Reversible Reactions & Equilibria (Edexcel GCSE Chemistry)

Hydrated copper sulfate, CuSO₄.5H₂O, is a blue solid.
Anhydrous copper sulfate, CuSO₄, is a white solid.

Heat energy is needed to convert hydrated copper sulfate to anhydrous copper sulfate.
This is a reversible reaction.

CuSO₄.5H₂O CuSO₄ + 5H₂O 

Devise an experiment to show that this is a reversible reaction.

Hydrogen reacts with iodine to form hydrogen iodide.
Iodine gas is purple and hydrogen iodide gas is colourless.

H₂ (g) + I₂ (g)  2HI (g)

Hydrogen and iodine are placed in a sealed container.

The container is left until equilibrium is reached.

The conditions are changed favouring the forward reaction.

Explain what you would see.

Higher Only

Calculate the number of atoms combined in one mole of copper iodide, CuI₂.

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
	☐	B
	☐	C
	☐	D	None of the above

In industry, ammonia is manufactured by reacting nitrogen with hydrogen.

The formula of ammonia is NH₃.

State what the formula of ammonia shows about the number of nitrogen atoms and the number of hydrogen atoms combined in a molecule of ammonia.

Most of the ammonia manufactured in industry is used to produce fertilisers.

Which is not true about the Haber process for making ammonia?

What does the graph show about the effect of pressure and temperature on the Haber process?

Higher Only

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?

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

Higher Only

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?

Higher Only

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.

Higher Only

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)

Higher Only

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)

Higher Only

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.

Hydrogen-oxygen fuel cells, rather than chemical cells, can be used to power some vehicles.

Give one advantage of using a hydrogen-oxygen fuel cell, rather than using a chemical cell, to power a vehicle.

Higher Only

Complete the half-equation for the reaction taking place at one of the electrodes in a hydrogen-oxygen fuel cell.

O₂ + .................. H⁺+ .................. → ............... H₂O

Higher Only

Calculate the volume of 48 g of oxygen at room temperature and pressure.

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

Higher Only

The hydrogen used in a hydrogen-oxygen fuel cell can be produced from methanol, CH₃OH.

CH₃OH + H₂O  CO₂ + 3H₂

In this reaction the forward reaction is endothermic and heat energy is taken in from the surroundings.
The conditions used for this reaction are

• a nickel catalyst
• a temperature of 220 °C

Explain, in terms of their effects on the rate of attainment of equilibrium and the equilibrium yield of hydrogen, why the reaction is carried out using a catalyst at 220 °C rather than without a catalyst at a lower temperature.

Higher Only

Chemistry Only

Ammonia is produced by reacting hydrogen and nitrogen in the Haber process.

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

The forward reaction is exothermic.
An iron catalyst is used.

State the effect of the iron catalyst on the:

[1]

[1]

The rate of the reaction increases with temperature.

Explain why.

Higher Only

A temperature of 450°C is used, even though a higher temperature would increase the rate of the reaction.

Explain why this temperature is chosen for the reaction.

Higher Only

A high pressure of 200 atmospheres is used.

Explain why, in terms of the equilibrium yield of ammonia.

Hydrated copper(II) sulfate contains copper(II) sulfate joined with water molecules.

The ratio of water to copper(II) sulfate can vary.

State the formula of the ions in copper(II) sulfate.

Higher Only

22.5g of hydrated copper(II) sulfate (CuSO₄.XH₂O) is heated to form 14.4 g of anhydrous copper(II) sulfate.

Calculate X to deduce the formula of the hydrated copper(II) sulfate.

(relative formula masses:CuSO₄ = 159.5; H₂O = 18)

Heating blue hydrated copper(II) sulfate to form white anhydrous copper(II) sulfate is a reversible reaction.

Devise a way of proving this.

Predict how temperature changes when water is added to anhydrous copper sulfate.

Give a reason for your answer.

The reaction between hydrogen (H₂) and iodine (I₂) to form hydrogen iodide is exothermic.

The balanced symbol equation for this reaction is:

H₂ (g)+I₂ (g) ⇌ 2HI (g)

Draw a dot and cross diagram to show the bonding in a molecule of hydrogen iodide.

Show the electrons on the outer shells only.

Higher Only

At a high temperature, an equilibrium mixture of the gases generates a dark purple color.  H₂ is colourless.

Which gas is purple: I₂ or HI?

Explain your answer.

Higher Only

Changing the pressure has no effect on the equilibrium yield of hydrogen iodide.

State why.

Higher Only

Explain the effect of increasing the concentration of hydrogen in the reaction mixture on the equilibrium yield of hydrogen iodide.