Observation

  This was a compulsory question.  Majority of the candidates that answered this question performed fairly.

In part (a), majority of the candidates stated the advantages of synthetic polymers over natural polymers.
In part (b), majority of the candidates could not draw the structure of the product formed when one mole of ethyne reacted with two moles of hydrogen bromide.
In part (c), majority of the candidates could not correctly explain why benzene is soluble in ethanol, but insoluble in water.
In part (d), majority of the candidates stated Le Chatelier’s principle correctly, and a large number of them identified the oxidized species in the given reaction equation.
In part (e), candidates made a fair attempt to explain dynamic equilibrium.
In part (f), majority of the candidates did not satisfactorily show the types of bond present in ammonium ion.
In part (g), majority of the candidates performed poorly in the question. A large number of them did not know that the reaction produced a precipitate of pbSO4 which coats the surface of the remaining Pb(N03)2.
In part (h), majority of the candidates were familiar with the laws of chemical combination and Aufbau’s principle.
In part (i), majority of the candidates could not explain why the first ionization energy of aluminum (13 Al) is less than that of magnesium (12Mg).
In part (j), majority of the candidates listed correctly three methods of separating a solid from a liquid.
In part (j), majority of the candidates were familiar with substitution and addition reactions of alkanes and alkenes respectively.

The expected answers include:

1. - cheaper
   - more durable
   - can be recycled
   - easily moulded
   - lighter
   - can be made on demand / tailor made
   - easily available
   - can be produced in larger quantity
   - can be formed faster
2. (i)
   (ii) 1, 1 – dibromoethane
3. Benzene and ethanol are organic compounds while water is an inorganic / polar compound / like dissolves like / organic compounds dissolve in organic solvents hence benzene is insoluble in water
4. (i) Le chatelier’s principle states that if a system which is in equilibrium is subjected to a change in conditions, (temperature, pressure or concentration) the position of the equilibrium adjusts itself to counteract / annul / cancel the effect of the change. (Accept any correct body of words)
   (ii) I. K2SO3 Accept potassium trioxosulphate (IV)
   II. Zn Accept Zinc
5. Dynamic equilibrium occurs when the forward and backward reactions are proceeding at the same rate (2) OR a reaction where the free energy change is equal to zero OR
   A reversible reaction in which the amount / concentration of reactants and products do not change with respect to time.
6. Positive charge shown on structure
   Coordinate / dative bond clearly shown
   All covalent bonds shown
7. The reaction produces insoluble lead (II) tetraoxosulphate (VI) which coats the surface of the remaining lead (II) trioxonitrate (V) preventing further reaction.

8. (i) - law of conservation of mass / indestructibility of matter
   - law of constant / definite composition
   - law of multiple proportions
   - law of reciprocal proportions

   (ii) Aufbau principle states that electrons always occupy the lowest energy level before filling higher energy level orbitals OR electrons occupy orbitals in order of increasing energy

9. The (outermost) electron in aluminium is in the 3p sub-shell while that of magnesium is in the 3s sub-shell the 3p sub-shell in Al is farther away from the nucleus than the 3s sub-shell of Mg, hence the I.E. of Al is less than that of Mg. OR
   3p orbital electrons are farther away and less strongly held than the 3s orbital electron which is closer and more strongly held
10. (i) Substitution (reaction)
    (ii) Addition (reaction)