Question 3
a) (i) Describe, using the kinetic theory of matter, what happens when potassium chloride dissolves in water.
(ii) Give a reason why the process in 3(a) (i) is endothermic. [4 marks]
(b) (i) An underground iron pipe is less likely to corrode if it is bonded at intervals with magnesium rods. Give reasons for this observation.
(ii) State the stages involved in the rusting of iron.
(iii) State the condition for the rusting of iron in water. [6 marks]
(c) (i) What is a spontaneous reaction?
(ii) State two conditions that could make a reaction spontaneous.
(iii) Explain briefly why one gramme of sodium reacts more rapidly with water at 250C than one gramme of calcium at the same temperature.
(iv) Write equations for the reactions in 3(c)(iii). [10 marks]
(d) What mass of lead (II) trioxocarbonate (IV) would contain 35.0 g of lead?
[C=12.0, O = 16.0, Pb = 207.0] [3 marks]
(e) Name the type of intermolecular force present in:
(i) fluorine;
(ii) hydrogen fluoride. [2 marks]
Observation
This question was not popular among the candidates.
In part (a), majority of the candidates could not explain the dissolution of potassium chloride using the kinetic theory of matter.
In part (b), majority of the candidates could give reasons why an underground pipe is less likely to corrode if it is bonded at intervals with magnesium rods. In addition to this, the process of rusting was not satisfactorily explained.
In part (c), majority of the candidates could not explain a spontaneous reaction and could not state the conditions that could make a reaction spontaneous. Also, few candidates were able to write eaquations for reactions of sodium and calcium with water.
In part (d), majority of the candidates could not determine the mass of lead (II) trioxocarbonate (IV) that would contain 35.0 g of lead.
In part (e), few candidates could name the type of intermolecular force present in fluorine and hydrogen chloride.
The expected answers include:
(a) (i) The cations and anions attract the polar water molecules the energy released as
the ion-dipole forces are formed provides the energy to break the electrostatic
forces of attraction between the oppositely charged ions in the potassium chloride.
OR
The water molecules split KCl into free ions with a release of energy known as lattice energy. The free ions are hydrated/surrounded by water molecules. This is known as the hydration energy.
(ii) More energy is needed to overcome the ion-ion attraction hence the
dissolution process is endothermic /
OR
lattice energy is greater than hydration energy.
(b) (i) Magnesium is more reactive than iron therefore magnesium reacts
preferentially / acts as a sacrificial anode.
(ii) Iron oxidizes to Fe2+ by oxygen and water to give Iron (II) hydroxide / Fe2+ and
OH- formed. Further oxidation gives hydrated Iron (III) oxide / Fe2O3.xH2O
(iii) Presence of (dissolved) oxygen / air
(c) (i) It is a reaction that occurs without being driven by some outside / external force.
(ii)
- G must be negative
- the products are more random, more disorganized than the reactants /
entropy of products is greater than entropy of reactants / entropy must increase
(iii) Sodium is in Group I and is more reactive metal than calcium that is in
Group II. / sodium is more electropositive than calcium / higher in the reactivity series
(iv)
(d) Mr (PbCO3) = 207 + 12 + 16(3)
= 267g mol-1
(e)
(i) van der Waal’s forces
(ii) hydrogen bonding