All 38 questions from the West African Examinations Council (WAEC) Chemistry 2014 Theory paper, with the correct answer and a full explanation for each. Free, no signup needed.
Advertisement
SECTION A (For all candidates) 1(a). Define ionic bond.
Model answer
An ionic bond is the electrostatic force of attraction that holds two oppositely charged ions together, formed by the complete transfer of one or more electrons from a metal atom to a non-metal atom.
1(a)(ii). What type of bond(s) exist(s) in: I. magnesium oxide; II. ammonium ion?
Model answer
I. Magnesium oxide (MgO): Ionic bond. II. Ammonium ion (NH₄⁺): Covalent bond and dative (coordinate) bond.
1(b). Determine the oxidation number of sulphur in Na₂S₂O₃.
Model answer
Let oxidation number of S = x. Na=+1, O=-2. 2(+1)+2(x)+3(-2)=0 ⟹ 2+2x-6=0 ⟹ 2x=4 ⟹ x=+2. The oxidation number of sulphur in Na₂S₂O₃ is +2.
1(c). State Faraday's first law.
Model answer
Faraday's first law of electrolysis states that 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.
1(d). Give one example each of: (i) acid salt; (ii) base salt.
Model answer
(i) Acid salt: Sodium hydrogen sulphate (NaHSO₄) (ii) Basic salt: Basic lead(II) carbonate / Zinc hydroxide chloride (or similar basic salt example).
1(e). Name the type of energy change that occurs in each of the following processes: (i) I₂(s)→I₂(g); (ii) Cl+e⁻→Cl⁻(g)
Model answer
(i) I₂(s)→I₂(g): Enthalpy/latent heat of sublimation (energy absorbed — endothermic). (ii) Cl+e⁻→Cl⁻(g): Electron affinity (energy released — exothermic).
1(f). State the effect of each of the following aqueous solutions on litmus paper: (i) Na₂SO₄(aq); (ii) AlCl₃(aq)
Model answer
(i) Na₂SO₄(aq): No effect on litmus paper (neutral — salt of a strong acid and strong base). (ii) AlCl₃(aq): Turns blue litmus paper red (acidic — salt of a weak base and strong acid, undergoes hydrolysis).
1(g). Define the term efflorescence.
Model answer
Efflorescence is the process by which a hydrated crystalline salt loses part or all of its water of crystallization to the atmosphere, forming a powdery deposit on its surface, when exposed to air.
1(h). Give two uses of activated charcoal.
Model answer
(i) It is used for absorbing/removing colour from substances (decolourisation). (ii) It is used for absorbing gases (as in gas masks).
1(i). State one use of each of the following processes in the chemical industry: (i) hydrogenation of vegetable oil; (ii) cracking; (iii) esterification
Model answer
(i) Hydrogenation of vegetable oil: Used in the production of margarine. (ii) Cracking: Used for obtaining lighter and more useful petroleum fractions (e.g. petrol) from heavier fractions. (iii) Esterification: Used for producing esters/alkanoates (e.g. for perfumes and flavourings).
1(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 = 96500Cmol⁻¹]
Model answer
Ag⁺+e⁻→Ag(s). Since 1 mole of Ag takes 1 mole of electrons, 1F = 96500C is required to deposit 1 mole of Ag. If 10920C of electricity is used, mole of Ag produced = 10920/96500 = 0.1132 mole.
2(a). Define each of the following terms: (i) ion; (ii) isotopes.
Model answer
(i) An ion is an atom or group of atoms that possesses an electric charge (positive or negative). (ii) Isotopes are atoms of the same element with different relative atomic masses (i.e., atoms with the same number of protons but different numbers of neutrons).
2(b)(i). Consider the elements ₁₂Mg and ₁₃Al. Write the electron configuration of each element.
Model answer
Mg (12): 1s²2s²2p⁶3s² Al (13): 1s²2s²2p⁶3s²3p¹
2(b)(ii). Explain briefly why the first ionization energy of ₁₂Mg is greater than that of ₁₃Al.
Model answer
Higher stability is associated with an orbital that is either fully filled or exactly half-filled. Magnesium's 3s orbital is fully filled (3s²), making it more stable, whereas Aluminium's outer electron is in the 3p subshell (3p¹), a not-so-stable orbital. There is therefore less difficulty removing the single 3p electron from Al than removing an electron from the fully-filled, more stable 3s orbital of Mg, so Mg has a higher first ionization energy than Al.
2(c)(i). What are allotropes?
Model answer
Allotropes are the various physical forms in which an element can exist in the same physical state (e.g. solid), differing in their crystal structure/molecular arrangement (e.g. diamond and graphite for carbon).
2(c)(ii). Name the two crystalline allotropes of carbon.
Model answer
Diamond and Graphite.
2(c)(iii). Give one industrial use of each allotrope named in 2(c)(ii).
Model answer
Diamond: Used for making jewellery, and for cutting, drilling, and engraving tools (its hardness makes it useful as an abrasive/for cutting glass and metals, and as dies for drawing wires). Graphite: Used for making pencils and as a lubricant.
2(d). On warming crystals of sodium chlorate(VI) 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.
Model answer
(i) Physical properties of the gas (chlorine): it is denser than air; it turns moist blue litmus paper red (bleaches it). (ii) NaCl+H₂SO₄→NaHSO₄+HCl (or similar equation representing evolution of HCl gas, depending on the exact reactants).
2(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=28gmol⁻¹]
Model answer
Mass = 5.68mg = 5.68/1000 = 0.00568g. Mole = Mass/Molar mass = 0.00568/28 = 2.03×10⁻⁴ moles.
3(a)(i). Outline the steps involved in the purification of water for town supply.
Model answer
(i) Screening (ii) Aeration (iii) Coagulation (iv) Filtration (v) Hardness removal (vi) Chlorination
3(a)(ii). Give two cations that can cause hardness in water.
Model answer
Calcium ion (Ca²⁺) and Magnesium ion (Mg²⁺).
3(a)(iii). State two disadvantages of hardness of water.
Model answer
(i) Hard water does not form lather readily with soap, wasting soap. (ii) Hard water cannot be used in certain industrial processes (such as dyeing) because the ions present can interfere with the chemistry of those processes.
3(a)(iv). List two sources of water pollution.
Model answer
(i) Industrial waste (ii) Oil spillage
3(b)(i). Explain briefly why water is a good solvent for sodium chloride but not for oil.
Model answer
Water is a polar substance, and sodium chloride (an ionic compound) dissolves readily in polar solvents. Oil, on the other hand, is non-polar, and non-polar substances are not soluble/miscible in polar solvents like water.
3(b)(ii). State the function of H₂SO₄ in each of the reactions: I. C₂H₅OH→C₂H₄; II. MgO+H₂SO₄→MgSO₄+H₂O; III. C+conc.H₂SO₄→CO₂+SO₂+H₂O
Model answer
I. H₂SO₄ serves as a dehydrating agent (in the dehydration of ethanol to ethene). II. H₂SO₄ serves as a typical acid here (acid-base neutralization reaction). III. H₂SO₄ serves as an oxidizing agent (oxidizing carbon to CO₂).
3(c)(i). Give the two products formed when chlorine water is exposed to sunlight.
Model answer
HCl and O₂. Reason: Cl₂+H₂O→HCl+HOCl, then HOCl (sunlight) → HCl + ½O₂.
3(c)(ii). Consider the compounds Na₂CO₃ and NaOH. I. Which of the compounds is suitable for the preparation of a standard alkaline solution? II. Give a reason for your answer in 3(c)(ii)I.
Model answer
I. Na₂CO₃ is suitable for the preparation of a standard alkaline solution. II. Reason: Na₂CO₃ can be weighed accurately as opposed to NaOH, which is deliquescent (i.e. it absorbs water from the atmosphere), making it unsuitable for preparing an accurately-weighed standard solution.
4(a)(i). Explain briefly each of the following terms: I. anode; II. cathode.
Model answer
I. Anode: This is the electrode where oxidation occurs (i.e. loss of electrons). II. Cathode: This is the electrode where reduction occurs (i.e. gain of electrons).
4(a)(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.
Model answer
For sodium (electrolysis of molten NaCl): At the anode: 2Cl⁻(l)→Cl₂(g)+2e⁻ At the cathode: Na⁺(l)+e⁻→Na(s) For aluminium (electrolysis of molten Al₂O₃): At the anode: 2O²⁻(l)→O₂(g)+4e⁻ At the cathode: Al³⁺(l)+3e⁻→Al(s)
4(a)(iii). Explain briefly why extraction of aluminium is considered environmentally friendly while that of sodium is not.
Model answer
The anodic product during the electrolytic extraction of aluminium is oxygen, which does not constitute any harm to the surrounding environment. However, the anodic product from the extraction of sodium is chlorine gas, which is a dangerous, poisonous air pollutant.
4(b). Consider the reaction: K₂Cr₂O₇+HCl→KCl+CrCl₃+H₂O+Cl₂ (i) Explain briefly why this reaction is redox; (ii) Write balanced half equations for the reaction; (iii) Write the overall balanced reaction equation.
Model answer
(i) The oxidation number of chromium decreases from +6 in K₂Cr₂O₇ to +3 in CrCl₃ — this is reduction. The oxidation number of chlorine increases from −1 in HCl to 0 in Cl₂ — this is oxidation. Since both reduction and oxidation occur, the reaction is a redox reaction. (ii) Oxidation half equation: 2Cl⁻→Cl₂+2e⁻ Reduction half equation: Cr₂O₇²⁻+14H⁺+6e⁻→2Cr³⁺+7H₂O (iii) Overall equation: Cr₂O₇²⁻+14H⁺+6Cl⁻→2Cr³⁺+3Cl₂+7H₂O (or the fully balanced molecular form: K₂Cr₂O₇+14HCl→2KCl+2CrCl₃+3Cl₂+7H₂O)
4(c). During the electrolysis of molten Al₂O₃, a current of 6A was passed through the electrolyte for 1hr 30mins. Calculate the mass of aluminium deposited at the cathode.
Model answer
Al³⁺(l)+3e⁻→Al(s) Q=It, I=6A, t=1hr30mins=90mins=5400s. Q=6×5400=32400C. From the equation, 1 mole of Al³⁺ requires 3e⁻ (3F). Since 1F=96500C, quantity of electricity required for 1 mole (27g) of aluminium = 3×96500=289500C. If 289500C deposits 27g of aluminium, 32400C will deposit (27/289500)×32400 = 3.02g of aluminium.
5(a)(i). Define the term functional group.
Model answer
A functional group is an atom, group of atoms, or even a bond that gives an organic compound its distinct chemical properties.
5(a)(ii). Name the functional groups present in the following compound: CH₃-C(=CH?)-CH-COOCH₃ [as shown in the diagram, an ester/alkene-containing compound]
Model answer
Alkenyl (C=C, alkene) group and Alkanoate (ester, -COO-) group.
5(b). Consider the following structure of an organic compound, Q, shown as a cyclopropane-type ring structure (H₂C-CH₂-CH₂ triangular ring). (i) 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? (iv) Draw the structure of the alkene that is an isomer of compound Q.
Model answer
(i) Compound Q is Cyclopropane. (ii) 2C₃H₆+9O₂→6CO₂+6H₂O (iii) Q will undergo a substitution reaction with chlorine. (iv) The alkene isomer of Q (C₃H₆) is Propene: CH₂=CH-CH₃.
5(c). A chemistry student was provided with four samples of organic compounds, A, B, C, and D. Samples A and B each decolourised 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 why sample B gave a white precipitate with the solution of ammoniacal silver trioxonitrate(V); (iv) Give the name of the reaction between samples C and D; (v) State the two roles of the concentrated tetraoxosulphate(VI) acid in the reaction in 5(a)(iv).
Model answer
(i) A belongs to the alkene family; B belongs to the alkyne family; C belongs to the alkanol (alcohol) family; D belongs to the alkanoic acid (carboxylic acid) family. (ii) A and B reacted with bromine in tetrachloromethane because they are unsaturated compounds (a saturated compound like an alkane would not react this way). (iii) Sample B gave a white precipitate because it is a terminal alkyne (which reacts with ammoniacal silver trioxonitrate(V) to give a silver-alkynide precipitate). (iv) The reaction between samples C and D (an alcohol and a carboxylic acid, forming a fruity-smelling product) is Esterification. (v) In esterification, concentrated H₂SO₄ serves two roles: (1) as a catalyst; (2) as a dehydrating agent.
5(d)(i). Describe briefly the production of biogas using a biogas generator.
Model answer
Organic waste is first transferred into the biogas generator (digester), where some bacteria are present. Through the process of anaerobic decomposition (fermentation in the absence of oxygen) by these bacteria, biogas is produced.
5(d)(ii). State two uses of biogas.
Model answer
(i) It is used for cooking. (ii) It is used industrially for heating.
Advertisement
…and/or go further with premium services and No Ads.