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
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
- mass of reactants,
- volume of gaseous products,
- amount of precipitate formed,
- change in pH,
- 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
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.
The expected answers were
I - enthalpy of solution (exothermic)
II - enthalpy of combustion (exothermic)
III - enthalpy of neutralization (exothermic)