waecE-LEARNING
Chemistry Paper 2 (Essay) ,May/June 2014  
Questions:   1 2 3 4 5  Main
General Comments
Weakness/Remedies
Strength













































































Question 4

(a)        (i)         Explain briefly each of the following terms.
                                    I           anode;
                                    II         cathode.

                        (ii)        Sodium and aluminium are extracted by the electrolysis of molten
                                    sodium chloride and alumina respectively.  Write balance equations
                                    for the reactions at the anode and cathode during the extraction of:

                                    I.          sodium;
                                    II.        aluminum.

                        (iii)       Explain briefly why extraction of aluminum is considered                                             environmentally friendly while that of sodium is not.               [11 marks]

            (b)        Consider the reaction represented by the following equation:

                                    K2Cr2O7  +  HCl        KCI + CrCl3  + H2O  +Cl2

                                (i)         Explain briefly why this reaction is redox.

                        (ii)        Write balanced half equations for the reaction.

                        (iii)       Write the overall balanced reaction equation.                     [8 marks]

            (c)        During the electrolysis of molten Al2O3, a current of 6A was
                        passed through the electrolyte for 1 hr 30mins.  Calculate the
                        mass of aluminium deposited at the cathode.                                             [6 marks]

_____________________________________________________________________________________________________
OBSERVATION

This question was unpopular among the candidates as shown by the number
            and performance of the candidates who attempted it.

            In part (a), majority of the candidates could not correctly explain anode and
            cathode in terms or oxidation and reduction.  Also, majority of the candidates
            could not write half cell reactions of sodium and aluminium.

            In part (b), majority of the candidates were able to explain why the given reaction
            is redox but could not write balanced half equations for the reactions and therefore
            could not write the overall balance reaction equation.    

            In part (c), majority of the candidates calculated the quantity of electricity involved
            but could not go beyond that.  Most candidates could not write the half equation for         aluminium.

            The expected answers include:

            (a)        (i)         I .         Anode     - Electrode at which oxidation occurs / electron loss        
                                    II.        Cathode  - Electrode at which reduction occurs / electron gain        

 

                        (ii)        I.          Cathodic reaction
                                                Na+(l) + e-  →  Na(s)     
                                                                                                                                                                                                            Anodic reaction
                                                2Cl-(l)  →  Cl2(g) + 2e-              

                                                II.        Cathodic reaction
                                                Al3+(l) + 3e- → Al(s)                 

                                                Anodic reaction
                                                2O2-(l) → O2(g) + 4e-    

            (iii)       In the extraction of aluminium oxygen is produced  
 whereas in the extraction of sodium, chlorine is produced.
Chlorine is an environmental pollutant whereas oxygen is not
                                                                                                                                          
(b)        (i)         The reaction is redox because both oxidation and reduction
occur simultaneously.   The oxidation number of Cr changes
from +6 (in K2Cr207)  to +3 (in CrCl3).    Cr is therefore
reduced.  The oxidation number of Cl changes from -1 (in HCl)
to 0 ( in Cl2).    Cl is therefore oxidized.
                                                OR
                        Removal of hydrogen from HCl is oxidation 
                        Removal of oxygen from K2Cr2O7 is reduction  

            (ii)        Reduction                  
                                Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H2O          

                        Oxidation                   
                        2Cl- → Cl2 + 2e    

            (iii)       Cr2O72- + 14H+ + 6Cl- → 2Cr3+ + 7H2O + 3Cl2                     
                        Accept            K2Cr2O7   +  14HCl    →      2CrCl3  +7H2O  + 3Cl2
                                                                                                                                     
            (c)        Q = I x t                       
                           = 6 x (30 + 60) x 60
                           =  32400 C  
                              Al3+ + 3e- → Al(s) 

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