waecE-LEARNING
Chemistry Paper 2 (Essay) ,May/June 2013  
Questions:   1 2 3 4 5 6 7 8   Main
General Comments
Weakness/Remedies
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Question 3

  1. Define the term standard electrode potential.
            (ii)      State three factors that affect the discharge of ions during electrolysis.

    1.  State two functions of a salt bridge in an electrochemical cell                        [7 marks]

    (b)    Describe briefly what happens when a solution of copper (II) tetraoxosulphate (VI)
             iselectrolysed using copper electrodes.                                                                       [6 marks]

    (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, 1F = 96 =,500C]              [6 marks]

    (d)    (i)  State three characteristics of a catalyst.
            (ii)  Name one manufacturing process in which each of the following metals is
    used as catalysts:

    1. iron;
    2. nickel;
    3. platinum.                                                                                                       [6 marks]

 

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OBSERVATION

Majority of the candidates that attempted this question did poorly.

In (a), majority of the candidates could not properly define standard electrode potential. They could not also state the functions of a salt bridge.  However, some candidates were able to state factors affecting the discharge of ions during electrolysis.

In (b) majority of the candidates could not give all the ions present in CuSO4(aq) there by leading to loss of some marks.  Candidates’ explanation of the ionization of CuSO4 was fair.

In (c) majority of the candidates got the calculations correct.
In (d), majority of candidates correctly gave the characteristics of a catalyst.  However, only a few candidates were able to give the manufacturing processes involving the use of the named catalyst.

The expected answers include:

(a)  (i)  Standard electrode potential is the measure of the tendency of an element to form ions in solution relative to the tendency of hydrogen atoms to form ions in solution at standard conditions / Is the potential difference that exists between an electrode and the hydrogen electrode under standard conditions of 1 moldm-3 concentration at 298K and 1 atm

  (ii)      -           position of the ions in the electrochemical series/standard electrode potential value;
            -          nature or reactivity of electrode used;
            -           relative concentration of ions.
                       
(iii)       -           allows electrical contact between the two solutions/complete the electric
                        circuit             
            -           maintains electrical neutrality in each half cell/ allows ions flow
                        in and out of salt bridge                    
                                                                                                           
(b)        CuSO4(aq) contains Cu2+, SO42- , H+ / H3O+ , OH -     
            Cu2+ and H3O+ migrate to cathode    
            Cu2+ discharged / Cu metal deposited            /Cu cathode increases in mass
            Copper anode dissolves/ionizes/decreases in size to give Cu2+ ions
            Solution remains blue/concentration of Cu2+ ions is constant
                                                                                   
(c)        Cu2+ + 2e → Cu                     
            Q = It  = 0.2 x 35 x 60  = 420 C        
            2F  2e  1 mole Cu / 2 x 96500 C≡ 64 g
             420 C =  x 64 g Cu  
                         = 0.14 g Cu   
                                                                                                           
ALTERNATIVE SOLUTION

             Cu2+ + 2e → Cu   
                    n =It
                         ZF
              I= 0.2 A
            t= 35 x 60 = 2100 seconds
            Z = 2
            F = 96500
            n = 0.2 x 2100
                   2 x 96500
            n = 0.00218 mol
            n = mass
                  molar mass
           mass = n x molar mass
                     = 0.00218 x 64     
                     = 0.13952 g
                     = 0.139 g        

(d)  (i)              -           remains unchanged in chemical nature and mass
                        -           does not affect the types of product formed
                        -           alters (increase or decrease) the rate of chemical reaction
                        -           they are reactions specific
                        -           cannot initiate a reaction that is not feasible
                        -           has no effect on the equilibrium of a reversible reaction
                        -          required in small quantity
                                                                                               
   (ii)     I           iron – used in Haber process 
            II.        nickel– used in hydrogenation of vegetable oil / alkenes 
            III.       platinum – used in preparation of hydrogen iodide from hydrogen and  iodine/
                        contact process for H2SO4 production/manufacture of HNO3/used in
                         hydrogenation

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