NAME: __________________________
Read each question carefully and write your answers on a separate sheet of paper. Show all working.
PART A MULTIPLE CHOICE
Choose the best alternative and write A, B, C or D on your answer sheet.
1. The symbol [X] means:
(A) the number of moles of X
(C) the concentration of X in g L
-1
(B) the number of grams of X
(D) the concentration of X in mol L
-1
2. Which of the following statements below is the most correct? An equilibrium is a reaction:
(A) in which you see nothing happen
(B) which does not use up all the reactants
(C) where the product is not being formed any more
(D) in which the rate of the forward reaction is equal to the rate of the reverse reaction
3. Reactions which form equilibria generally
(A) release lots of energy (B) absorb lots of energy
(C) are easily reversed (D) involve gases
4. In general, if a reaction at equilibrium is upset it will
(A) automatically re-establish itself
(B) stop reacting
(C) proceed only in the forward direction
(D) proceed to completion
5. Which one of the following systems is most likely to be at equilibrium?
(A) a precipitation reaction
(B) a saturated salt solution
(C) an open beaker of water which has been standing for some time.
(D) a closed vessel containing some nitrogen gas and some helium gas.
6. A student wrote two equations to represent the equilibrium that forms when a mixture of SO
2
and O
2
reacts at a fixed temperature in a closed container. The equations are shown below:
Equation 1: SO
2(g)
+ ½O
2(g)
⇌
SO
3(g)
Equation 2: 2SO
2(g)
+ O
2(g)
⇌
2SO
3(g)
Choose the statement about these two equations that is correct. Both equations
(A) would not be affected by changes in temperature
(B) have a different ratio of moles reacting
(C) show complete reactions occurring
(D) show equilibria that would shift to the right if pressure was increased

Questions 7 and 8 refer to the graph shown below.
Two gases A and B are allowed to react in a 1 litre container until equilibrium is reached.
The graph of this equilibrium reaction is shown below.
Concentration
A
B
t
1
t
2
t
3
Time
7. The system was at equilibrium at time
(A) t
1
only (B) t
2
only (C) t
3
only (D) t
1
and t
3
8. Which of the following factors was most probably changed at time t
2
?
(A) concentration (B) pressure (C) volume (D) temperature
9. Nitrogen dioxide, a brown gas, and dinitrogen tetroxide, a colourless gas are both forms of
nitrogen (IV) oxide. They are in equilibrium according to the equation:
2NO
2(g)
⇌
N
2
O
4(g)
An equilibrium mixture of the two gases at room temperature is light brown but at higher temperatures
the colour becomes a much deeper brown. From this observation one can conclude that
(A) the forward reaction in the above reaction is endothermic
(B) the reverse reaction in the above reaction is endothermic
(C) both forward and reverse reactions produce a brown colour
(D) the equilibrium concentration of N
2
O
4(g)
is not dependent on temperature
10. Choose the correct statement
(A) strong acids form an equilibrium with their ions in water
(B) strong acids normally ionise completely
(C) the ionisation of strong acids is not a chemical reaction
(D) strong acids do not react with water molecules
PART B SHORT ANSWER QUESTIONS
11. Sketch a generalised energy profile diagram for an endothermic equilibrium reaction. On the same
diagram, sketch a profile for the same reaction but this time a catalyst has been added.

12. Consider the decomposition of phosphorus pentachloride into phosphorus trichlorlde and chlorine gas.
This forms an equilibrium as shown in the following equation:
PCl
5(g)
⇌
PCl
3(g)
+ Cl
2(g)
(a) Describe how fast the forward and reverse reactions will be occurring
(i) at the instant the reaction begins
(ii) a short time after the reaction begins
(iii) at equilibrium
(b) When does the reaction stop?
13. When heated in a closed vessel under certain conditions, solid ammonium chloride will form an
equilibrium with ammonia gas and hydrogen chloride gas.
(a) Write an equation that represents this equilibrium.
(b) Suggest an apparatus which would be suitable to study this equilibrium in your lab.
(c) What is one way you could prove that ammonia gas was formed in this reaction?
(d) What is one way you could prove that hydrogen chloride gas was formed in this reaction?
(e) Could an equilibrium be set up if some NH
4
Cl
(s)
was placed in an open crucible and heated?
Explain.
14. Is the water cycle on earth an example of a natural chemical equilibrium? Comment.
15. Ammonia is an extremely important substance which is used in the manufacture of fertilisers.
The process of making ammonia industrially is called the Haber process. A chemist is employed
to consider the Haber process reaction shown below:
N
2(g)
+ 3H
2(g)
⇌
2NH
3(g)

H = -92 kJ
The chemist is told that the reaction is far too slow to make a decent profit in a reasonable time.
The chemist also knows that increasing the temperature will speed up the reaction.
(a) Should the chemist increase the temperature of the reaction? Comment.
(b) What could the chemist do to improve profits?
16. Examine the following equilibrium reaction: 4NH
3(g)
+ 5O
2(g)
⇌
4NO
(g)
+ 6H
2
O
(g)
What would be the effect on the concentrations of all four chemicals of
(a) adding more O
2
to the system?
(b) reducing the concentration of NO?
I7. Examine the following equilibrium reaction:
2CO
(g)
+ O
2(g)
⇌
2CO
2(g)

H = -564 kJ
What would be the effect on the concentrations of all three chemicals of
(a) increasing the volume of the system?
(b) increasing the pressure on the system?
(c) increasing the temperature?
(d) adding a catalyst before the system had reached equilibrium?
(e) adding a catalyst after the system had reached equilibrium?

18. From your knowledge of the following reactions, where would the equilibrium concentrations
position be found at 100 kPa and 25 o
C – far to the right, far to the left or near the middle.
(a) 2H
2(g)
+ O
2(g)
⇌
2H
2
O
(g)
(b) 2CO
2(g)
⇌
2CO
(g)
+ O
2(g)
K = 2 x 10
K = 4 x 10
81
-92
19. Acetic acid (ethanoic acid) is commonly found in vinegar. It has the formula CH
3
COOH .
It does not ionise completely but forms an equilibrium with its ions in solution.
(a) Write down the full equilibrium equation for acetic acid, including water as a reactant.
(b) Draw a beaker containing an acetic acid solution and use a key to indicate all chemicals
present in the beaker.
(c) The ionisation of acetic acid is an endothermic process. Predict what would happen
to the equilibrium if the temperature was to increase.
20. In a 2.0 L flask, 0.7 mol of PCl
3
(phosphorus trichloride gas) and 0.5 mol of Cl
2
(chlorine gas)
are allowed to come to equilibrium according to the equation:
PCl
3(g)
+ Cl
2(g)
⇌
PCl
5(g)
When equilibrium has been reached 0.3 mol of PCl
3
remains.
(a) When equilibrium has been reached how many moles of Cl
2
and PCl
5
are present?
(b) What is the concentration of all three chemical at equilibrium?
(c) Explain why all of the chlorine gas did not react even though the PCl
3
is in excess.
(d) What would happen to this equilibrium if pressure was applied to the system.
*******************************
DH