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Chemistry Paper 2 (Essay) ,May/June 2008  
Questions:   1 2 3 4 5 6 7 8   Main
General Comments

Question 3

(a)        Consider the following reaction sequence.


(i) What process leads to the formation of K?
(ii)   Write the formula of K.
(iii) Write the structural formula of L and name L.
(iv) Name An.
(v) Write the structure of M and name M.   [7 marks]
(b) (i) What are carbohydrates?

(ii) Give one example each of a

  1. monosaccharide;
  2. disaccharide;
  3. polysaccharide.   [5 marks]
(c) Consider the following structure of a simple sugar.

                                   C    =   O                                   
                        HO -     C    -   OH                                           
                        HO -     C    -   H
                          H   -   C   -   OH                                            
                          H   -   C   -   OH

(i) Which functional group makes the compound a reducing agent?

(ii) State what would be observed when
(I) the compound is mixed with Fehling’s solution and boiled;
(II) few drops of concentrated H2SO4 is added to the sample of the compound.

(iii)  Write an equation for the reaction in 3(c)(ii)(II).  [6marks]

(d) A hydrocarbon Z with molecular mass 78 on combustion gave 3.385 g of CO2 and 0.692 g of H2O.  Determine the molecular formula of Z.
[H  =  1, C = 12, O  =  16]    [7 marks]


This question was attempted by few candidates and the performance was good.

In (a), candidates correctly responded to the questions thus:
(i) Dehydration
(ii) C2H4    

(iii)              H     H                                     
                     |    |          ethane – 1,2-diol/ethylene glycol                                  
               H – C –  C – H                                  
                     |    |                                                   
                  OH    OH          

(iii) Polyethene/polythene.

In (b)(i), candidates correctly stated that carbohydrates are (naturally occurring) organic compounds containing carbon, hydrogen and oxygen with the hydrogen and oxygen present in the ratio of 2:1 (as in water).

In (b)(ii), they correctly gave example of each of monosaccharide, disaccharide and polysaccharide thus:

            I.          monosaccharide:  glucose/fructose
            II.         disaccharide:  sucrose/lactose/maltose
            III.       polysaccharide:  starch/glycogen/cellulose

In (c)(i), candidates gave  C = O /CHO as the functional group that makes the compound a reducing sugar.

In (c)(ii), candidates correctly stated what would be observed in I and II thus:

  1. brick red precipitate
  2. black/charred mass (of carbon is observed)

In (c)(iii), most candidates correctly wrote an equation for the reaction in 3(c)(ii) (II) as follows:
                        C6H12O6  →  6C + 6H2O

In (d), majority of the candidates correctly determined the molecular formula of Z thus:

           mass of hydrogen in Z = 2  x 0.692 = 0.077g

            mass of carbon in Z = 12  x 3.385 = 0.923g

             C                        H                            
             0.923                  0.077                       
                12                       1                           
             0.077                  0.077
             0.077                  0.077                       
             0.077                  0.077                       

                1                          1                           

            Empirical formula of Z    =     CH.

(CH)n   =   78 
13n      =   78
    n      =   78      =    6
                     Molecular formula of Z    =    C6H6

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