Equilibrium II (Edexcel A Level Chemistry)

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.

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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.

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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

CH3COOH(l)	+	CH3CH2CH2OH(l)		CH3COOCH2CH2CH3(l)	+	H2O(l)
ethanoic acid		propan-1-ol		propyl ethanoate

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.

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

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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.

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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

	☐	A
	☐	B
	☐	C
	☐	D