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

Question 4

(a) (i) Define covalent bond.
(ii)  Give two properties of covalent compounds
(iii) With the aid of a diagram, show how ammonia molecule is formed.
(iv) Illustrate with a diagram the formation of ammonium ion.
(v) What type of bond(s) exist(s) in

    1. ammonia,
    2. ammonium ion? [ 1H,  7 N]  [10 marks]

(b) (i) Write three subatomic particles with their corresponding relative masses.
(ii) Name the possible states in which water can exist.  [9 marks]

(c) (i) State Graham’s law of diffusion.
(ii)  Arrange the following gases, He, CH4 and N2 in order of increasing rates of diffusion.  Give a reason for the order.

[ H  =  1, He  =  4, C  =  12, N  =   14 ]   [4 marks]

(d)  Draw the structures of the following compounds:
(i) 2, 3-dimethylbutane;

(ii) 1, 4-dibromocyclohexane.    [2 marks]

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

In (a)(i), most candidates correctly defined covalent bond however, some of them lost marks because the definition given was not complete.  The expected response from candidates was that covalent bond is a bond between two atoms in which each of the atoms contributes to the shared pair of electrons.

In (a) (ii), candidates correctly gave two properties of covalent compounds from the following:

  1. non- conductors of electricity.
  2. insoluble in water/soluble in non-polar solvents.
  3. have low melting/boiling point.

In (a)(iii), most candidates could not correctly show with the aid of a diagram how ammonia molecule is formed.  The required answer to the question was

The candidates were also unable to correctly illustrate the formation of ammonium ion with a diagram in (a)(iv).

The expected diagram was as follows:

In (a)(v), candidates correctly stated that the type of bond which exists in each of ammonia and ammonium ion were covalent bond and covalent bond/dative or coordinate covalent bond respectively.   

In (b)(i), candidates could not correctly write the three subatomic particles with their corresponding relative masses as they did not understand the demand of the question. 

The expected answers were as shown in the table below.









In (b)(ii), candidates correctly named the possible states in which water can exist as, solid, liquid and gaseous.

In (c)(i), most candidates correctly stated Graham’s law of diffusion however, some of them lost marks as they left the conditions under which the law holds while others wrote density instead of vapour density.  The expected response from candidates was at constant temperature and pressure, the rate of diffusion of a gas is inversely proportional to the square root of its vapour density/molecular mass.

In (c)(ii) only few candidates correctly arranged the gases in increasing order of rates of diffusion with a reason as follows:  N2, CH4,  He: the smaller the molar mass, the faster the rate at which the gas diffuses or the larger the molar mass, the slower the rate at which gas diffuses.

In (d), most candidates could not draw correctly the structures of the organic compounds.  The expected structures from candidate were as drawn below.                                                  
                               H  -  C  - H                 
                       H  -  H     |     H             
                       |      |     |      |                                     
                 H  -C  -  C  -  C  -  C  - H 
                       |      |     |      |
                       H     |      H     H
                       H –  C –  H                 
2,3 -dimethylbutane                     1,4-dibromocyclohexane

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