5.1 Equilibrium II

1a2 marks

Ammonia is manufactured by the Haber Process.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

K_p= $\frac{p {({NH}_{3})}^{2}}{p (N_{2}) p {(H_{2})}^{3}}$

The pressure used in the Haber Process is 200 atm.

Explain the effect, if any, of increasing the pressure on the equilibrium yield of ammonia.

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4 marks

The equilibrium constants for K_p and K_c are related by the equation

K_p = _{$\frac{K_{c}}{{(R T)}^{∆ n}}$}

where Δn is the number of moles of reactants minus the number of moles of products.

Calculate the value of K_c at 500K when the value of K_p = 3.55 × 10⁻² atm⁻².
Include the units for K_c.
[Use the value of R = 0.0821 dm³ atm K⁻¹ mol⁻¹]

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5 marks

A mixture of 1.0 mol of nitrogen and 3.0 mol of hydrogen is left to reach equilibrium at 700 K.

Calculate the total pressure, in atmospheres, needed to produce a yield of 0.30 mol of ammonia at 700 K.
Give your answer to an appropriate number of significant figures.

You must show your working.
[K_p = 7.76 × 10⁻⁵ atm⁻² at 700 K]

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4 marks

The value of the equilibrium constant, K_p, varies with temperature.

The equation relating the values of the equilibrium constant at two temperatures is

In $[\frac{K_{2}}{K_{1}}] = \frac{∆ H}{R} [\frac{1}{T_{1}} - \frac{1}{T_{2}}]$

The equilibrium constant, K₁, for the formation of ammonia is 6.76 × 10⁵atm⁻² when the temperature T₁ = 298 K.
The enthalpy change ∆H = −92400 J mol⁻¹.

Calculate the value of the equilibrium constant for this reaction at 310 K.
[Use the value of R = 8.31 J mol⁻¹ K⁻¹]

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1a1 mark

This question is about the solubility of metal hydroxides.

Which of these metal hydroxides is the most soluble in water?

☐	A	barium hydroxide
☐	B	calcium hydroxide
☐	C	magnesium hydroxide
☐	D	potassium hydroxide

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9 marks

When excess magnesium hydroxide is added to water and shaken, a saturated solution is formed and the mixture reaches equilibrium.

Mg(OH)₂(s) ⇌ Mg²⁺(aq) + 2OH⁻(aq)

The equilibrium constant, K_c, for this reaction is

K_c = [Mg²⁺(aq)][OH⁻(aq)]²

Give a reason why the magnesium hydroxide is not included in the expression for K_c.

(1)

ii)

Give the units for K_c.

(1)

iii)

Calculate the enthalpy change of solution of magnesium hydroxide, using the following data.

Energy or enthalpy change	Value / kJ mol⁻¹
Lattice energy of Mg(OH)₂(s)	−2842
∆_hydH (Mg²⁺(aq))	−1920
∆_hydH (OH⁻(aq))	−460

(2)

iv)

Which graph shows the change in the concentration of the Mg²⁺(aq) ions when some solid magnesium hydroxide is shaken with water and left to reach equilibrium?

(1)

q6biv-9cho-al-1-june-2018-qp-edexcel-a-level-chem

Predict the effect, if any, of adding each of the following to a saturated solution of magnesium hydroxide in contact with solid magnesium hydroxide.
Justify your answers in terms of the effect on the equilibrium.

Mg(OH)₂(s) ⇌ Mg²⁺(aq) + 2OH⁻(aq)

(4)

Magnesium sulfate solution

Dilute hydrochloric acid

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3a1 mark

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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3 marks

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)

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4 marks

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..........................................................................

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5 marks

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)

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3e2 marks

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

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3f2 marks

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

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3g2 marks

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).

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Substance	SO₂(g)	O₂(g)	SO₃(g)
Number of moles /mol	0.0160	0.0120	0.772

Edexcel A Level Chemistry

Topic Questions

5.1 Equilibrium II

☐	A	180° and 90°
☐	B	120° and 90°
☐	C	120° and 109.5°
☐	D	109.5° and 109.5°

☐	A	$K_{c} = \frac{[Fe] \times [H_{2} O]}{[{Fe}_{3} O_{4}] \times [H_{2}]}$
☐	B	$K_{c} = \frac{{[Fe]}^{3} \times {[H_{2} O]}^{4}}{[{Fe}_{3} O_{4}] \times {[H_{2}]}^{4}}$
☐	C	$K_{c} = \frac{[H_{2} O]}{[H_{2}]}$
☐	D	$K_{c} = \frac{{[H_{2} O]}^{4}}{{[H_{2}]}^{4}}$