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 Chemistry Paper 2,Nov/Dec 2009
 Questions: 1 2 3 4 5 6 7 8 Main
General Comments
Weakness/Remedies
Strength

Question 8

1. Consider the equilibrium reaction represented by the following equation:

2SO2(g) + O2(g)  ⇌  2SO3(g);     ΔH  =  - xkJmol-1

1. Write an expression for the equilibrium constant Kp.
2. State the effect of each of the following factors on the yield of SO3(g):

I.          decrease in pressure;
II.        increase in temperature;

1. liquefaction of SO3(g).
1. Sketch the energy profile diagram for the forward reaction indicating the position of:
2. reactants and product;
3. activated complex;
4. enthalpy change;
5. activation energy.

[10 marks]

1.   (i)   Mention two properties of carbon (IV) oxide that are responsible for its

use in fire extinguishers.
(ii)         Write a balanced equation for the reaction of carbon (IV) oxide with lime
water.
(iii)        State the function of carbon (IV) oxide in a nuclear reactor.
[5 marks]

1. Give the reason why
1. dilute H2SO4 is not suitable for the preparation of CO2(g) from CaCO3(s);
2. concentrated H2SO4 cannot be used to dry ammonia gas.              [4 marks]
1. On heating 13.15 g of NiSO4.yH2O, 7.75 g of the anhydrous salt was obtained.  Determine the value of y.

[H = I, O = 16, S = 32, Ni = 59)   [6 marks]

_____________________________________________________________________________________________________
OBSERVATION

The question was attempted by few candidates and performance as fair.

In part (a)(i), candidates could not correctly write an expression for the equilibrium constant Kp for the equilibrium reaction.  The expected answer from candidates was

Kp  =       P2SO2
P2SO2 . P2O2

In (a)(ii) I – III, candidates correctly stated the effect of each of the factor on the yield of SO2.
However, some of them lost marks because they confused the yield with the equilibrium position.  The expected answers from candidates were:
I.          decrease in the yield of SO3 (g)
II.        decrease in the yield of SO3 (g)
III.       more of SO3 (g) will be formed / increase the yield of SO3 (g)

In (a) (iii), candidates correctly sketch the energy profile diagram as follows:

In part (b), candidates correctly mentioned two properties of carbon (IV) oxide that are responsible for its use as fire extinguisher, wrote a balanced equation for the reaction between carbon (IV) oxide and lime water and stated the function of carbon (IV) oxide in nuclear reactor.

In part (c), candidates knew that dilute H2SO4 would form an insoluble salt which covers the CaCO3 thus preventing further reaction and that conc. H2SO4 reacts with ammonia to form a salt.

In part (d), candidates correctly determined the number of moles of water molecules in the given hydrated salt.

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