waecE-LEARNING
Chemistry Paper 2 (Essay) ,May/June 2012  
Questions:   1 2 3 4 5 6 7 8   Main
General Comments
Weakness/Remedies
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Question 7

  1. (a)  (i)     Define standard electrode potential
           (ii)    State two factors that affect the value of standard electrode potential
          (iii)    Give two uses of the values of standard electrode potential
          (iv)    Draw and label a diagram for an electrochemical cell made up of
                   Cu2+/Cu;   = + 0.34
                   Zn2+/Zn;   = - 0.76
          (v)     Calculate the e.m.f of the cell in 7(a)(iv) above                                                     [12marks]

    (b)   (i)  In terms of electron transfer, define
                  I.  oxidation;
                 II.  oxidizing agent.

         (ii)    Balance the following redox reaction:
                                              
                  MnO4-    +  I- H+   I2Mn2+                                                                              [7 marks]

    (c)   Classify each of the following oxides as basic, amphoteric, acidic or neutral:
           (i)   Carbon (II) oxide;
          (ii)   Sulphur(IV) oxide;
         (iii)   Aluminium oxide;
         (iv)   Lithium oxide.                                                                                                              [4 marks ]

    (d)   What is hydrogen bonding?                                                                                            [2marks ]

 

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OBSERVATION

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

In (a)(i)(ii) and (iii) most candidates correctly define standard electrode potential, state factors that affect standard electrode potential and gave uses of the values of standard potential.  However, some candidates could not properly define standard electrode potential.

In (a)(iv)  most  candidates could not draw and label correctly electrochemical cell.
In (b) (ii) and (III) only a few candidates could define oxidation in terms of electron transfer.  Also majority of the candidates could not balance redox reaction.

In (c) most candidates correctly classified oxides as basic, amphoteric, acidic or neutral.
In (d) most candidates could not correctly define hydrogen bonding.

The expected answers were:

(a)        (i)         Is a 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
                        (at 298k and  1 atm. pressure and 1 molar concentration)                                                                                                     
            (ii)        Condition, temperature, pressure.
                                                                                                                            
            (iii)       -     To predict the direction/ feasibility of a chemical reaction.

  • To calculate the e. m. f. of a cell.
  • To predict the standard potential of unknown elements

 

                       

(v)        Emf   = Eo (Right)  -  Eo ( left)
                                  + 0 .34V –  (- 0. 76V)    
                                  = + 1. 10V                                   

(b)        (i)         I . Oxidation - a process of election loss.       
                        II. Oxidation agent – Is an electron acceptor.  

            (ii)        Separate into half reactions
                        2I-   ®  I2 + 2e       
                        5e+ 8H++ MnO4- ® Mn2+ +  4H2O     
                        Balancing 10I-    ® 5I2  x 10e  
                        10e+ 16H+ + 2MnO4-  ® 2Mn2+ + 8H2O  
                        10I- +16H+ + 2MnO4- ®  5I2 +2Mn2+ + 8H2O
                                                                                                                                             
 
(c)        (i)         Neutral                         
            (ii)        Acidic              
            (iii)       Amphotenic     
            (iv)       Basic              
                                                                                                                                            
(d)       Hydrogen bonding is an intermolecular force between molecules in which hydrogen is
            attached to a very electronegative  atom which has a small size e.g N, O, F
           

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