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 1

  1.  (i)  What is the structure of the atom as proposed by Rutherford?

(ii)   Distinguish between the atomic number and the mass number of an element.
(iii)  Explain briefly why the relative atomic mass of chlorine is not a whole number.  [ 7 marks]

  •  (i)   What is meant by first ionization energy?

(ii)   List three properties of electrovalent compounds
(iii)  Consider the following pairs of elements:

  •  9F and 17CL;
  • 12Mg and 20Ca.

Explain briefly why the elements in each pair have similar chemical properties.  [7marks ]

  •  Explain briefly the following terms using an appropriate example in each case
  •  homologous series;
  • heterolytic fission.                                                                                                       [5 marks ]
  •   State the indicator(s) which could be used to determine the end-point of the following titrations:
  •  dilute hydrochloric acid against sodium hydroxide solution;
  • dilute hydrochloric acid against ammonium hydroxide solution;
  • ethanoic acid against sodium hydroxide solution.                                                [3 marks ]
  •      A solid chloride E which sublimed on heating reacted with an alkali F to give a

     choking gas G. G turned moist red litmus paper blue.  Identify E,F and G.                [3 marks]

              In part (a) majority of the candidates could not draw the structure of atom proposed by
              Rutherford.
              Many candidates could distinguished between the atomic number and the mass number
               of an element and they were able to explain why the relative number of chlorine is not a
              whole number.
              In part (b) most candidates could not well state what is meant by first ionization energy
              In part (c) majority of the candidates were able to explain the term homologous series.   
              but almost all the  candidates could not explain the term heterolytic fission.

                The expected answers were:
               
1.  (a)  (i)         The atom has small/tiny positively charged centre/nucleus with electrons surrounding the space around  the centre.  

(ii)         Atomic number of an element is the number of protons/electrons in an atom of the element while mass number is the sum of the protons and neutrons in the atom of the element.  

(iii)        Chlorine atom is made up of a mixture of isotopes and the relative atomic mass of chlorine is the average of its isotopic masses  
                                                                                                                                             
(b)    (i)         Is the (minimum) energy required to remove one mole of an electron from one
                     mole of gaseous atom (to form one mole gaseous charged ion)        

                                                                                                                                                   
            (ii)        -           High melting/boiling point;
                        -           Ability to conduct electricity in the molten state or in solution;
                        -           Solid at room temperature;
                        -           Soluble in water or polar solvents/insoluble in non-polar solvents.

  (iii)     Atoms of the elements in each pair have the same number of electrons in their
            outer-most shell therefore similar chemical properties.
                                                                                                               
(c)        (i)         Is a family of organic compounds:     
                        -           where successive members differ by –CH2 of  molar mass of 14;  
                        -           with similar chemical properties;
                        -           which conform to the same general formula;
                        -           which show gradation of physical properties;
                        -           which have the same general method of preparation.
                                    e.g  alkanes, alkenes , alkanols.  
                                     
(ii)        Is a process in which a (covalent) bond is broken in such a way that the electron pair is completely transferred to one of the atoms (resulting in the formation of ions)
                        H ÷ CI     →      H+ + Cl-/ HCl ® H+ + Cl-                                    

(d)       (i)         -   Methyl orange/methyl red/phenolphthalein;   
            (ii)        -   Methyl orange/methyl red;   
            (iii)       -   Phenolphthalein.   
                                                                                                                                            
(e)                    E          -           NH4Cl    
                        F          -           NaOH, KOH, or Ca (OH)2,  Li OH, CsOH, Ba(oH)2,
Mg(OH)2  
                        G         -           NH3                                                                  

 

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OBSERVATION

Many candidates did not attempt this question.  In part (a), most candidates could not deduce the mathematical relationship between volume and pressure rather, most candidates state that P∝ I/V or PV = K.  Most candidates were able to name the law but a good number of the responding candidates could not calculate the pressure of the gas when the volume is 3.20 dm3.

In part (b), most responding candidates failed to realize the role of a salt bridge in an electrochemical cell, rather, they saw the role as that of “completing the circuit”.  The type of ions that migrates to the cathode and why it does so also posed a challenge to the candidates. 

Most candidates  could not write the overall cell reaction and therefore could not determine the standard cell potential although a few of them were able to calculate this correctly.  Many of the candidates could not also calculate the number of moles of electrons and the amount of coulomb of charges.

In part (c), many candidates were able to define activation energy and exothermic reaction but could not give the significance of activated complex in a chemical reaction nor give an example of endothermic process.
The expected answers are:
(a)(i)   Volume (V) decreases as pressure (P) increases. V∝ 1/P
          OR  PV = K where K is a constant of proportionality.
  (ii)    Boyles law
  (iii)   P1 V1 = P2 V2
           P2 = P1 V1  =   1atm x 4.00  OR  2 x 2  OR 4 x 1
                            V2                  3.20                   3.2           3.2
                = 1.25 atm
(b)(i)    Salt bridge is to maintain electrical neutrality in each half cell/to allow cations and anions              move from one electrode compartment to the other.
   (ii)     negative ions  because the cathode is positively charged
  (iii)     I.   2Ag+(aq)  +   Zn(s)→  Ag(s)  +  Zn2+(aq)                                   
                  Eθ  =   Eθ  cathode - Eθ  anode
      =   +   0.80 – (- 0.76 V)
      =  1.56V

            II.  Ag+  + e →    Ag 
                        108g of Ag = 1 mole of electrons
      Thus 3.3g of Ag – 1 x 33 mol electrons
                                   108
                     =  0.03 mol
III. 1 mole  e- = 96500C
       0.03 mol e- = 96500 x 0.003
         =  2.89 x 10# C

(c) (i)   I.   Activation energy is the minimum amount of energy required to effect a chemical
                 reaction/minimum amount of energy required by reactants to form products.
          II.   Exothermic reaction is a reaction in which energy is released to the surrounding.    
         III.   Activated complex must be attained/formed for reactants to form products

 

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