Reversible Reactions & Equilibria Practice Questions

Question 1

5 marks

Ammonia is manufactured industrially from nitrogen and hydrogen using an iron catalyst.

(a)

State the raw material from which each of the two reactants is obtained.

(i) nitrogen

(ii) hydrogen

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(b)

Write the balanced equation for the formation of ammonia from nitrogen and hydrogen. State symbols are not required. Use the $\rightleftharpoons$ symbol in your equation.

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(c)

State the temperature and the pressure used in this industrial process.

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Question 2

4 marks

Hydrated cobalt(II) chloride is a pink solid. When it is heated, blue anhydrous cobalt(II) chloride is formed.

$\mathrm{CoCl_2.6H_2O(s)} \rightleftharpoons \mathrm{CoCl_2(s)} + 6\mathrm{H_2O(g)}$

The forward reaction is endothermic.

(a)

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

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(b)

State the colour change seen when hydrated cobalt(II) chloride crystals are heated in a test tube.

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(c)

Water is added to the blue anhydrous cobalt(II) chloride. Describe and explain what would be observed and felt.

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Question 0

5 marks

Nitrogen dioxide, $\mathrm{NO_2}$ , is a brown gas. It reacts to form dinitrogen tetroxide, $\mathrm{N_2O_4}$ , which is a colourless gas.

$2\mathrm{NO_2(g)} \rightleftharpoons \mathrm{N_2O_4(g)}$

A sample of nitrogen dioxide is sealed in a gas syringe and left until equilibrium is reached.

(a)

Explain what is meant by the term dynamic equilibrium.

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(b)

The plunger of the syringe is pushed in, increasing the pressure of the gas mixture. Predict, giving a reason, the effect of this on the equilibrium yield of dinitrogen tetroxide.

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(c)

When the plunger is first pushed in, the brown colour darkens because the gases are compressed. State how the colour of the gas mixture then changes as the new equilibrium is established.

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Question 0

5 marks

Ammonium chloride is a white solid. When it is heated strongly it decomposes to form two colourless gases.

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

A student heats some ammonium chloride at the bottom of a test tube, as shown below.

(a)

Name the type of reaction taking place at the bottom of the test tube, where the ammonium chloride is being heated.

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(b)

Explain why a white solid forms on the cool glass near the top of the test tube.

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(c)

The student's test tube is open at the top. Explain why a dynamic equilibrium cannot be reached in this apparatus.

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Question 0

4 marks

Iron(III) ions react with thiocyanate ions in aqueous solution to form an ion that has a deep red colour.

$\mathrm{Fe^{3+}(aq)} + \mathrm{SCN^{-}(aq)} \rightleftharpoons \mathrm{FeSCN^{2+}(aq)}$

$\mathrm{Fe^{3+}(aq)}$ is pale yellow, $\mathrm{SCN^{-}(aq)}$ is colourless and $\mathrm{FeSCN^{2+}(aq)}$ is deep red.

Solutions containing iron(III) ions and thiocyanate ions are mixed and left until equilibrium is reached. The red mixture is shared equally between three test tubes, A, B and C.

Test tube	Change made	Observation
A	a few drops of iron(III) nitrate solution added	colour becomes a darker red
B	a few drops of potassium thiocyanate solution added	to be predicted
C	a few drops of a solution that removes $\mathrm{Fe^{3+}}$ ions added	colour becomes much paler

(a)

Explain the observation made in test tube A.

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(b)

Predict the observation made in test tube B.

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(c)

Explain, in terms of the position of equilibrium, why the colour in test tube C becomes much paler.

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Question 0

5 marks

The equilibrium yield of ammonia in the reaction below depends on the temperature and the pressure used.

$\mathrm{N_2(g)} + 3\mathrm{H_2(g)} \rightleftharpoons 2\mathrm{NH_3(g)}$

The table shows the percentage yield of ammonia obtained at equilibrium under different conditions.

Temperature in °C	Yield at 100 atm in %	Yield at 200 atm in %	Yield at 400 atm in %
350	53	67	80
450	25	39	55
550	11	18	31

(a)

Give the percentage yield of ammonia at 450 °C and 200 atm.

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(b)

Describe the effect of increasing the pressure on the equilibrium yield of ammonia and explain this effect using the equation.

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(c)

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

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Question 0

6 marks

Methanol, $\mathrm{CH_3OH}$ , is manufactured from carbon monoxide and hydrogen. The reaction is reversible and reaches a dynamic equilibrium in a sealed reaction vessel.

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

Heat energy is given out in the forward reaction.

A manufacturer can change the conditions used in the reaction vessel:

increase the pressure from 50 atm to 100 atm
increase the temperature from 250 °C to 400 °C
use hydrogen in excess, rather than mixing the gases in the ratio shown in the equation.

Explain the effect that each of these three changes has on the equilibrium yield of methanol.

(You do not need to write about the rate of the reaction.)

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Question 0

6 marks

Hydrogen for the Haber process can be made by reacting carbon monoxide with steam in a sealed reactor. The reaction is reversible.

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

Heat energy is given out in the forward reaction.

(a)

Explain why increasing the pressure in the reactor has no effect on the equilibrium yield of hydrogen.

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(b)

The reactor can be operated at 200 °C or at 400 °C. Predict which temperature gives the greater equilibrium yield of hydrogen and explain your answer.

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(c)

In the industrial process the carbon dioxide is removed from the gas mixture as soon as it is formed. Explain why this increases the amount of hydrogen produced.

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Atomic Structure

States of Matter & Mixtures

Chemical changes

Extracting metals & equilibria

Separate Chemistry 1

Groups in the periodic table

Rates of reaction & energy changes

Fuels & Earth science

Separate Chemistry 2

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