Reversibility & Equilibrium Practice Questions

Question 1

3 marks

Ammonium chloride is a white solid. When it is heated in a sealed tube the following reaction takes place:

$\mathrm{NH_4Cl(s) \rightleftharpoons NH_3(g) + HCl(g)}$

(a)

State what the symbol $\rightleftharpoons$ in the equation tells you about this reaction.

/ 1

(b)

The forward reaction is endothermic.

Complete the two sentences below.

The reverse reaction is ______________ .

The amount of energy transferred in the reverse reaction is ______________ the amount of energy transferred in the forward reaction.

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Question 2

3 marks

A student heated blue crystals of hydrated copper(II) sulfate in a test tube, as shown in the diagram.

(a)

State the colour change of the solid that the student observed as the crystals were heated.

/ 1

(b)

After heating, the student allowed the solid to cool and then added a few drops of water to it.

Describe two observations the student would make.

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Question 0

5 marks

Chromate ions and dichromate ions form an equilibrium mixture in aqueous solution.

$\mathrm{2CrO_4^{2-}(aq) + 2H^+(aq) \rightleftharpoons Cr_2O_7^{2-}(aq) + H_2O(l)}$

Ion	Colour of solution
$\mathrm{CrO_4^{2-}}$	yellow
$\mathrm{Cr_2O_7^{2-}}$	orange

(a)

A few drops of dilute sulfuric acid are added to the equilibrium mixture. This increases the concentration of $\mathrm{H^+}$ ions.

Predict the colour change observed and explain your answer.

/ 3

(b)

Sodium hydroxide solution is then added to the mixture. The hydroxide ions remove $\mathrm{H^+}$ ions from the solution.

Predict and explain the effect of this on the equilibrium mixture.

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Question 0

5 marks

Sulfur trioxide is made industrially from sulfur dioxide and oxygen. The reaction reaches equilibrium in a sealed reaction vessel.

$\mathrm{2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)}$

(a)

State Le Chatelier's Principle.

/ 1

(b)

Predict and explain the effect of increasing the pressure on the amount of sulfur trioxide in the equilibrium mixture.

/ 2

(c)

The forward reaction is exothermic.

Explain the effect of increasing the temperature on the yield of sulfur trioxide at equilibrium.

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Question 0

5 marks

Hydrogen and iodine vapour form an equilibrium mixture with hydrogen iodide in a sealed container at 450 °C.

$\mathrm{H_2(g) + I_2(g) \rightleftharpoons 2HI(g)}$

Hydrogen and hydrogen iodide are colourless gases. Iodine vapour is purple.

(a)

Explain why increasing the pressure has no effect on the position of this equilibrium.

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(b)

Some of the hydrogen iodide is removed from the equilibrium mixture at constant temperature and pressure.

Predict and explain what happens to the purple colour of the mixture.

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Question 0

5 marks

Carbon monoxide and steam were sealed in a rigid container at a constant temperature. The gases reacted to reach equilibrium.

$\mathrm{CO(g) + H_2O(g) \rightleftharpoons CO_2(g) + H_2(g)}$

The rates of the forward and reverse reactions were monitored. The results are shown in the table.

Time in minutes	Rate of forward reaction in mol/dm³ per minute	Rate of reverse reaction in mol/dm³ per minute
0	0.60	0.00
2	0.42	0.14
4	0.30	0.24
6	0.26	0.26
8	0.26	0.26

(a)

Explain why the reaction must take place in a sealed container for equilibrium to be reached.

/ 1

(b)

Use the table to give the time at which equilibrium was first reached. Give a reason for your answer.

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(c)

After equilibrium had been reached, both the forward and the reverse reactions were still taking place.

Explain why the concentration of hydrogen in the container stayed constant.

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Question 0

5 marks

Hydrogen can be manufactured by reacting methane with steam. The reaction is reversible.

$\mathrm{CH_4(g) + H_2O(g) \rightleftharpoons CO(g) + 3H_2(g)}$

The table shows the percentage of hydrogen in the equilibrium mixture at different temperatures and two different pressures.

Temperature in °C	% hydrogen in equilibrium mixture at 1 atmosphere	% hydrogen in equilibrium mixture at 20 atmospheres
400	30	11
600	63	37
800	89	70

(a)

Use the data in the table to deduce whether the forward reaction is exothermic or endothermic.

Explain your answer.

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(b)

Use the equation to explain why the percentage of hydrogen in the equilibrium mixture is lower at 20 atmospheres than at 1 atmosphere.

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Question 0

6 marks

Methanol, $\mathrm{CH_3OH}$ , is manufactured from carbon monoxide and hydrogen.

$\mathrm{CO(g) + 2H_2(g) \rightleftharpoons CH_3OH(g)}$

The forward reaction is exothermic.

In industry, the reaction is carried out at a temperature of about 250 °C and a pressure of about 60 atmospheres.

Explain how the temperature and the pressure used affect the rate of the reaction and the yield of methanol at equilibrium, and why the conditions chosen are a compromise.

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Question 0

6 marks

Ethene reacts with hydrogen to form ethane. The reaction is reversible and reaches equilibrium in a sealed vessel.

$\mathrm{C_2H_4(g) + H_2(g) \rightleftharpoons C_2H_6(g)}$

The table shows the percentage of ethane in the equilibrium mixture at different temperatures. The pressure was kept constant.

Temperature in °C	% ethane in the equilibrium mixture
100	97
300	74
500	40

(a)

Use the data in the table to deduce whether the forward reaction is exothermic or endothermic.

Explain your answer.

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(b)

Predict and explain the effect of increasing the pressure on the percentage of ethane in the equilibrium mixture.

/ 2

(c)

A catalyst is used in this reaction. Explain why the catalyst does not increase the percentage of ethane in the equilibrium mixture.

/ 1

Atomic Structure & the Periodic Table

Bonds, Structure & Properties of Matter

Quantitative Chemistry

Chemical Changes

Energy Changes

Chemical Change: Rate & Extent

Organic Chemistry

Chemical Analysis

Chemistry of the Atmosphere

Using Resources

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