WAEC SSCE - Chemistry - 2014 (Essay)
Question 1 Report
(a)(i) Outline the steps involved in the purification of water for town supply.
(ii) Give two cations that can cause hardness in water.
(iii) State two disadvantages of hardness of water.
(iv) List two sources of water pollution.
(b)(i) Explain briefly why water is a good solvent for sodium chloride but not for oil.
(ii) State the function of H\(_2\)SO\(_4\) in each of the following reaction equations:
(i) C\(_2\)H\(_5\)OH\(_{(l)}\) \(\to\) C\(_2\)H\(_{4(g)}\)
(ii) MgO\(_{(s)}\) + H\(_2\)SO\(_4\)\(_{(aq)}\) —> MgSO\(_{4(aq)}\) + H\(_2\)O\(_{(l)}\)
(iii) C\(_{(s)}\) + conc. H\(_2\)SO\(_4\) --> CO\(_{(g)}\) + SO\(_{2(g)}\) + H\(_2\)O\(_{(l)}\)
(c) (i) Give the two products formed when chlorine water is exposed to sunlight.
(ii) I. Which of the compounds is suitable for the preparation of standard alkaline solution?
II. Give a reason for your answer in (c)(ii)I
(a)(i) Purification of water for town supply
- Screening to remove large floating debris.
- Sedimentation/coagulation: a coagulant such as alum is added to make fine suspended particles clump and settle.
- Filtration through beds of sand and gravel to remove remaining suspended matter.
- Chlorination (sterilization) to kill harmful micro-organisms.
- Storage in covered reservoirs and distribution.
(ii) Cations causing hardness: Ca2+ and Mg2+.
(iii) Disadvantages of hard water: it wastes soap (forms scum); it deposits scale/fur in kettles, boilers and pipes.
(iv) Sources of water pollution: industrial effluents; sewage/domestic waste (also agricultural run-off, oil spillage).
(b)(i) Water is a polar solvent. Sodium chloride is ionic, so its Na+ and Cl- ions are strongly attracted to and become surrounded (solvated) by the polar water molecules, and it dissolves. Oil is non-polar (covalent) and has no charged centres for water to attract, so water does not dissolve it (like dissolves like).
(ii) Function of H2SO4
- C2H5OH → C2H4: dehydrating agent (removes water).
- MgO + H2SO4 → MgSO4 + H2O: acid (undergoes neutralization).
- C + conc. H2SO4 → CO + SO2 + H2O: oxidizing agent.
(c)(i) When chlorine water stands in sunlight the hypochlorous acid decomposes and the two products are hydrochloric acid (HCl) and oxygen (O2):
2Cl2 + 2H2O → 4HCl + O2
(ii) Neither of these two products (an acid and oxygen) gives an alkaline solution, so neither is suitable for preparing a standard alkaline solution; both leave an acidic/neutral solution. A primary standard alkali (e.g. anhydrous Na2CO3) would instead be needed, which is not among these products.
Answer Details
(a)(i) Purification of water for town supply
- Screening to remove large floating debris.
- Sedimentation/coagulation: a coagulant such as alum is added to make fine suspended particles clump and settle.
- Filtration through beds of sand and gravel to remove remaining suspended matter.
- Chlorination (sterilization) to kill harmful micro-organisms.
- Storage in covered reservoirs and distribution.
(ii) Cations causing hardness: Ca2+ and Mg2+.
(iii) Disadvantages of hard water: it wastes soap (forms scum); it deposits scale/fur in kettles, boilers and pipes.
(iv) Sources of water pollution: industrial effluents; sewage/domestic waste (also agricultural run-off, oil spillage).
(b)(i) Water is a polar solvent. Sodium chloride is ionic, so its Na+ and Cl- ions are strongly attracted to and become surrounded (solvated) by the polar water molecules, and it dissolves. Oil is non-polar (covalent) and has no charged centres for water to attract, so water does not dissolve it (like dissolves like).
(ii) Function of H2SO4
- C2H5OH → C2H4: dehydrating agent (removes water).
- MgO + H2SO4 → MgSO4 + H2O: acid (undergoes neutralization).
- C + conc. H2SO4 → CO + SO2 + H2O: oxidizing agent.
(c)(i) When chlorine water stands in sunlight the hypochlorous acid decomposes and the two products are hydrochloric acid (HCl) and oxygen (O2):
2Cl2 + 2H2O → 4HCl + O2
(ii) Neither of these two products (an acid and oxygen) gives an alkaline solution, so neither is suitable for preparing a standard alkaline solution; both leave an acidic/neutral solution. A primary standard alkali (e.g. anhydrous Na2CO3) would instead be needed, which is not among these products.
Question 2 Report
(a) Define each of the following terms:
(i) ion; (ii) isotopes.
(b)(i) Consider the:element \(_{12}\)Mg and \(_{3}\)Al
I. Write the electron configuration of each element. II. Explain briefly why the first ionization energy of \(_{12}\)Mg is greater than that of \(_{13}\)Al.
(ii) Write the formulae of three different oxides of period 3 elements that react with water.
(c)(i) What are allotropes?
(ii) Name the two crystalline allotropes of carbon.
(iii) Give one industrial use of each allotrope named in (c)(ii).
(d) On warming crystals of sodium chloride with concentrated tetraoxosulphate (VI) acid, a gas was evolved.
(i) List two physical properties of the gas produced.
(ii) Write a balanced equation for the reaction.
(e) A certain chip W used in a circuit of a microcomputer has a mass of 5.68mg. Calculate the amount of W in the microcomputer. [ W = 28 gmol\(^{-1}\) ].
(a)(i) An ion is an atom or group of atoms that carries a net electrical charge because it has lost or gained one or more electrons. (ii) Isotopes are atoms of the same element having the same atomic number (number of protons) but different mass numbers (different numbers of neutrons).
(b)(i) I. Electron configurations
- 12Mg: 1s2 2s2 2p6 3s2 (2, 8, 2)
- 13Al: 1s2 2s2 2p6 3s2 3p1 (2, 8, 3)
II. The first ionization energy of Mg is greater than that of Al because in Mg the outermost electron is removed from a stable, completely filled 3s2 sub-shell, whereas in Al the outermost electron is removed from the higher-energy 3p sub-shell, which is partly shielded by the 3s electrons and so is more easily removed. Hence less energy is needed for Al.
(ii) Three oxides of period 3 elements that react with water: Na2O, SO3, P4O10.
(c)(i) Allotropes are different structural forms of the same element existing in the same physical state. (ii) Diamond and graphite. (iii) Diamond: used in cutting/drilling tools and as an abrasive; graphite: used as a lubricant and as electrodes.
(d) Warming NaCl with concentrated H2SO4 evolves hydrogen chloride gas.
(i) Two physical properties of HCl: colourless gas; pungent (choking) smell; very soluble in water; fumes in moist air. (ii)
NaCl(s) + H2SO4(l) → NaHSO4(s) + HCl(g)
(e) Amount of W: (5.68 mg = 0.00568 g)
\[ n=\frac{0.00568}{28}=2.03\times10^{-4}\ \text{mol} \]Answer Details
(a)(i) An ion is an atom or group of atoms that carries a net electrical charge because it has lost or gained one or more electrons. (ii) Isotopes are atoms of the same element having the same atomic number (number of protons) but different mass numbers (different numbers of neutrons).
(b)(i) I. Electron configurations
- 12Mg: 1s2 2s2 2p6 3s2 (2, 8, 2)
- 13Al: 1s2 2s2 2p6 3s2 3p1 (2, 8, 3)
II. The first ionization energy of Mg is greater than that of Al because in Mg the outermost electron is removed from a stable, completely filled 3s2 sub-shell, whereas in Al the outermost electron is removed from the higher-energy 3p sub-shell, which is partly shielded by the 3s electrons and so is more easily removed. Hence less energy is needed for Al.
(ii) Three oxides of period 3 elements that react with water: Na2O, SO3, P4O10.
(c)(i) Allotropes are different structural forms of the same element existing in the same physical state. (ii) Diamond and graphite. (iii) Diamond: used in cutting/drilling tools and as an abrasive; graphite: used as a lubricant and as electrodes.
(d) Warming NaCl with concentrated H2SO4 evolves hydrogen chloride gas.
(i) Two physical properties of HCl: colourless gas; pungent (choking) smell; very soluble in water; fumes in moist air. (ii)
NaCl(s) + H2SO4(l) → NaHSO4(s) + HCl(g)
(e) Amount of W: (5.68 mg = 0.00568 g)
\[ n=\frac{0.00568}{28}=2.03\times10^{-4}\ \text{mol} \]Question 3 Report
(a)(i) Define the term functional group.
(ii) Name the functional groups present in the following compound:
(b) Consider the following structure of an organic compound, Q.
(1) Name compound Q.
(ii) Write the balanced equation for the complete combustion of compound Q.
(iii) What type of reaction will compound Q undergo with chlorine?
(vi) Draw the structure of-the alkene that is an isomer of compound Q.
(c) A chemistry student was provided with four samples of organic compounds, A, B, C, and D. Samples A and B each decolourized bromine in tetrachloromethane but only sample B reacted with a solution of ammoniacal silver trioxonitrate (V) to give a white precipitate. Sample D reacted with sodium trioxocarbonate (IV) to liberate carbon (IV) oxide. When heat was applied to the mixture of samples C and D in a test tube and drops of concentrated tetraoxosulphate (VI) acid added, a product with a fruity odour was formed.
(i) Name the family of organic compounds to which samples A, B, C, and D belong.
(ii) State why samples A and B reacted with bromine in tetrachloromethane.
(iii) State the reason why sample B gave a white precipitate with the solution of ammoniacal silver trioxocarbonate (V).
(iv) Give the name o f the reactions between' samples C and D.
(v) State the two roles of the concentrated tetraoxosulphate (VI) acid in the reaction in (a)(iv).
(d) (i) Describe briefly the production of biogas using a biogas generator.
(ii) State two uses of biogas.
(a)(i) A functional group is an atom, a bond, or a group of atoms in an organic compound which determines its characteristic chemical properties.
(a)(ii) The functional groups present are:
- hydroxyl (alkanol) group, −OH;
- alkanoate (ester) group, −COO−.
(b)(i) Compound Q is cyclopropane.
(b)(ii) The balanced equation for its complete combustion is:
\[2C_3H_6 + 9O_2 \rightarrow 6CO_2 + 6H_2O\]
(b)(iii) Cyclopropane undergoes a substitution reaction with chlorine.
(b)(iv) The alkene isomer of cyclopropane is propene, with structural formula:
(c)(i)
| Sample | Family of organic compound |
|---|---|
| A | Alkene |
| B | Alkyne |
| C | Alkanol |
| D | Alkanoic acid |
(c)(ii) Samples A and B decolourize bromine in tetrachloromethane because they are unsaturated compounds. They contain carbon-to-carbon multiple bonds which add bromine.
(c)(iii) Sample B is a terminal alkyne. Its terminal hydrogen is replaced by silver ions in ammoniacal silver trioxonitrate(V) solution to form an insoluble white silver alkynide precipitate.
(c)(iv) The reaction between C and D is esterification.
(c)(v) Concentrated tetraoxosulphate(VI) acid acts as:
- a catalyst; and
- a dehydrating agent, removing water formed and favouring ester formation.
(d)(i) Organic waste such as animal dung and plant materials is mixed with water and fed into an airtight biodigester. In the absence of air, anaerobic bacteria decompose the waste and produce biogas, mainly methane with carbon(IV) oxide. The gas collects in the dome of the digester and is taken out through a gas outlet. The residual slurry can be used as manure.
(d)(ii) Uses of biogas include:
- as a fuel for cooking or heating;
- for lighting or generating electricity.
Answer Details
(a)(i) A functional group is an atom, a bond, or a group of atoms in an organic compound which determines its characteristic chemical properties.
(a)(ii) The functional groups present are:
- hydroxyl (alkanol) group, −OH;
- alkanoate (ester) group, −COO−.
(b)(i) Compound Q is cyclopropane.
(b)(ii) The balanced equation for its complete combustion is:
\[2C_3H_6 + 9O_2 \rightarrow 6CO_2 + 6H_2O\]
(b)(iii) Cyclopropane undergoes a substitution reaction with chlorine.
(b)(iv) The alkene isomer of cyclopropane is propene, with structural formula:
(c)(i)
| Sample | Family of organic compound |
|---|---|
| A | Alkene |
| B | Alkyne |
| C | Alkanol |
| D | Alkanoic acid |
(c)(ii) Samples A and B decolourize bromine in tetrachloromethane because they are unsaturated compounds. They contain carbon-to-carbon multiple bonds which add bromine.
(c)(iii) Sample B is a terminal alkyne. Its terminal hydrogen is replaced by silver ions in ammoniacal silver trioxonitrate(V) solution to form an insoluble white silver alkynide precipitate.
(c)(iv) The reaction between C and D is esterification.
(c)(v) Concentrated tetraoxosulphate(VI) acid acts as:
- a catalyst; and
- a dehydrating agent, removing water formed and favouring ester formation.
(d)(i) Organic waste such as animal dung and plant materials is mixed with water and fed into an airtight biodigester. In the absence of air, anaerobic bacteria decompose the waste and produce biogas, mainly methane with carbon(IV) oxide. The gas collects in the dome of the digester and is taken out through a gas outlet. The residual slurry can be used as manure.
(d)(ii) Uses of biogas include:
- as a fuel for cooking or heating;
- for lighting or generating electricity.
Question 4 Report
(a)(i) Explain briefly each of the following terms: I. anode; II. cathode.
(ii) Sodium and aluminium are extracted by the electrolysis of molten sodium chloride and alumina respectively. Write balanced equations for the reactions at the anode and cathode during the extraction of: I. sodium; II. aluminium.
(iii) Explain briefly why extraction of aluminium is considered environmentally friendly while that of sodium is not.
(b) Consider the reaction represented by the following equation:
K\(_2\)Cr\(_2\)O\(_7\) + HC1--> KCI + CrCl\(_3\) + H\(_2\)O + Cl\(_2\)
(i) Explain briefly why this reaction is redox.
(ii) Write balanced half equations for the reaction.
(iii) Write the over-all balanced reaction equation.
(c) During the electrolysis of molten Al\(_2\)O\(_3\), a current of 6A was passed through the electrolyte for 1 hr. 30 mins. Calculate the mass of aluminium deposited at the cathode.
(a)(i) I. Anode: the electrode at which oxidation (loss of electrons) occurs; in electrolysis it is the positive electrode, where anions are discharged. II. Cathode: the electrode at which reduction (gain of electrons) occurs; in electrolysis it is the negative electrode, where cations are discharged.
(ii) Extraction equations
I. Sodium (molten NaCl):
Cathode: Na+ + e- → Na
Anode: 2Cl- → Cl2 + 2e-
II. Aluminium (molten Al2O3):
Cathode: Al3+ + 3e- → Al
Anode: 2O2- → O2 + 4e-
(iii) In the extraction of aluminium the gas discharged at the anode is oxygen, which is harmless. In the extraction of sodium the gas discharged at the anode is chlorine, which is poisonous and pollutes the environment. Hence the sodium process releases a toxic gas whereas the aluminium process does not.
(b)(i) The reaction is redox because chromium is reduced (its oxidation number falls from +6 in Cr2O72- to +3 in CrCl3) while chlorine is oxidized (from -1 in HCl to 0 in Cl2); oxidation and reduction occur together.
(ii) Half equations
Reduction: Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H2O
Oxidation: 2Cl- → Cl2 + 2e-
(iii) Overall:
K2Cr2O7 + 14HCl → 2KCl + 2CrCl3 + 7H2O + 3Cl2
(c) Time = 1 h 30 min = 5400 s, current = 6 A, Al3+ + 3e- → Al.
\[ Q=6\times5400=32400\ \text{C} \] \[ n(e^-)=\frac{32400}{96500}=0.3358\ \text{mol};\quad n(\text{Al})=\frac{0.3358}{3}=0.1119\ \text{mol} \] \[ \text{mass}=0.1119\times27=3.02\ \text{g} \]Answer Details
(a)(i) I. Anode: the electrode at which oxidation (loss of electrons) occurs; in electrolysis it is the positive electrode, where anions are discharged. II. Cathode: the electrode at which reduction (gain of electrons) occurs; in electrolysis it is the negative electrode, where cations are discharged.
(ii) Extraction equations
I. Sodium (molten NaCl):
Cathode: Na+ + e- → Na
Anode: 2Cl- → Cl2 + 2e-
II. Aluminium (molten Al2O3):
Cathode: Al3+ + 3e- → Al
Anode: 2O2- → O2 + 4e-
(iii) In the extraction of aluminium the gas discharged at the anode is oxygen, which is harmless. In the extraction of sodium the gas discharged at the anode is chlorine, which is poisonous and pollutes the environment. Hence the sodium process releases a toxic gas whereas the aluminium process does not.
(b)(i) The reaction is redox because chromium is reduced (its oxidation number falls from +6 in Cr2O72- to +3 in CrCl3) while chlorine is oxidized (from -1 in HCl to 0 in Cl2); oxidation and reduction occur together.
(ii) Half equations
Reduction: Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H2O
Oxidation: 2Cl- → Cl2 + 2e-
(iii) Overall:
K2Cr2O7 + 14HCl → 2KCl + 2CrCl3 + 7H2O + 3Cl2
(c) Time = 1 h 30 min = 5400 s, current = 6 A, Al3+ + 3e- → Al.
\[ Q=6\times5400=32400\ \text{C} \] \[ n(e^-)=\frac{32400}{96500}=0.3358\ \text{mol};\quad n(\text{Al})=\frac{0.3358}{3}=0.1119\ \text{mol} \] \[ \text{mass}=0.1119\times27=3.02\ \text{g} \]Question 5 Report
(a)(i) Define ionic bond.
(ii) What type of bond (s) exist (s) in: I. magnesium oxide; II. ammonium ion?
(b) Determine the oxidation number of sulphur in \( \mathrm{Na_2S_2O_2} \).
(c) State Faraday's first law.
(d) Give one example each of: (i) acid salt; (ii) base salt.
(e) Name the type of energy change that occurs in each of the following processes
(i) \( \mathrm{I_{2(s)} \to I_{2(g)}} \)
(ii) \( \mathrm{Cl_{(g)} + e^- \to Cl^-_{(g)}} \)
(f) State the effect of each of the following aqueous solutions on litmus paper: (i) \( \mathrm{Na_2SO_{4(aq)}} \) (ii) \( \mathrm{AlCl_{3(aq)}} \)
(g) Define the term efflorescence.
(h) Give two uses of activated charcoal.
(i) State one use of each of the following processes in the chemical industry: (i) hydrogenation of vegetable oil; (ii) cracking; (iii) esterification.
(j) Calculate the amount of silver deposited in moles when 10920 coulombs of electricity is passed through a solution of a silver salt. [ Faraday constant-- 96500 \( \mathrm{C\ mol^{-1}} \)]
(a)(i) An ionic (electrovalent) bond is the electrostatic force of attraction between oppositely charged ions formed by the complete transfer of one or more electrons from a metal atom to a non-metal atom.
(ii) I. Magnesium oxide (MgO): ionic (electrovalent) bond. II. Ammonium ion (NH4+): covalent bonds, one of which is a coordinate (dative) covalent bond from N to the extra H+.
(b) Oxidation number of sulphur in sodium thiosulphate, Na2S2O3
\[ 2(+1)+2(S)+3(-2)=0 \Rightarrow 2S=+4 \Rightarrow S=+2 \](Average oxidation number of S = +2. If the formula is read strictly as Na2S2O2, the same method gives S = +1.)
(c) 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.
(d)(i) Acid salt: sodium hydrogentrioxocarbonate(IV), NaHCO3 (or NaHSO4). (ii) Basic salt: magnesium hydroxychloride, Mg(OH)Cl (or basic lead carbonate).
(e)(i) I2(s) → I2(g): sublimation, energy is absorbed (endothermic, enthalpy of sublimation). (ii) Cl(g) + e- → Cl-(g): energy is released (exothermic, electron affinity/electron gain enthalpy).
(f)(i) Na2SO4(aq): neutral salt, no effect on litmus. (ii) AlCl3(aq): acidic (salt hydrolyses), turns blue litmus red.
(g) Efflorescence is the process by which a hydrated crystalline substance loses its water of crystallization to the atmosphere on exposure to air, becoming powdery.
(h) Two uses of activated charcoal: as an adsorbent to decolourize/purify (e.g. sugar solutions, water); in gas masks to adsorb poisonous gases.
(i) (i) Hydrogenation of vegetable oil: manufacture of margarine (solid cooking fats). (ii) Cracking: production of petrol and of alkenes for making plastics. (iii) Esterification: manufacture of esters used as flavourings, perfumes and solvents.
(j) Silver deposited (Ag+ + e- → Ag, so 1 mol needs 96500 C):
\[ n(\text{Ag})=\frac{Q}{F}=\frac{10920}{96500}=0.113\ \text{mol} \]Answer Details
(a)(i) An ionic (electrovalent) bond is the electrostatic force of attraction between oppositely charged ions formed by the complete transfer of one or more electrons from a metal atom to a non-metal atom.
(ii) I. Magnesium oxide (MgO): ionic (electrovalent) bond. II. Ammonium ion (NH4+): covalent bonds, one of which is a coordinate (dative) covalent bond from N to the extra H+.
(b) Oxidation number of sulphur in sodium thiosulphate, Na2S2O3
\[ 2(+1)+2(S)+3(-2)=0 \Rightarrow 2S=+4 \Rightarrow S=+2 \](Average oxidation number of S = +2. If the formula is read strictly as Na2S2O2, the same method gives S = +1.)
(c) 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.
(d)(i) Acid salt: sodium hydrogentrioxocarbonate(IV), NaHCO3 (or NaHSO4). (ii) Basic salt: magnesium hydroxychloride, Mg(OH)Cl (or basic lead carbonate).
(e)(i) I2(s) → I2(g): sublimation, energy is absorbed (endothermic, enthalpy of sublimation). (ii) Cl(g) + e- → Cl-(g): energy is released (exothermic, electron affinity/electron gain enthalpy).
(f)(i) Na2SO4(aq): neutral salt, no effect on litmus. (ii) AlCl3(aq): acidic (salt hydrolyses), turns blue litmus red.
(g) Efflorescence is the process by which a hydrated crystalline substance loses its water of crystallization to the atmosphere on exposure to air, becoming powdery.
(h) Two uses of activated charcoal: as an adsorbent to decolourize/purify (e.g. sugar solutions, water); in gas masks to adsorb poisonous gases.
(i) (i) Hydrogenation of vegetable oil: manufacture of margarine (solid cooking fats). (ii) Cracking: production of petrol and of alkenes for making plastics. (iii) Esterification: manufacture of esters used as flavourings, perfumes and solvents.
(j) Silver deposited (Ag+ + e- → Ag, so 1 mol needs 96500 C):
\[ n(\text{Ag})=\frac{Q}{F}=\frac{10920}{96500}=0.113\ \text{mol} \]
