Chemistry 2 Nov/Dec 2010

Chemistry Paper 2,Nov/Dec 2010

Questions:		1	2	3	4	5	6	7	8		Main

General Comments
Weakness/Remedies
Strength

Question 3
(a) Consider the following reaction equation: PCl₃(g) + Cl₂(g) → PCl₅(g); ΔH=-92.5 KJmol^-1 (i) State: I two factors that affect both the rate of reaction and the equilibrium; II one factor that affects only the rate of reaction but not the equilibrium position Give a reason for your answer. (ii) Explain what would be the effect on the equilibrium concentration of Cl₂(g) if: I the total pressure is increased; II the temperature is increased; III. more PCl₃(g) is added [ 10 marks] (b) (i) The standard electrode potential of copper is + O.34V. Explain this statement. (ii) Consider the following standard electrode potentials: Zn²⁺(aq) + 2e- → Zn(S) ; E^O = -0.76V; CU²⁺(aq) + 2e- → CU(S) ; E^O = +0.34V. When the two half cells are connected: I. write the reaction equation at each electrode; II. state the type of reaction occurring at each electrode; III. write the overall cell reaction; IV. calculate the e.m.f of the cell. [9 marks] (c) (i) Name one chemical industry. (ii) Give three factors that should be considered when sitting a chemical industry. (iii) State two effects of a chemical industry on the community in which it is sited. [ 6 marks]
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OBSERVATION
The question was not popular among the candidates and the performance was fair. In(a)(i), candidates correctly responded to I and II as follows: I - pressure/volume of vessel temperature II - Catalyst Catalyst affects both forward and backward reactions equally In (a) (ii], most candidates gave the effect on the equilibrium position instead of the effect on the equilibrium concentration of Cl2• They therefore lost the marks. The required answers for I - III were as follows: (a) (ii) I - Equilibrium concentration of Cl2 decreases. Increase in pressure shifts equilibrium position to the right where there is lower number of moles of species. II - Equilibrium concentration of Clz increases Increase in temperature shifts equilibrium to the left where the reaction is endothermic. III - Equilibrium concentration of Cl2 decreases Increase in concentration of PCl3 (thereby decreasing the concentration of Cl2) In (b)(i), most candidates could not explain that when a standard hydrogen electrode is connected to a standard copper half cell, the voltmeter reads an emf of O.34V and that the positive sign indicates the flow of electrons from the hydrogen electrode to the copper electrode in the external circuit. In (b) (ii) I - IV, candidates showed shallow knowledge of electrochemistry and hence, lost most marks for the question. The expected answers from candidates were: (iii) At Anode Zn(s) ----+ Zn2+(aq) + 2e- At Cathode II Anode - oxidation Cathode reduction III Zn(s) + Cu2+ (aq) → Zn2+ (aq) + CU(s) (iv) E^O cell = E^Oreduction - E^O oxidation = 0.34V - (- 0.76V) = 1.10V Alternative method + 0.76V Cu2+ 2e- → Cu(s) + 0.34V e.m.f. = +0.76 + 0.34 =1.10 V In part (c), candidates correctly named one chemical industry, gave three factors that should be considered when sitting a chemical industry and stated two effects of a chemical industry on the community in which it is sited in (i) - (iii) as follows: (i) Named chemical industry e.g Textile Tannery Brewery Bottling Company Soaps and detergents Infant formula/Baby food Food seasoning. (ii) - nearness to raw material source/feedstock - nearness to market - labour supply - transportation - nearness to power supply - government policy - away from residential areas. (ii) - improvement in the standard of living of people - employment opportunities - development in that community - pollution (air, land water etc) .