Exam-style Questions: Chemical Equilibria

1.

Esters are a useful group of compounds due to their distinctive smells. One example
of an ester is ethyl ethanoate, its formation is shown below.

CH3COOH(aq) + C2H5OH(aq)λCH3COO C2H5(aq)
+ H2O(l)

a) Systems like this are described as being a 'dynamic equilibrium'. Explain
the term 'dynamic equilibrium'

b) Write down the expression for the equilibrium constant, Kc, for this reaction.

c) Calculate the value of Kc for this reaction given the equilibrium concentrations below.

[CH3COOH] = 0.08 moldm-3

[C2H5OH] = 0.08 moldm-3

[CH3COO C2H5] = 0.25 moldm-3

[H2O] = 0.1 moldm-3

d) Concentrated sulphuric acid is added to the reaction mixture as it removes water molecules. What effect would this have on the equilibrium position of this system?

(Marks available: 7)

Answer

Answer outline and marking scheme for question: 1

a)

- concentrations of the species are constant

- forward and backward reactions are continually taking place but at the same rate.

b) Kc =[CH3COO C2H5] [H2O]

           [CH3COOH]
[C2H5OH]

c) Kc = [CH3COO C2H5] [H2O] = (0.25) (0.1) = 3.91 (no units)


            [CH3COOH] [C2H5OH]    (0.08)(0.08)

(1 mark numerical answer, 1 mark stating no units)

d) Concentrated sulphuric acid is added to the reaction mixture as it removes water molecules. What effect would this have on the equilibrium position of this system?

(Marks available: 7)

2. The Haber-Bosch process is used for the large-scale production of ammonia from
nitrogen and hydrogen gas. The reaction is shown below:

N2(g) + 3H2(g)λ2NH3(g)

a) Write an expression for Kp for this reaction.

b) What are the units of Kp for this reaction? (Assume pressure is measured in kPa)

c) When the temperature is raised for this process the proportion of NH3(g) in the mixture decreases. Explain this observation.

d) What effect will an increase in the total pressure have on the equilibrium position?

(Marks available: 5)

Answer

Answer outline and marking scheme for question: 2

a) Kp = (p NH3)2 (1)

            (p N2)(pH2)2

b) Units = kPa2 =
1 / kPa2 = kPa-2

           kPa x kPa3


c) Increase in temperature leading to decrease in NH3 means reaction
towards right is exothermic.

d) An increase in pressure will lead to an increase in the proportion of NH3(g) found in the mixture.

(Marks available: 5)