All 6 questions from the West African Examinations Council (WAEC) Chemistry 2013 Theory paper, with the correct answer and a full explanation for each. Free, no signup needed.
1. (a)(i) What is the common name given to the group VII elements? (ii) Name the hydrides of the first two elements in group VII. (iii) State three chemical properties of group VII elements.
(b) Copy and complete the following table:
Particle | Number of neutrons | Number of electrons | Number of protons | Mass number
W²⁻ | 12 | ? | 8 | 24
X²⁺ | ? | 8 | ? | 16
Y | 14 | 13 | ? | 27
Z | 12 | ? | 11 | 23
(c)(i) Define each of the following processes: I. nuclear fission; II. nuclear fusion. (ii) Give one use of each process in (c)(i) above.
(d)(i) List three types of radiation that are produced during radioactivity. (ii) Arrange the radiations listed in 1(d)(i) in order of increasing: I. penetrating power; II. ionizing power.
Model answer
(a)(i) Halogens. (ii) The first two elements in group VII are fluorine and chlorine; their hydrides are hydrogen fluoride and hydrogen chloride. (iii) Group VII elements: They react with metals under appropriate thermal conditions to give salts. They react with alkalis to give salts. They are good oxidizing agents.
(b) Completed table (using: for a neutral atom, protons=electrons; for a negative ion, electrons exceed protons by the magnitude of the charge; for a positive ion, electrons are fewer than protons by the magnitude of the charge; mass number = protons+neutrons):
W²⁻: protons=8, neutrons=12, electrons=10 (8+2), mass number=24
X²⁺: protons=8, electrons=8... (per the printed source, X has 8 protons and mass number 16, so neutrons=8)
Y: protons=13, neutrons=14, electrons=13, mass number=27
Z: protons=11, neutrons=12, electrons=11 (neutral), mass number=23
(c)(i) I. Nuclear fission is a process by which a large nuclide splits into two smaller nuclides of comparable masses with the release of energy and radiations. II. Nuclear fusion refers to a process by which smaller nuclides combine to give a large nuclide with the release of energy and radiation.
(ii) Nuclear fission is used in power stations for generating power. Nuclear fusion can be harnessed in the production of the hydrogen bomb.
(d)(i) Alpha particles, beta particles and gamma rays.
(ii) I. Increasing penetrating power: Alpha < beta < gamma. II. Increasing ionizing power: Gamma < beta < Alpha.
2. (a)(i) Define each of the following terms: I. normal salt; II. acid salt. (ii) Tetraoxosulphate (VI) acid and sodium hydroxide react to produce salt and water. Write a balanced chemical equation for the formation of: I. a normal salt; II. an acid salt.
(b)(i) Explain briefly the term acid-base indicator. (ii) Copy and complete the following table.
Indicator | Colour in acidic medium | Colour in basic medium
Methyl orange | ? | ?
Phenolphthalein | ? | ?
(iii) For each of the following titrations, state the most suitable indicator: I. strong acid against strong base; II. strong acid against weak base; III. weak acid against strong base.
(c) Baking soda and hydrochloric acid react according to the equation: NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(g). Calculate the mass of baking soda that would produce 10g of carbon (IV) oxide. [H=1.00, C=12.0, O=16.0, Na=23.0]
(d) Give a reason why a given mass of sodium hydroxide pellets cannot be used to prepare a standard solution.
Model answer
(a)(i) I. A normal salt is formed when all the replaceable hydrogen ions of an acid are replaced by metallic or ammonium ion. Examples are NaCl, KNO3, Na2SO4 etc. II. An acid salt is formed when only part of the replaceable hydrogen ions are replaced by metallic or ammonium ion. Examples are NaHSO4, NaHSO3, Na2HPO4 etc.
(ii) I. 2NaOH+H2SO4 → Na2SO4+2H2O (normal salt). II. NaOH+H2SO4 → NaHSO4+H2O (acid salt).
(b)(i) An acid-base indicator is a dye which changes colour according to the pH of the medium.
(ii) Methyl orange: Red (acidic medium), Yellow (basic medium). Phenolphthalein: Colourless (acidic medium), Red/Pink (basic medium).
(iii) I. Strong acid against strong base: any indicator (methyl orange, litmus, phenolphthalein etc.). II. Strong acid against weak base: Methyl orange. III. Weak acid against strong base: Phenolphthalein.
(c) Mass of CO2 = 10g. Molar mass of CO2 = 12+16(2) = 44 g/mol. Mole of CO2 = 10g/44gmol⁻¹ = 0.2273 mol. According to the equation, 1 mole of CO2 is generated from 1 mole of baking powder (NaHCO3). Thus, 0.2273 mol of CO2 will come from 0.2273 mol of NaHCO3. Mass of NaHCO3 = (0.2273×84)g = 19.09g.
(d) Sodium hydroxide pellets cannot be used to prepare a standard solution because it is deliquescent (it absorbs moisture from the atmosphere and its mass changes, so an accurately weighed amount does not remain accurate).
3. (a)(i) Define the term standard electrode potential. (ii) State three factors that affect the discharge of ions during electrolysis. (iii) State two functions of a salt bridge in an electrochemical cell.
(b) Describe briefly what happens when a solution of copper (I) tetraoxosulphate (VI) is electrolyzed using copper electrodes.
(c) Calculate the mass of copper deposited at the cathode when a current of 0.2A is passed through a solution of copper (II) tetraoxosulphate (VI) for 35 minutes using copper electrodes. [H=1.00, O=16.0, S=32.0, Cu=64.0, F=96,500 C]
Model answer
(a)(i) The standard electrode potential of an electrode is the potential difference between the electrode and the hydrogen electrode under standard conditions. The standard conditions are: A temperature of 25°C (298K); A concentration of 1 moldm⁻³; A pressure of 1 atm.
(ii) Factors affecting the discharge of ions during electrolysis: Position of ions in the electrochemical series; Concentration of ions; Nature of electrode.
(iii) Functions of a salt bridge: It enhances ionic contact between the two half-cell compartments; It maintains electrical neutrality in the compartments.
(b) When an aqueous solution of CuSO4 is electrolyzed using copper electrodes, the colour of the CuSO4 (blue) is not discharged. This is because as copper ions leave the electrolyte for the cathode, more copper anode dissolves to replace the one just removed from the electrolyte. The whole electrolysis can therefore be interpreted as dissolution of copper anode and subsequent deposition on the copper cathode: Cu²⁺ + 2e⁻ → Cu(s).
(c) Q = It. I=0.2A, t=35mins=(35×60)s=2100s. Q=(0.2×2100)C=420C. 1 mole of Cu (64g) requires 2F (2×96,500C) of electricity. Thus, 420C will only produce (64/(2×96500))×420g = 0.139g.
4. (a)(i) Define the term hygroscopic. (ii) Give two differences between a physical change and a chemical change. (iii) Using the kinetic theory of gases, explain briefly the Charles' law.
(b)(i) Arrange the following compounds in order of increasing boiling points: CS2; CO2; NaH. Give reasons for your answer.
(ii) Write a balanced chemical equation to illustrate the reaction of chlorine gas with cold dilute sodium hydroxide.
(c) In a certain reaction, 15.0g of impure magnesium sample reacted with excess hydrochloric acid liberating 8.6dm³ of hydrogen gas at s.t.p.
(i) Write a balanced equation for the reaction.
(ii) Calculate the: I. mass of pure magnesium in the sample; II. percentage purity of the magnesium sample; III. number of Cl⁻ ions produced in the reaction. [Mg=24.0; volume at s.t.p.=22.4dm³; Avagadro's constant=6.02×10²³mol⁻¹]
Model answer
(a)(i) A substance is said to be hygroscopic if when exposed to the atmosphere, it can absorb moisture to become wet or sticky. Examples of hygroscopic substances are CaO, CI0, H2SO4 etc.
(ii) A physical change is easily reversible while a chemical change is not easily reversible. A physical change does not produce any new substance while a chemical change produces new substance(s).
(iii) At a constant pressure, when the temperature of a gas is increased, the molecules of the gas gain more kinetic energy. They therefore move faster with the distance between the molecules increasing, leading to an apparent increase in volume.
(b)(i) Increasing boiling point: CO2 < CS2 < NaH. Reason: NaH is an ionic substance. Thus, it has the highest boiling point. CS2 has stronger Van der Waals forces than CO2. Hence, it has a higher boiling point than CO2.
(ii) Cl2(g) + 2NaOH(aq) → NaCl(aq) + NaOCl(aq) + H2O(l). (Note: if the alkali is hot and concentrated, the equation will instead be 3Cl2+6NaOH(aq) → 5NaCl+NaClO3+3H2O.)
(c)(i) Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g).
(ii) I. Mole of H2 = Volume of hydrogen at s.t.p./GMV = (8.6/22.4) mol = 0.383 mol. Since both magnesium and hydrogen are equal in mole (from the balanced equation), mole of magnesium used = 0.3839 mol. Molar mass of magnesium = 24g/mol. Mass of pure magnesium = (0.3839×24)g = 9.2136 g.
II. % purity of the magnesium sample = (Mass of pure sample/Mass of impure sample)×100% = (9.2136g/15g)×100% = 61.42%.
III. To calculate number of chloride ions produced: since both MgCl2 and H2 are equal in mole (from the balanced equation), mole of MgCl2 = 0.3839 mol. 1 mole of MgCl2 produces 2 moles of Cl⁻. Thus, mole of Cl⁻ = (0.3839×2) mol = 0.7678 mol. Number of chloride ions = mole × Avogadro's constant = 0.7678×6.02×10²³ = 4.622×10²³ ions.
7. (Questions 5 and 6 in the original paper are not for Nigeria and are omitted.)
(a)(i) Explain briefly the term chemical industry. (ii) State three factors that should be considered in siting a chemical industry.
(b)(i) Describe briefly how tin is extracted from its ore. (ii) Give two uses of tin.
(c)(i) Name the constituents of cement. (ii) How does mortar set?
(d)(i) Explain briefly the term pollution. (ii) Give two examples of air pollutants.
(e) Consider the following reversible reaction which occurred at the temperature of 298k: N2+3H2(g) ⇌ 2NH3(g); ΔH=−92.37 kJ. List two factors that would increase the yield of NH3(g).
Model answer
(a)(i) A chemical industry simply refers to an industry that utilizes chemical processes for converting raw materials into useful products.
(ii) Factors to be considered in siting a chemical industry: Nearness to the source of raw material; Government policy; Nearness of market.
(b)(i) The tin ore (cassiterite) is first concentrated and then roasted in air to remove impurities. The roasted ore is heated in a furnace to a very high temperature after being mixed with coke. Limestone is added to remove impurities. The tin is collected in molten form.
(ii) Tin is used in making alloys; It is used for coating other metals.
(c)(i) The major components of cement are: Calcium carbonate, silicon oxide and aluminum oxide.
(ii) Mortar which is a mixture of sand, cement and water hardens by the absorption of CO2 from the atmosphere. This leads to the production of CaCO3.
(d)(i) Pollution is the process of introducing harmful and/or unwanted substances into environment.
(ii) Examples of air pollutants: Carbon monoxide; Dust.
(e) The two factors that will increase the yield of ammonia are: Decrease in temperature; Increase in pressure.
8. (a)(i) Members of the same homologous series have a general molecular formula. State two other characteristics of a homologous series.
(ii) The compound CH3(CH2)2CH3 belongs to the alkane family (butane). I. Which homologous series does the compound belong to? II. Write the structures of the three possible isomers of the compound. III. Name the three possible isomers in 8(a)(ii)II.
(b) Write the structure of the major product formed in each of the following reactions:
(i) ethanol with excess acidified potassium tetraoxomanganate (VII);
(ii) excess ethane with chlorine in the presence of sunlight;
(iii) ethanol with propanoic acid with few drops of concentrated tetraoxosulphate (VI) acid.
(c) Name the major product formed in each reaction in 8(b).
(d) Consider the following organic compounds:
Y - (CH3)3COH
X - C6H4COOH (a benzene ring with a –COOH substituent, plus –CH3, –CH3, –CH3 groups as shown in the source figure)
(i) Give the IUPAC name of each compound. (ii) State a chemical test for the functional group in each compound.
(e) An organic compound with a relative molecular mass of 136 contains 70.57% carbon, 5.90% hydrogen and 23.53% oxygen. Determine its: (i) empirical formula; (ii) molecular formula. [H=1.00, C=12.0, O=16.0]
Model answer
(a)(i) Members of the same homologous series: Consecutive members differ in their molecular formula by a –CH2 group and in their relative molecular mass by 14.
(ii) I. The compound CH3(CH2)2CH3 (i.e. CH3CH2CH2CH3, butane) belongs to the alkane family.
II./III. The possible isomers of the compound are: CH3–CH2–CH2–CH3 (Normal butane); and CH3–CH(CH3)–CH3 (2-methylpropane / isobutane).
(b)(i) The major product when ethanol reacts with excess acidified potassium tetraoxomanganate (VII) is ethanoic acid: CH3–C(=O)–OH.
(ii) The major product is chloroethane: H–C(H)(H)–C(H)(Cl)–H, i.e. CH3CH2Cl.
(iii) The major product is ethylpropanoate: CH3CH2–C(=O)–O–CH2CH3.
(c) (i) Ethanoic acid. (ii) Chloroethane. (iii) Ethylpropanoate.
(d)(i) Y is 2-methylpropan-2-ol; X is benzoic acid (benzenecarboxylic acid).
(ii) Test for Y: Make use of Lucas reagent and add a few drops. A cloudy or milky appearance indicates Y (a tertiary alcohol). Test for X: Evolution of colourless, odourless gas (CO2) alongside vigorous effervescence upon adding a hydrogen carbonate indicates X (a carboxylic acid).
(e) C=70.57%, H=5.90%, O=23.53%. Dividing by atomic mass: C=70.57/12=5.88, H=5.90/1=5.9, O=23.53/16=1.4725. Dividing by smallest (1.4725): C=4, H=4, O=1. Thus, the empirical formula is C4H4O.
(ii) (C4H4O)n=136. (48+4+16)n=136. 68n=136; n=136/68=2. Molecular formula = (C4H4O)2 = C8H8O2.
Advertisement
Sign up free to unlock
Score tracking
Practice history
Saved questions
Progress dashboard
Personalized sessions
Weak-topic breakdown
…and/or go further with premium services and No Ads.