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Chemistry Paper 2 (Essay) ,May/June 2009  
Questions:   1 2 3 4 5 6 7 8   Main
General Comments

Question 1

(a) (i)     Outline a suitable procedure for the preparation of  ZnCl2 crystals starting  from zinc granules.
     (ii)   Write a balanced equation for the reaction in 1(a)(i).                               [7 marks]

(b) Consider the following solutions:

              Na2SO4(aq),  CH3COOK(aq),  Pb(NO3)2(aq)    and MgCl2(aq)

                     Which of them has/have a pH
                     (i)     greater than 7;
                     (ii)    equal to 7;
                     (iii)   less than 7?                            [4 marks]

  (c)   (i)  Determine the oxidation number of Mn in

  1. MnO2;
  2. KMnO4.

      (ii)  State one laboratory use of each of the compounds in 1(c)(i).                 [6 marks]

(d)     Explain why oxidation and reduction processes are complementary.            [3 marks]

(e)      Consider the reaction represented by the equation:
Fe(s)   +   2Ag+ (aq)    →     Fe2 + (aq)   +   2Ag(s)                               

(i)      Write a balanced ionic half equations for the reaction.
(ii)     Which of the species is

                                  I.    oxidized;
                                  II.   reduced?
(iii)   State the change in the oxidation number of silver during the reaction.    [5 marks]


The question was attempted by majority of the candidates and the performance was only fair.

In part (a)(i), candidates could not outline the procedure for the preparation of ZnCl2 crystals from zinc granules.  Most of them only indicated the addition of HCl and could not go further.  The expected response from candidates was as follows:

  1. to some dil HCl in a beaker, add Zn granules and shake.
  2. continue to add Zn granules until effervescence stops and the Zn is in excess.
  3. filter off undissolved Zn granules.
  4. put the filtrate in an evaporating dish and heat to concentrate
  5. cool to obtain crystals.
  6. filter and dry the crystals between filter papers.

In part (a)(ii), most candidates correctly wrote balanced equation for the reaction thus:
            Zn(s) + 2 HCl(aq)   →    ZnCl2(aq)  + H2(g)
In (b), candidates correctly indicated that:
            CH3COOK(aq)   had pH greater than 7;
             NaSO4(aq) had pH equal to 7;
            Pb(NO3)2 and MgCl2 had pH less than 7.
In part (c)(i) and (ii), candidates correctly determined the oxidation number of Mn and stated one laboratory use of each of the given compounds.  However, some of them lost marks because they did not assign magnitude (sign) to the oxidation number.
The expected responses were:


 Mn + (-2 x 2) = 0    

 Mn - 4 = 0   

 Mn  = + 4

 1 + Mn + (-2 x 4) = 0  

 1 + Mn - 8 = 0 

Mn  = + 7
             MnO2       -     as a catalyst
                              - as an oxidizing agent

           KMnO4       -     as an oxidizing agent
In (d), candidates knew that oxidation is a process of electron loss while reduction is a process of electron gain and that a species can only be oxidized if there is another species that would accept the donated electrons.

In part (e), candidates wrote balanced ionic equations for the reaction as follows:

                                    Fe(s)     →       Fe2 +(aq)   +     2e-     
                                   Ag2+(aq)    +     e-     →     Ag(s)

Also, they knew that Fe was oxidized, Ag+ was reduced and that the change in oxidation number of silver was from +1 to 0. 

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