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 Chemistry Paper 2,Nov/Dec 2008
 Questions: 1 2 3 4 5 6 Main
Weakness/Remedies
Strength

Question 2

(a)        (i)    Define the terms     I.   rate of reaction;
II.   reversible reaction in a state of dynamic equilibrium.

(ii)   Mention three factors that could affect the rate of a reaction.
(iii)  State two properties of a reaction system that could be used to determine the
reaction rate.                                                                                       [9 marks]

(b)        Consider the following equilibrium reaction:

PC15(1)    ⇌    PC13(l)   +    C12(g);      ∆H   =    +x kJmol-1

(i)     Write an expression for the equilibrium constant Kc.
(ii)     What would happen to the equilibrium position when the

I.   pressure of the system is reduced;
II.   temperature of the system is increased;
III.  concentration of the reactant is increased?

(iii)     Calculate the amount (moles) of chlorine that would be produced from
5.10g of phosphorus (V) chloride
[P = 31, Cl = 35.5]                                                               [8 marks]

(c)        (i)      What is meant by enthalpy change of a reaction?
(ii)      Sketch an energy profile diagram for an endothermic reaction.
(iii)      Name the enthalpychanges involved in each of the following processes:

I.     dissolution of sodium hydroxide in water;
II.    burning of carbon in air;
III.   reaction of dilute sodium hydroxide with dilute hydrochloric acid.

_____________________________________________________________________________________________________
OBSERVATION

This question was attempted by most candidates but the performance was poor.
In(a)(i), candidates could not correctly define the terms rate of reaction and reversible reaction in a state of dynamic equilibrium .  The correct definitions of the terms are:

I.          Rate of reaction is a measure of the change in concentration of reactants/products per unit time or the number of moles of reactants used or products formed with time or it is equal to dx
dt
where dx is a measure of a very small quantity of chemical change taking place in a very small time dt.

II.            Reversible reaction is in a state of dynamic equilibrium when the rate of forward reaction equals the rate of backward/reverse reaction.

In (a)(ii), majority of the candidates could not correctly state two properties of a reaction system that could be used to determine the  reaction rate.  The expected answers were any two from

1. mass of reactants,
2. volume of gaseous products,
3. amount of precipitate formed,
4. change in pH,
5. change in colour intensity.

In(b)(i), candidates could not write the correct expression for the equilibrium constant Kc because they did not know that it is the concentration of the gaseous substance written as Kc  = [Cl2].

In (b)(ii) candidates did not know that the equilibrium position for each of I, II and III shifts to the right.

In (b)(iii), most candidates correctly calculated the amount (moles) of chlorine that would be produced from 5.10g of phosphorus (V) chloride thus:

Molar mass of PCl2      =   (31 + 35.5  x  5)     =     208.5gmol    .
From given equation,
208.5g   PCl5             Ξ        1 mole  Cl2
∴ 5.10g  PCl5             Ξ     1 x 5.10  Cl2
208.5
In (c)(i); only few candidates correctly stated that enthalpy of a reaction is the heat change that occurs in a reaction.

In (c)(ii), candidates could not correctly draw the energy profile diagram of an endothermic reaction as follows.

In (c)(iii), candidates could not name the enthalpy changes involved in each of the processes in I – III.