Reversible Reactions & Equilibria (Edexcel GCSE Chemistry: Combined Science)

Topic Questions

16 marks

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.

set of conditions	pressure in atm	temperature in °C	catalyst
A	2	680	no catalyst used
B	4	425	catalyst used

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.

(6)

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1a4 marks

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.

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1b2 marks

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.

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2 marks

Higher Only

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

(Avogadro constant = 6.02 × 10²³)

number of atoms = ..............................................................

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☐	A	The energy change of the reverse reaction is endothermic
☐	B	The equilibrium occurs within a closed system
☐	C	The reaction has reached completion and stopped
☐	D	The rate of the forward reaction is equal to the rate of the reverse reaction

☐	A
☐	B
☐	C
☐	D	None of the above

☐	A	ammonia nitrate
☐	B	ammonia nitric
☐	C	ammonium nitrate
☐	D	ammonium nitric

☐	A	Platinum is used as a catalyst
☐	B	The process is carried out at a temperature if 450 ^oC
☐	C	The process takes place at a pressure of about 200 atmospheres
☐	D	Natural gas is a raw material

☐	A	Less ammonia is produced at higher pressures
☐	B	The yield of ammonia decreases with increased temperature
☐	C	The optimum conditions are low pressure and high temperature
☐	D	At 500 atmospheres and 400 ^oC the yield of ammonia is 45%

		Change 1	Change 2
☐	A	Increase pressure	Decrease temperature
☐	B	Decrease pressure	Decrease temperature
☐	C	Increase pressure	Increase temperature
☐	D	Decrease pressure	Increase temperature

		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

☐	A	Increase the surface area to volume ratio of the carbon
☐	B	Increase the pressure
☐	C	Increase the temperature
☐	D	Add a catalyst