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













































































Question 3

  1. (a)    (i)    Define nuclear fission
            (ii)    A certain natural decay series starts with   and ends with .
                     Each step involves the loss of an alpha or a beta particle.  Using the given information,
                    deduce how many alpha and beta particles were emitted.                                       [5marks]

    (b)    Consider the equilibrium reaction represented by the following equation:
             A2(g)  +  3B2(g)                    2AB3(g);    H   =  +  kJmol-1
             Explain briefly the effect of each of the following changes on the equilibrium composition:

    •  increase in concentration of B;
    •  decrease in pressure of the system;
    • addition of catalyst.                                                                            [5marks]

    (c)    The lattice energies of three sodium halides are as follows:

     

    Compound

     

    NaF

     

    NaBr

     

    NaI

     Lattice energy/kJmol-1

    890

    719

    670

               Explain briefly the trend.                                                                                                       [3 marks]

    (d)      State the property exhibited by nitrogen (IV) oxide in each of the following reactions:

               (i)          4Cu  +  2NO2          4CuO + N2;    
              (ii)          H2O+ 2NO2                HNO3   + HNO2.                                                         [3marks]

     

    (e)      Iron is manufactured in a blast furnace using iron ore (Fe2O3), coke and limestone.
               Write the equation for the reaction(s) at the:

    •  top of the furnace;
    • middle of the furnace;
    • bottom of the furnace.                                                                                        [5 marks]

    (f)        (i)   Name two products of destructive distillation of coal.
               (ii)    Give one use of each product in 3(f)(i).                                                         [4 marks]

 

_____________________________________________________________________________________________________
OBSERVATION

         3(a)(i) and (ii): This question was attempted by majority of the candidates and the performance was poor.  In this section most candidates were unable to correctly define nuclear fission.  Most candidates omitted the word “heavy” nucleus and ‘large’ energy in the definition.  Most candidates could not write the balanced nuclear equation but mention the numbers of alpha and beta particles emitted.

In b(i), (ii) and (III) majority of the candidates were not able to respond appropriately to questions on equilibrium. Most candidates lost marks by not further stating the effect of each increase, decrease and addition of catalyst.

In (c) majority of the candidates had no answer to this.  In (d)(i) and (ii) most candidates got (i) correctly but lost mark in (ii) because they only mention oxidizing agents.
In (e) most candidates lost marks because they could not identify the equation at the top, middle and bottom of the furnace.

In (f) most candidates correctly name two product of the destructive distillation of coal and gave one use of each product.

The expected answers were:

(a)        (i)         Is the splitting up of  heavy nucleus into smaller nuclei  accompanied by
the release of  large amount of energy

            (ii)        Let x and y represent number of alpha  and  beta particles emitted respectively.

                           U Th  + x He + y e
                         
                   By inspection
 u Th  + 2 He + 2 e                            
  
                       
                      \    a  particles         =  2              
                                Beta particles    =  2            
                                                                                                                                            

(b)        (i)         Equilibrium position shifts to the right and more of AB3 is formed/ 
                        decrease in concentration of A2.
(ii)        Equilibrium shift to the left and more of A2 and B2 formed/ decrease in                          concentrate of AB3
            (iii)       No effect. Catalyst affects both forward backward reactions equally.  
                                                                                                                                            
(c)         For a constant cation Na+, lattice energy decreases with increase in size of the anion/ decrease in electronegativity of anion.
             F-,Br-, I-   increasing order of size.  Hence the decreasing order of lattice energy.         
                ®                                                                                                       
                                                                                                                                                                             
(d)       (i)         NO2 is an oxidizing agent.  
           (ii)         NO2 is acting as an oxidizing as well as reducing agents or acid anhydride. 
                         
                                                                                                                                            

 

(e)      (i)          Fe2O3  +  3CO  ®  2Fe + 3CO2    
          (ii)          CaCO3    ®     CaO   + CO2  
                        CaO  +  SiO2  ®  CaSiO3      
          (iii)         C + O2      ®  CO2     
                        CO2 + C   ®  2CO    
                                                                                                             
(f)      (i)           Coke/Coal tar/Coal gas/Ammonical liquor
                                                                                                                         
          (ii)          Coke        -       solid fuels/production of water gas, extraction of metals.
Coal tar   -       synthesis of  chemicals - perfumes, dyes, paints, drugs, plastic and explosives etc.
Coal gas  -       as fuels in homes and industries.
                        Ammonical liquor -  production  of (NH4)2 SO4 for fertilizer.

Powered by Sidmach Technologies(Nigeria) Limited .
Copyright © 2015 The West African Examinations Council. All rights reserved.