Equilibrium II (Edexcel A Level Chemistry): Exam Questions

A mixture of ethanoic acid, ethanol and a catalyst was left for several days to reach equilibrium.

CH₃COOH (l) + CH₃CH₂OH (l)  ⇌  CH₃COOCH₂CH₃ (l) + H₂O (l)

The equilibrium constant, K_c , under these conditions, was 0.28.

i) Write the expression for the equilibrium constant, K_c .

(1)

ii) The initial amounts of ethanol and ethanoic acid used were 1.2 mol of each reactant.

Use this information, your expression for the equilibrium constant, K_c , and the value for K_c, to find the amounts of each product at equilibrium, in moles.

(3)

Amount of CH₃COOCH₂CH₃ = ................................................................................................ Amount of H₂O                        = ................................................................................................

Another ester, methyl methanoate, can be formed by the reaction between methanol and carbon monoxide in the gaseous phase.

i) The two O−C−H bond angles, x and y, in the ester are approximately

(1)

ii) The reaction often forms an equilibrium mixture. Which could be the units for the equilibrium constant, K_p?

(1)

iii) Describe what effect, if any, increasing the pressure would have on the equilibrium constant, K_p. Justify your answer.

(2)

This question is about an experiment to determine the equilibrium constant, K_c , for an esterification reaction producing propyl ethanoate. The equation for the reaction is

In an experiment to determine the equilibrium constant, K_c , the following steps were carried out.

• 6.0 cm³ of ethanoic acid (0.105 mol), 6.0 cm³ of propan-1-ol (0.080 mol) and 2.0 cm³ of dilute hydrochloric acid were mixed together in a sealed boiling tube. In this pre-equilibrium mixture, there is 0.111 mol of water

• The mixture was left for one week, at room temperature and pressure, to reach equilibrium

• The equilibrium mixture and washings were transferred to a volumetric flask and the solution made up to exactly 250.0 cm³ using distilled water

• 25.0 cm³ samples of the diluted equilibrium mixture were titrated with a solution of sodium hydroxide, concentration 0.200 mol dm⁻³, using phenolphthalein as the indicator

• The mean titre was 23.60 cm³ of 0.200 mol dm⁻³ sodium hydroxide solution.

State the role of the hydrochloric acid in the esterification reaction.

i) Calculate the total amount, in moles, of acid present in the volumetric flask in the equilibrium mixture.

(2)

ii) The 2.0 cm³ of dilute hydrochloric acid contained 0.00400 mol of H⁺ (aq) ions. Use this and your answer to part (b)(i) to calculate the amount, in moles, of ethanoic acid present in the equilibrium mixture.

(1)

i) The initial mixture in the boiling tube contained 0.105 mol of ethanoic acid. Use your answer to (b)(ii) to calculate the amount, in moles, of ethanoic acid that reacted to form the ester in the equilibrium mixture.

(1)

ii) Use information given in the method, and your answer to (c)(i), to calculate the amounts, in moles, of propan-1-ol, propyl ethanoate and water that are present in the equilibrium mixture.

(3)

Moles of propan-1-ol at equilibrium.............................................................
Moles of propyl ethanoate at equilibrium...................................................
Moles of water at equilibrium..........................................................................

i) Write the expression for the equilibrium constant, K_c, for this reaction.

CH₃COOH(l) + CH₃CH₂CH₂OH(l)   CH₃COOCH₂CH₂CH₃(l) + H₂O(l)

(1)

ii) Explain why it is possible, in this case, to calculate K_c using equilibrium amounts in moles, rather than equilibrium concentrations.

(2)

iii) Calculate the value of K_c.

Give your answer to an appropriate number of significant figures.

(2)

The pink colour of the phenolphthalein fades after the end-point of the titration has been reached. Give a possible explanation for this observation.

Explain what you could do to confirm that one week is sufficient time for the mixture to reach equilibrium.

A student repeated the experiment, but left the mixture in a water bath at 40 °C until equilibrium was reached.

CH₃COOH (l) + CH₃CH₂CH₂OH (l)  CH₃COOCH₂CH₂CH₃ (l) + H₂O (l)   ∆_rH = +21.4 kJ mol⁻¹

Deduce the effect, if any, on this student’s value for K_c compared with that obtained in part (d)(iii).

This question is about equilibrium systems.

Sulfur dioxide and oxygen form an equilibrium with sulfur trioxide.

2SO₂ (g) + O₂ (g) 2SO₃ (g)

The composition of an equilibrium mixture at 698 K and a total pressure of 2.40 atm is shown in the table.

i) Calculate the value of K_p at this temperature. Include units, if appropriate.

(5)

ii) Calculate the number of sulfur dioxide molecules present in this equilibrium mixture.

(1)

iii) Deduce, by referring to K_p , how the number of sulfur dioxide molecules will change if more oxygen is added to the equilibrium mixture.

(2)

An equilibrium exists in aqueous solution between the chromate(VI) ions and the dichromate(VI) ions.

Explain any change in the position of equilibrium if a few drops of sodium hydroxide solution are added to this equilibrium system.

The equilibrium for the reaction between hydrogen gas and an oxide of iron is

The K_c expression for this equilibrium is