ii) Consider the following reaction equation: \( \mathrm{C_{12}H_2 \to X + C_8H_{18}} \)
(iii) Draw the structure of two isomers of X.
(iv) Name the isomers drawn in (a)(iii).
(v) Write a balanced equation for the reaction between X and hydrogen.
(b) Describe one test for fats.
(c) Sulphur (IV) oxide is converted to tetraoxosulphate (VI) acid according to the following equation: \( \mathrm{2SO_{2(g)} + O_{2(g)} + 2H_2O_{(l)} \to 2H_2OSO_{4(aq)}} \). If 1.5 moles of oxgen reacts with sulphur (IV) oxide, calculate the mass of tetraoxosulphate (VI) acid produced. [H = 1.0; O = 16.0; S = 32.0].
(d) Consider the following neutralization reaction:
\( \mathrm{CH_3COOH + NaOH \to CH_3COONa + H_2O;} \ \bigtriangleup H_1 \)
\( \mathrm{CH_3COOH + NH_4OH \to CH_3COONH_4 + H_2O;} \ \bigtriangleup H_2 \)
\( \mathrm{NaOH + HCl \to NaCl + H_2O} \ \bigtriangleup H_3 \)
(i) Arrange the enthalphy changes for the reactions in order of increasing magnitude.
(ii) Explain briefly your order in (d)(i).
(e) Consider the following substances. \( \mathrm{Cu_{(s)}} \), \( \mathrm{BeCl_2} \), \( \mathrm{NaH_{(s)}} \), \( \mathrm{HF_{(s)}} \)and \( \mathrm{CCl_{4(l)}} \). State the substance(s) which;
(i) can conduct electricity;
(ii) is/are soluble in water.
The equation in (a)(ii) is the catalytic cracking of dodecane, \(C_{12}H_{26} \to X + C_8H_{18}\). Balancing atoms: carbon \(12-8=4\); hydrogen \(26-18=8\). Therefore X is butene, \(C_4H_8\), an alkene.
(a) Molecular formula of X
\[ X = C_4H_8 \]
(a)(i) Type of reaction
It is catalytic cracking (thermal decomposition of a long-chain alkane into a shorter alkane and an alkene).
(a)(iii) Structures of two isomers of X (\(C_4H_8\))
Two chain/positional isomers of butene are drawn below, showing every bond and hydrogen atom:
(a)(iv) Names of the isomers
Isomer A is but-1-ene (\(CH_2=CH{-}CH_2{-}CH_3\)) and Isomer B is but-2-ene (\(CH_3{-}CH=CH{-}CH_3\)).
(a)(v) Reaction of X with hydrogen
Butene undergoes catalytic addition (hydrogenation) with hydrogen to give butane:
\[ C_4H_{8} + H_2 \xrightarrow{\;Ni\;} C_4H_{10} \]
(b) One test for fats
Translucent (grease) spot test: rub a small quantity of the substance onto a piece of filter paper and allow it to dry (warm gently if solid). A permanent translucent (oily) spot that does not disappear on drying confirms the presence of fat. (Alternatively, adding Sudan III reagent gives a red-stained oily layer, confirming fat.)
(c) Mass of tetraoxosulphate (VI) acid produced
Equation: \(2SO_{2(g)} + O_{2(g)} + 2H_2O_{(l)} \to 2H_2SO_{4(aq)}\)
Molar mass of \(H_2SO_4 = (2\times1.0) + 32.0 + (4\times16.0) = 2 + 32 + 64 = 98\ \text{g mol}^{-1}\).
From the equation, \(1\) mole of \(O_2\) produces \(2\) moles of \(H_2SO_4\).
\[ 1.5\ \text{mol } O_2 \Rightarrow 2 \times 1.5 = 3\ \text{mol } H_2SO_4 \]
\[ \text{Mass} = \text{moles} \times \text{molar mass} = 3 \times 98 = \mathbf{294\ g} \]
(d)(i) Enthalpy changes in order of increasing magnitude
\[ \Delta H_2 < \Delta H_1 < \Delta H_3 \]
(d)(ii) Explanation
The enthalpy of neutralisation depends on the energy released when \(H^+\) and \(OH^-\) ions combine to form water; the more completely the acid and base ionise, the more \(H^+\) and \(OH^-\) are available and the greater the heat evolved. \(\Delta H_3\) (strong acid HCl + strong base NaOH) is largest because both are fully ionised. \(\Delta H_1\) (weak acid \(CH_3COOH\) + strong base NaOH) is smaller because some energy is absorbed to ionise the weak acid. \(\Delta H_2\) (weak acid + weak base \(NH_4OH\)) is smallest because energy is absorbed to ionise both the weak acid and the weak base.
(e)(i) Substance(s) that can conduct electricity
\(Cu_{(s)}\) only, because it is a metal with a sea of mobile (delocalised) electrons.
(e)(ii) Substance(s) soluble in water
\(BeCl_2\), \(NaH_{(s)}\) and \(HF\) dissolve in water (they are ionic/polar and interact with the polar water molecules), whereas the non-polar covalent \(CCl_4\) is insoluble.
Answer Details
The equation in (a)(ii) is the catalytic cracking of dodecane, \(C_{12}H_{26} \to X + C_8H_{18}\). Balancing atoms: carbon \(12-8=4\); hydrogen \(26-18=8\). Therefore X is butene, \(C_4H_8\), an alkene.
(a) Molecular formula of X
\[ X = C_4H_8 \]
(a)(i) Type of reaction
It is catalytic cracking (thermal decomposition of a long-chain alkane into a shorter alkane and an alkene).
(a)(iii) Structures of two isomers of X (\(C_4H_8\))
Two chain/positional isomers of butene are drawn below, showing every bond and hydrogen atom:
(a)(iv) Names of the isomers
Isomer A is but-1-ene (\(CH_2=CH{-}CH_2{-}CH_3\)) and Isomer B is but-2-ene (\(CH_3{-}CH=CH{-}CH_3\)).
(a)(v) Reaction of X with hydrogen
Butene undergoes catalytic addition (hydrogenation) with hydrogen to give butane:
\[ C_4H_{8} + H_2 \xrightarrow{\;Ni\;} C_4H_{10} \]
(b) One test for fats
Translucent (grease) spot test: rub a small quantity of the substance onto a piece of filter paper and allow it to dry (warm gently if solid). A permanent translucent (oily) spot that does not disappear on drying confirms the presence of fat. (Alternatively, adding Sudan III reagent gives a red-stained oily layer, confirming fat.)
(c) Mass of tetraoxosulphate (VI) acid produced
Equation: \(2SO_{2(g)} + O_{2(g)} + 2H_2O_{(l)} \to 2H_2SO_{4(aq)}\)
Molar mass of \(H_2SO_4 = (2\times1.0) + 32.0 + (4\times16.0) = 2 + 32 + 64 = 98\ \text{g mol}^{-1}\).
From the equation, \(1\) mole of \(O_2\) produces \(2\) moles of \(H_2SO_4\).
\[ 1.5\ \text{mol } O_2 \Rightarrow 2 \times 1.5 = 3\ \text{mol } H_2SO_4 \]
\[ \text{Mass} = \text{moles} \times \text{molar mass} = 3 \times 98 = \mathbf{294\ g} \]
(d)(i) Enthalpy changes in order of increasing magnitude
\[ \Delta H_2 < \Delta H_1 < \Delta H_3 \]
(d)(ii) Explanation
The enthalpy of neutralisation depends on the energy released when \(H^+\) and \(OH^-\) ions combine to form water; the more completely the acid and base ionise, the more \(H^+\) and \(OH^-\) are available and the greater the heat evolved. \(\Delta H_3\) (strong acid HCl + strong base NaOH) is largest because both are fully ionised. \(\Delta H_1\) (weak acid \(CH_3COOH\) + strong base NaOH) is smaller because some energy is absorbed to ionise the weak acid. \(\Delta H_2\) (weak acid + weak base \(NH_4OH\)) is smallest because energy is absorbed to ionise both the weak acid and the weak base.
(e)(i) Substance(s) that can conduct electricity
\(Cu_{(s)}\) only, because it is a metal with a sea of mobile (delocalised) electrons.
(e)(ii) Substance(s) soluble in water
\(BeCl_2\), \(NaH_{(s)}\) and \(HF\) dissolve in water (they are ionic/polar and interact with the polar water molecules), whereas the non-polar covalent \(CCl_4\) is insoluble.
Question 2 Report
(a)(i) What is an acid-base indicator?
(ii) Give one example of an acid-base indicator.
(b) State the property exhibited by nitrogen(IV) oxide in each of the following equations:,
(i) 4Cu + 2NO\(_2\) -> 4CuO + N\(_2\) (ii) H\(_2\)O + 2NO\(_2\) --> HNO\(_3\) + HNO\(_2\)
(c)(i) Define enthalpy of combustion..
(ii) State why the enthalpy of combustion is always negative.
(d)(i) Distinguish between a primary cell and a secondary cell.
(ii) Give an example of each of the cells stated in I (d)(i).
(e) Define the term mole.
(f) Calculate the amount of hydrochloric acid in 40.0 cm\(^3\) of 0.40 moldm\(^{-3}\) dilute HCl.
(g) Name two substances which can be used as electrodes during the electroylsis of acidified water.
(h) List two forces of attraction that can exist between covalent molecules.
(i) Name the products formed when butane undergoes incomplete combustion.
(j) Write the electron configuration of \(_{26}\)Fe\(^{3+}\)
(a)(i) An acid-base indicator is a substance (usually a weak acid or weak base) that shows different colours in acidic and alkaline media and is used to detect the end point of a neutralization.
(ii) Example: litmus (or methyl orange, or phenolphthalein).
(b)(i) In \(4Cu + 2NO_2 \rightarrow 4CuO + N_2\), nitrogen(IV) oxide acts as an oxidizing agent (it is reduced and oxidizes the copper).
(ii) In \(H_2O + 2NO_2 \rightarrow HNO_3 + HNO_2\), nitrogen(IV) oxide undergoes disproportionation, acting as both an oxidizing and a reducing agent (N goes from +4 to +5 and to +3).
(c)(i) Enthalpy of combustion is the heat change when one mole of a substance is completely burnt in excess oxygen under standard conditions.
(ii) It is always negative because combustion is exothermic - heat is released to the surroundings.
(d)(i) A primary cell cannot be recharged; its chemical reaction is irreversible. A secondary cell can be recharged because its reaction is reversible.
(ii) Primary cell: dry (Leclanche) cell. Secondary cell: lead-acid accumulator.
(e) A mole is the amount of a substance that contains as many elementary entities as there are atoms in 12 g of carbon-12 (that is, \(6.02 \times 10^{23}\) entities).
(f) Amount of HCl \(= C \times V = 0.40 \times \dfrac{40.0}{1000} = 0.016\ \text{mol}\).
(g) Platinum and carbon (graphite) - inert electrodes.
(h) Van der Waals (dispersion) forces and hydrogen bonds.
(i) Carbon(II) oxide (carbon monoxide) and water (with some carbon/soot).
(j) \(_{26}\)Fe is \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 4s^2\); Fe3+ loses the two 4s and one 3d electron:
\[1s^2 2s^2 2p^6 3s^2 3p^6 3d^5\]
Answer Details
(a)(i) An acid-base indicator is a substance (usually a weak acid or weak base) that shows different colours in acidic and alkaline media and is used to detect the end point of a neutralization.
(ii) Example: litmus (or methyl orange, or phenolphthalein).
(b)(i) In \(4Cu + 2NO_2 \rightarrow 4CuO + N_2\), nitrogen(IV) oxide acts as an oxidizing agent (it is reduced and oxidizes the copper).
(ii) In \(H_2O + 2NO_2 \rightarrow HNO_3 + HNO_2\), nitrogen(IV) oxide undergoes disproportionation, acting as both an oxidizing and a reducing agent (N goes from +4 to +5 and to +3).
(c)(i) Enthalpy of combustion is the heat change when one mole of a substance is completely burnt in excess oxygen under standard conditions.
(ii) It is always negative because combustion is exothermic - heat is released to the surroundings.
(d)(i) A primary cell cannot be recharged; its chemical reaction is irreversible. A secondary cell can be recharged because its reaction is reversible.
(ii) Primary cell: dry (Leclanche) cell. Secondary cell: lead-acid accumulator.
(e) A mole is the amount of a substance that contains as many elementary entities as there are atoms in 12 g of carbon-12 (that is, \(6.02 \times 10^{23}\) entities).
(f) Amount of HCl \(= C \times V = 0.40 \times \dfrac{40.0}{1000} = 0.016\ \text{mol}\).
(g) Platinum and carbon (graphite) - inert electrodes.
(h) Van der Waals (dispersion) forces and hydrogen bonds.
(i) Carbon(II) oxide (carbon monoxide) and water (with some carbon/soot).
(j) \(_{26}\)Fe is \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 4s^2\); Fe3+ loses the two 4s and one 3d electron:
\[1s^2 2s^2 2p^6 3s^2 3p^6 3d^5\]
Question 3 Report
(a)(i) 1. State the periodic law.
2. What is meant by the term periodic property of elements?
(ii) List three properties of an element which show periodicity.
(iii) Explain briefly how each of the properties listed in (a)(i) in varies across the period.
(b) Defulle relative atomic mass.
(c)(i) What phenomenon is exhibited by an element Z which exist as \(^{35}_{17}Z\) and \(^{37}_{17}X\)
(ii) What accounts for the difference in the mass numbers of the element Z?
(iii) Calculate the relative atomic mass of Z if the percentage abundance of \(^{37}_{17}Z\) is 75%
(d)(i) State the method used for collecting each of the following gases: I. CO II. HCI III. H\(_2\)
(ii) Give a reason for your answer stated in (d)(i) I and II
Answer Details
None
Question 4 Report
(a)(i) Describe briefly the industrial preparation of ammonia.
(ii) Write a balanced equation for the reaction in (a)(i).
(iii) State one way of increasing the yield of ammonia in 4(a)(i).
(iv) State two uses of ammonia.
(b) Describe briefly, one chemical test for each of the following gases in the laboratory: (i) hydrogen; (ii) carbon (IV) oxide; (iii) oxygen.
(c)(i) State the composition of water gas.
(ii) List two uses of water gas.
(d) Describe briefly a simple experiment to determine the type of hardness in a sample of water.
(a)(i) Industrial preparation of ammonia (Haber process): nitrogen (from air) and hydrogen (from natural gas) are mixed in the ratio 1:3 and passed over a finely divided iron catalyst at about 450 °C and about 200 atmospheres pressure.
(ii) \[N_2 + 3H_2 \rightleftharpoons 2NH_3\]
(iii) Increase the yield by increasing the pressure (or by lowering the temperature, or removing ammonia as it forms).
(iv) Uses of ammonia: manufacture of fertilizers (ammonium salts) and manufacture of trioxonitrate(V) acid.
(b) Chemical tests
- (i) Hydrogen: a lighted splint applied to the gas burns it with a squeaky "pop" sound.
- (ii) Carbon(IV) oxide: the gas turns limewater (calcium hydroxide solution) milky.
- (iii) Oxygen: the gas rekindles (relights) a glowing splint.
(c)(i) Water gas is a mixture of carbon(II) oxide and hydrogen (CO + H2).
(ii) Uses of water gas: as a fuel; as a source of hydrogen in industry (for example in the manufacture of methanol/ammonia).
(d) Type of hardness: boil a sample of the water and then add soap solution. If the water now lathers readily (hardness removed by boiling), the hardness is temporary. If it still does not lather easily after boiling, the hardness is permanent. Comparing lathering before and after boiling distinguishes the two.
Answer Details
(a)(i) Industrial preparation of ammonia (Haber process): nitrogen (from air) and hydrogen (from natural gas) are mixed in the ratio 1:3 and passed over a finely divided iron catalyst at about 450 °C and about 200 atmospheres pressure.
(ii) \[N_2 + 3H_2 \rightleftharpoons 2NH_3\]
(iii) Increase the yield by increasing the pressure (or by lowering the temperature, or removing ammonia as it forms).
(iv) Uses of ammonia: manufacture of fertilizers (ammonium salts) and manufacture of trioxonitrate(V) acid.
(b) Chemical tests
- (i) Hydrogen: a lighted splint applied to the gas burns it with a squeaky "pop" sound.
- (ii) Carbon(IV) oxide: the gas turns limewater (calcium hydroxide solution) milky.
- (iii) Oxygen: the gas rekindles (relights) a glowing splint.
(c)(i) Water gas is a mixture of carbon(II) oxide and hydrogen (CO + H2).
(ii) Uses of water gas: as a fuel; as a source of hydrogen in industry (for example in the manufacture of methanol/ammonia).
(d) Type of hardness: boil a sample of the water and then add soap solution. If the water now lathers readily (hardness removed by boiling), the hardness is temporary. If it still does not lather easily after boiling, the hardness is permanent. Comparing lathering before and after boiling distinguishes the two.
Question 5 Report
(a)(i) Describe briefly how trioxonitrate (V) ions could be tested for in the laboratory.
(ii) State two uses of each of the following compounds: I. sodium chloride; II. sodium trioxocarbonate (IV).
(b) Write balanced equations for the reactions involved in the extraction of iron in the blast furnace.
(ii) State Faraday's first law of electrolysis.
(iii) State two applications of electrolysis.
(c) Concentrated tetraoxosulphate (VI) acid is added to sugar crystals in a beaker. State what would be observed. Explain briefly your answer.
(d) Write an equation for the reaction of zinc powder with:
(i) dilute tetraoxosulphate (VI) acid;
(ii) concentrated tetraoxosulphate (VI) acid.
(e) What property of concentrated tetraoxosulphate (VI) acid is shown in (d)(ii)
(a)(i) Test for trioxonitrate(V) (nitrate) ion - brown ring test: add freshly prepared iron(II) sulphate solution to the test solution, then carefully pour concentrated tetraoxosulphate(VI) acid down the side of the tube so it forms a layer below. A brown ring forms at the junction of the two liquids.
(ii)
- I. Sodium chloride: seasoning/preserving food; raw material for making chlorine, sodium hydroxide and sodium trioxocarbonate(IV).
- II. Sodium trioxocarbonate(IV): softening hard water; manufacture of glass and soap.
(b)(i) Extraction of iron in the blast furnace
\[C + O_2 \rightarrow CO_2\]
\[CO_2 + C \rightarrow 2CO\]
\[Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2\]
\[CaCO_3 \rightarrow CaO + CO_2\]
\[CaO + SiO_2 \rightarrow CaSiO_3 \text{ (slag)}\]
(ii) Faraday's first law: the mass of a substance liberated or deposited at an electrode during electrolysis is directly proportional to the quantity of electricity (charge) passed through the electrolyte.
(iii) Applications: electroplating; purification (refining) of metals; extraction of reactive metals.
(c) The white sugar turns black (a spongy black mass of carbon rises up), with much heat and steam given off. Concentrated tetraoxosulphate(VI) acid is a powerful dehydrating agent: it removes hydrogen and oxygen from the sucrose as water, leaving carbon:
\[C_{12}H_{22}O_{11} \xrightarrow{conc.\ H_2SO_4} 12C + 11H_2O\]
(d)(i) \[Zn + H_2SO_4\ (dilute) \rightarrow ZnSO_4 + H_2\]
(ii) \[Zn + 2H_2SO_4\ (conc.,\ hot) \rightarrow ZnSO_4 + SO_2 + 2H_2O\]
(e) In (d)(ii) the concentrated acid acts as an oxidizing agent.
Answer Details
(a)(i) Test for trioxonitrate(V) (nitrate) ion - brown ring test: add freshly prepared iron(II) sulphate solution to the test solution, then carefully pour concentrated tetraoxosulphate(VI) acid down the side of the tube so it forms a layer below. A brown ring forms at the junction of the two liquids.
(ii)
- I. Sodium chloride: seasoning/preserving food; raw material for making chlorine, sodium hydroxide and sodium trioxocarbonate(IV).
- II. Sodium trioxocarbonate(IV): softening hard water; manufacture of glass and soap.
(b)(i) Extraction of iron in the blast furnace
\[C + O_2 \rightarrow CO_2\]
\[CO_2 + C \rightarrow 2CO\]
\[Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2\]
\[CaCO_3 \rightarrow CaO + CO_2\]
\[CaO + SiO_2 \rightarrow CaSiO_3 \text{ (slag)}\]
(ii) Faraday's first law: the mass of a substance liberated or deposited at an electrode during electrolysis is directly proportional to the quantity of electricity (charge) passed through the electrolyte.
(iii) Applications: electroplating; purification (refining) of metals; extraction of reactive metals.
(c) The white sugar turns black (a spongy black mass of carbon rises up), with much heat and steam given off. Concentrated tetraoxosulphate(VI) acid is a powerful dehydrating agent: it removes hydrogen and oxygen from the sucrose as water, leaving carbon:
\[C_{12}H_{22}O_{11} \xrightarrow{conc.\ H_2SO_4} 12C + 11H_2O\]
(d)(i) \[Zn + H_2SO_4\ (dilute) \rightarrow ZnSO_4 + H_2\]
(ii) \[Zn + 2H_2SO_4\ (conc.,\ hot) \rightarrow ZnSO_4 + SO_2 + 2H_2O\]
(e) In (d)(ii) the concentrated acid acts as an oxidizing agent.
