Question

1(a)(i) State Faraday's first law of electrolysis. (ii) Distinguish between a strong electrolyte and weak electrolyte. (b) State one chemical property of ethyne. (c)(i) What is meant by the term unsaturated hydrocarbon? (ii) Complete the following reaction equation: CH3COOH + CH3OH -> (iii) Name the major product formed in the reaction stated in 1(c)(ii). (d) State one way by which the rate of esterification could be increased. (e) Consider the reaction represented by the following equation: Zn(s) + H2SO4(aq) -> ZnSO4(aq) + H2(g). If 3.75 g of Zn dust was added to excess H2SO4, calculate the number of molecules of hydrogen gas produced. [Zn=65.0, NA=6.02x10^23] (f) State one effect of global warming. (g) Consider the following reaction equations: A. Pb(NO3)2 + H2S -> PbS + 2HNO3 B. H2 + C2H4 -> C2H6 C. Zn(OH)2 + 2OH- -> [Zn(OH)4]2- (i) Which of the equations represent(s) a redox process? (ii) State the change in oxidation number of the species that are oxidized or reduced. (h)(i) State two of the main concepts of Bohr's model of the atom. (ii) State the limitation of Bohr's model. (i) List three factors that could influence the equilibrium position of a reversible reaction. (j) Calcium trioxocarbonate (IV) powder is added to separate equimolar solutions of hydrochloric acid and ethanoic acid. State one: (i) similarity in the observation in both reactions; (ii) difference in the observation in both reactions.

Accepted Answer

(a)(i) Faraday's first law of electrolysis states that the mass of an element produced during electrolysis is directly proportional to the quantity of electricity passing through the electrolyte. M is proportional to Q; M=ZQ, Q=IT. Then, M=ZIT, where M=mass of element produced (g), Q=Quantity of electricity (C), I=current (A), t=time (sec), Z=Electrochemical equivalent (gC^-1).
(ii) A strong electrolyte ionizes/dissociates completely in solution while a weak electrolyte ionizes/dissociates only partially in solution.
(b) Ethyne reacts with excess hydrogen as at 200 C in the presence of nickel as a catalyst to produce ethane: C2H2+2H2 --(Ni,200C)--> C2H6. Ethyne also reacts very readily with excess hydrogen iodide at room temperature to form 1,1-diiodoethane: C2H2+2HI->C2H4I2.
(c)(i) An unsaturated hydrocarbon is one that has at least one carbon to carbon double or triple bond.
(ii) CH3COOH + CH3OH --(H+)--> CH3COOCH3 + H2O.
(iii) The name of the major product is methyl ethanoate.
(d) The rate of the reaction (esterification) could be increased by adding some drops of concentrated H2SO4.
(e) Zn(s)+H2SO4(aq)->ZnSO4(aq)+H2(g). Nzn=(3.75/65)mol=0.0577 mol. From the balanced equation of reaction, 1 mole of Zn produces 1 mole of H2. Thus, 0.0577 mol of H2. Number of H2 molecules = moles of H2 x Avogadro's constant = 0.0577 x 6.02x10^23 = 3.47x10^22.
(f) It may eventually cause flooding (as the polar ice caps melt).
(g)(i) Reaction 'B' is redox.
(ii) The oxidation number of hydrogen has increased from zero to +1 (oxidation) while that of carbon has decreased from -2 to -3 (reduction).
(h)(i) Main concepts of Bohr's model of the atom: Electrons orbit the nucleus at fixed energy levels; Orbits further from the nucleus exist at higher energy levels; When electrons return to a lower energy level, they emit energy in the form of light.
(ii) Limitation of Bohr's model: Poor spectral predictions when a larger atom is involved; Bohr's model does not explain the Zeeman effect.
(i) Factors that could influence the equilibrium position of a reversible reaction: Position in the electrochemical series; Concentration of the ions; Nature of the electrode.
(j)(i) Effervescence is produced in both cases.
(ii) The reaction is faster with hydrochloric acid than with ethanoic acid.

WAEC Chemistry 2020 Theory — Question 1

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