Reversibility & Equilibrium (AQA GCSE Chemistry: Combined Science)

A company manufactures ethanol (C₂H₅OH) for sale to other chemical companies as a raw ingredient for many products.

The reaction for the process is as follows:

C₂H₄ (g) + H₂O (g)    ⇌    C₂H₅OH (g)            

The forward reaction is exothermic.

The temperature and pressure of the reaction can be adjusted to increase the yield of ethanol when the reaction is at equilibrium.

Explain what is meant by the term equilibrium.

Explain what effect increasing the temperature would have on the yield of ethanol at equilibrium.

Explain what effect increasing the pressure would have on the yield of ethanol at equilibrium.

How can the direction of a reversible reaction be changed?

Figure 1 shows the effect of changing pressure on the percentage yield of ethanol at three different temperatures.

Figure 1

Figure 2 shows the effect of changing temperature on the rate of reaction at three different pressures.

Figure 2

The conditions for the reaction of ethene with steam in one of the processes are: 

• 65 atm
• 300 ^oC
• a catalyst.

Use your knowledge and the information in Figure 1 and Figure 2 to justify this choice of conditions.

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.

The Haber process is an example of a reversible reaction where the forward reaction is exothermic:

N₂ (g) + 3H₂ (g)  2NH₃ (g)

What is not true about this system when it is at equilibrium?

Which graph shows the highest yield of products?

A pink coloured compound of cobalt reacts with hydrochloric acid. 

The word equation for the reaction is:

pink cobalt compound + hydrochloric acid ⇌ blue cobalt compound + water

The forward reaction is endothermic. 

If carried out under correct conditions the reaction reaches equilibrium. 

The mixture at equilibrium contains both cobalt compounds and the mixture is purple.

The equilibrium mixture is cooled.

Explain what happens to the concentration of the pink cobalt compound on cooling.

A chemist adds more hydrochloric acid to the reaction.

Explain what happens to the colour of the mixture.

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?

Which statement is not true about Le Chatelier's Principle?

Hydrogen gas and iodine vapour can react to form hydrogen iodide:

H₂ (g) + I₂ (g) 2HI (g)

Which statement is not true?

Which option shows two correct statements about reversible reactions and equilibrium?

	Statement 1	Statement 2
A	The sign for a reversible reaction is	Reversible reactions are slower than non-reversible ones
B	At equilibrium the rate of the forward reaction and the rate of the reverse reaction are equal	An endothermic reaction is exothermic if reversed
C	Reversible reactions are not useful in industry as they do not go to completion	To reach an equilibrium the reaction must occur in a closed system
D	The rate of the reaction is zero at equilibrium	The concentration of the product is constant at equilibrium

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, with reasons, what happens to the amount of ammonia produced at equilibrium if the pressure is increased.

Methanol is produced by reacting carbon monoxide with hydrogen as shown in the equation:

CO (g) + 2H₂ (g)   ⇌   CH₃OH (g)

The reaction is normally performed at a temperature of 250 °C and a pressure of 100 atm.

The forward reaction is exothermic.

Explain what would happen to the yield of methanol if a temperature higher were used.

Explain why the high pressure gives a greater yield of methanol and an increased rate of reaction.

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.

This question is about reversible reactions.

The decomposition of hydrogen iodide into hydrogen and iodine is reversible.

2HI (g)          H₂ (g)     +     I₂ (g)

The forward reaction is endothermic. 

Explain the effect of increasing temperature on the amount of hydrogen iodide, HI. 

Suggest what effect, if any, increasing the pressure will have on the amount of hydrogen iodide at equilibrium.

Give a reason for your answer.

An endothermic reaction can be represented using the following reaction profile in Figure 1. 

Label

• Activation energy
• Overall energy change

Figure 1

How does the diagram show that the reaction is endothermic?

Methanol, CH₃OH, is a fuel that can be produced from carbon monoxide and hydrogen. 

Complete the dot and cross diagram to show the bonding in methanol. 

The reaction between carbon monoxide and hydrogen to produce methanol is shown below. 

The forward reaction is exothermic. 

CO(g)          +          2H₂(g)             CH₃OH(g)

Explain why the highest equilibrium yield of methanol is obtained at high pressure. 

Methanol can undergo combustion to release energy. 

The equation for this reaction is shown below:

Bond energies are shown in Table 1. 

Calculate the overall energy change for the reaction using the diagram and table above. 

A catalyst is used in the reaction to produce methanol to increase the rate of the reaction. 

Explain how it increases the rate of reaction. 

Ammonia is produced from the reaction of nitrogen with hydrogen in the Haber process:

N₂ (g) + 3H₂ (g)   ⇌    2NH₃ (g)

The forward reaction is exothermic.

The conditions normally used in the Haber process are:

• 450 ^oC
• Pressure of 200 atm
• Iron catalyst

Use your knowledge of reversible reactions and the equation to explain why these conditions are used.

To gain full marks you must consider both rate of reaction and yield in your answer.

Hydrogen can be manufactured on an industrial scale by reacting methane with steam. 

CH₄ (g)   +   H₂O (g)      CO (g)   +   3 H₂   (g)

The forward reaction is endothermic. 

Give the conditions of temperature and pressure that would give the maximum yield of hydrogen. 

Give one hazard associated with this reaction.