Observation

This question was compulsory for all candidates and their performance was average.

In part (a), majority of the candidates could not define a transition element.

In part (b), majority of the candidates stated the element which forms a doubly charged cation and soluble trioxocarbonate (IV).

In part (c), majority of the candidates explained why there is a general increase in the irst ionization energies o the elements across the period in the periodic table.

In part (d), majority of the candidates gave two examples of an aliphatic compound.

In part (e), majority of the candidates could not explain why alkanols are stronger bases than water.

In part (f), majority of the candidates stated the major raw materials used in the solvay process.

In part (g), majority of the candidates could not define geometric isomerism.

In part (h), majority of the candidates gave a reason why water gas is a better fuel than producer gas.

In part (i), majority of the candidates stated Faraday’s second law of electrolysis. However, few candidates were able to calculate the amount of silver deposited when 10920 coulombs of electricity is passed through a solution of a silver salt if 1F =96500 C mol-1).

           

The expected answers include:

(a) A transition element is one which has incompletely filled d-orbitals.

 

(b) (i) B
(ii) C

 

(c) Across a period (from left to right) there is a gradual increase in the number of protons in     the nucleus / effective nuclear-charge. This increases the force of attraction between the nucleus and the electrons, hence more energy is needed to remove the outermost electron.       

 

(d) - methane
- ethene
- ethyne
- ethanol
- ethanoic acid

(e)   - Alkanols are formed by replacing one hydrogen of water by an alkyl group (1)
       - Alkyl groups have a positive inductive effect / are electron releasing/ electron donating      
- This increases the electron density of the oxygen

 

(f) - NaCl
- CaCO3
- NH3
- C

      (g) Geometric isomerism is the existence of two compounds with the same molecular formula but differ in the arrangement of groups attached to the carbon containing the double bond

 

(h)  This is because constituents of water gases (CO and H2) are both combustible and give a higher calorific / heat value than producer gas.

 

(i) It is defined as the heat change/ released when 1 mole of a substance is burnt in excess oxygen
OR
It is defined as the heat change/ released when 1 mole of a substance is completely burnt in air

 

(j) (i) When the same quantity of electricity is passed through different electrolytes, the relative number of moles of the elements discharged are inversely proportional to the charges on the ions of the element.

         (ii)   96500 C liberates 1.0 mol of Ag(s)
∴ 10920 C will liberate 10920
                                     96500
= 0.113 mole