Reversible Reactions & Equilibria (Edexcel IGCSE Chemistry)

This question is about ammonia gas, NH₃

Ammonia can be prepared in a laboratory from the reaction between ammonium chloride, NH₄Cl, and sodium hydroxide. The other products of the reaction are sodium chloride and water.

i) Give a chemical equation for this reaction.

(1)

ii) Give a test for ammonia gas.

(2)

In industry, ammonia is produced from nitrogen and hydrogen. The equation for this reaction is

N₂ (g) + 3H₂ (g)  2NH₃ (g)

In a sealed container, the reaction can reach a position of dynamic equilibrium. Explain the meaning of the term dynamic equilibrium.

The graph shows the percentage yield of ammonia at equilibrium for different temperatures and pressures.

Using the graph, explain if the forward reaction is exothermic or endothermic.

The decomposition of ammonium chloride is endothermic:

NH₄Cl (s) NH₃ (g) + HCl (g)

Which changes to the conditions would both decrease the amount of product in this reaction?

Hydrogen gas can be produced by reacting a mixture of methane and steam in the presence of a nickel catalyst.

The reaction conditions are a temperature of 700°C and a pressure of 5 atmospheres.

The equation for the reaction is

CH₄ (g) + H₂O (g) CO (g) + 3H₂ (g)                 ΔH = +206 kJ/mol

What does the symbol  represent?

i) The mixture of methane and steam is heated to a temperature greater than 700 °C but the pressure is kept at 5 atmospheres.

Predict the effect of this change on the yield of hydrogen at equilibrium, giving a reason for your answer.

(2)

ii) The mixture of methane and steam is kept at the same temperature of 700 °C but the pressure is increased to more than 5 atmospheres.

Predict the effect of this change on the yield of hydrogen at equilibrium, giving a reason for your answer.

(2)

Calculate the volume, in dm³, of hydrogen gas at rtp that is produced when 10 tonnes of methane gas completely react with steam.

[molar volume of hydrogen at rtp is 24 dm³]

Give your answer in standard form.

The gasification of carbon is a reversible endothermic reaction:

C (s) + H₂O (g)  CO (g) + H₂ (g)

Which of the following conditions could be changed to increase the relative amount of H₂ gas produced?

This question is about chemical equilibrium and reversible reactions.

If carried out in a closed system, reversible reactions can reach a state of equilibrium. 

Explain what a closed system is.

When a reversible reaction reaches equilibrium, it appears to have stopped. 

Explain why.

In the Haber process, the reaction of nitrogen with hydrogen to produce ammonia is reversible.

                                  N₂ (g)     +     3H₂ (g)     ⇌      2NH₃ (g)

Explain what happens to the amount of ammonia produced at equilibrium if the pressure is increased.

Describe the effect, if any, of using a catalyst on:

• the rate of reaction

• the position of equilibrium 

Ammonia is manufactured on a large scale and is used to make fertilisers such as ammonium nitrate (NH₄NO₃).

The first stage in the manufacture of ammonium nitrate is to react ammonia gas with oxygen gas.

4NH₃ (g) + 5O₂ (g)  4NO (g) + 6H₂O (g)    ΔH = −907 kJ/mol

The reaction is carried out at a pressure of about 10 atm and at a temperature of 800 °C, in the presence of a catalyst.

If the mixture is left for long enough in a sealed container, the reaction reaches a position of dynamic equilibrium.

Graph A shows how the percentage of nitrogen monoxide (NO) in the equilibrium mixture varies with temperature at constant pressure.

Graph B shows how the percentage of nitrogen monoxide (NO) in the equilibrium mixture varies with pressure at a constant temperature.

i) Explain why the percentage of NO at equilibrium decreases in each case.

Graph A:

Graph B:

(4)

ii) Explain why the use of a catalyst has no effect on the position of equilibrium in a reversible reaction.

(2)

The second stage in the manufacture of ammonium nitrate is to convert the nitrogen monoxide into nitric acid. The nitric acid is then reacted with concentrated aqueous ammonia as shown in this equation.

HNO₃ (aq) + NH₃ (aq) → NH₄NO₃ (aq)

i) State, in terms of proton transfer, why this reaction is classified as an acid-base reaction.

(1) 

ii) Calculate the volume of 14.8 mol/dm³ aqueous ammonia that is required to exactly neutralise 150 dm³ of a solution of nitric acid of concentration 15.8 mol/dm³.

(2)

Dinitrogen tetraoxide, N₂O₄, is a colourless gas.

 Nitrogen dioxide, NO₂, is a brown gas.

The two gases can exist together in dynamic equilibrium according to the equation:

N₂O₄(g)         2NO₂(g)

Explain what is meant by the term dynamic equilibrium.

Some N₂O₄ and some NO₂ are put into a sealed gas syringe and allowed to form an equilibrium mixture. 

This equilibrium mixture is brown.

i) The pressure of the gas in the syringe is increased by pushing in the piston. The mixture is then allowed to reach a new equilibrium at the same temperature as before.

Explain why the new equilibrium mixture contains less NO₂ than the original equilibrium mixture.

(2)

ii) A student suggests that the new equilibrium mixture would be lighter in colour than the original equilibrium mixture, as there is now less NO₂ present.

Suggest why the new equilibrium mixture is actually darker than the original.

(1)

Carbon monoxide, CO, and oxides of nitrogen are produced in a car engine when petrol is burned.

These oxides can be partly removed by using a catalytic converter fitted to the car’s exhaust system.

i)State how oxides of nitrogen are produced in the car engine.

(1)

ii) Give a disadvantage of allowing oxides of nitrogen to escape into the atmosphere.

(1)

iii) Write a chemical equation for the reaction between nitrogen monoxide, NO, and carbon monoxide to form carbon dioxide and nitrogen.

(1)

This question is about reactions involving gases.

Potassium carbonate reacts with dilute hydrochloric acid to produce carbon dioxide gas.

The equation for the reaction is

K₂CO₃ (s) + 2HCl (aq) → 2KCl (aq) + H₂O (l) + CO₂ (g)

Calculate the volume, in cm³, of carbon dioxide gas produced when 6.9 g of potassium carbonate reacts with excess dilute hydrochloric acid.

[M_r of K₂CO₃ = 138]

[molar volume of CO₂ at rtp = 24 dm³]

volume = .............................................................. cm³

This reaction involving gases is in dynamic equilibrium at a temperature of 225°C.

H₂ (g) + CO₂ (g) → CO (g) + H₂O (g) ΔH = + 41 kJ/mol

i) Predict the effect on the yield of CO (g) at equilibrium when the temperature is increased without changing the pressure.

Give a reason for your answer.

(2)

ii) Predict the effect on the yield of CO (g) at equilibrium when the pressure is increased without changing the temperature.

Give a reason for your answer.

(2)

The industrial production of sulfuric acid involves several steps.

One of these steps is the reaction of sulfur dioxide, SO₂, with oxygen to form sulfur trioxide, SO₃.

2SO₂ (g) + O₂ (g) 2SO₃ (g)

What volume of sulfur trioxide, in dm³, is produced by the complete reaction of 750 dm³ of sulfur dioxide? (all volumes of gases are measured under the same conditions of temperature and pressure)

Calculate the volume of oxygen needed to react completely with 750 dm³ of sulfur dioxide. (all volumes of gases are measured under the same conditions of temperature and pressure)

volume of oxygen = .............................. dm³

Calculate the mass, in kilograms, of 750 dm³ of sulfur dioxide, measured at room temperature and pressure.

(relative formula mass: SO₂ = 64;
1 mol of any gas at room temperature and pressure occupies 24 dm³)

The reaction to produce sulfur trioxide reaches an equilibrium.

 2SO₂ (g) + O₂ (g) 2SO₃ (g) 

 The forward reaction is exothermic. The rate of attainment of equilibrium and the equilibrium yield of sulfur trioxide are affected by pressure and temperature.

A manufacturer considered two sets of conditions, A and B, for this reaction.

In each case sulfur dioxide is mixed with excess oxygen.

The manufacturer changed the temperature and the pressure and only used a catalyst in B.

The sets of conditions A and B are shown in Figure 7.

Figure 7

The manufacturer chooses set of conditions B rather than set of conditions A.

Explain, by considering the effect of changing the conditions on the rate of attainment of equilibrium and on the equilibrium yield of sulfur trioxide, why the manufacturer chooses the set of conditions B rather than the set of conditions A.

This question is about equilibrium. 

An ionic equation for the reaction between iron (III), Fe³⁺ and thiocyanate ions, SCN²⁺ is shown below:

At room temperature, the mixture of the equilibrium solution is orange. 

 Which solvent would be used to dissolve the ions in this reaction?

Explain the effect, if any, of changing pressure on the colour of the equilibrium mixture. 

A few drops of a colourless solution containing a high concentration of Fe³⁺ ions are added to the orange equilibrium mixture.

Explain the colour change observed.

A water bath is set up at a temperature below room temperature.

When a test tube containing the orange equilibrium mixture is placed in the water bath, the mixture becomes more red.

Explain what this shows about the energy change for the forward reaction.