AQA GCSE Chemistry (8462), Higher tier · The rate and extent of chemical change › Reversible reactions and dynamic equilibrium
Practise Effect of pressure changes. 13 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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How many molecules of gas are shown on the left-hand side of the equation, and how many on the right-hand side?
3 on the left and 2 on the right
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy3 marks
Sulfur dioxide reacts with oxygen in a reversible reaction to form sulfur trioxide: 2SO2(g) + O2(g) ⇌ 2SO3(g)
(a) How many molecules of gas are shown on the left-hand side of the equation, and how many on the right-hand side?[1]
(b) The pressure is increased. What happens to the amount of sulfur trioxide at equilibrium? Tick (✓) one box.[1]
It decreases.
It increases.
It stays the same.
(c) Complete the sentence. Increasing the pressure shifts the position of equilibrium towards the side of the equation with .................... molecules of gas.[1]
Show the answer and mark scheme
(a)Answer: 3 on the left and 2 on the right
3 on the left and 2 on the right
(b)Answer: It increases.
(c)Answer: fewer
fewer / a smaller number of
Question 2Medium2 marks
(a) For which of these reactions at equilibrium does a change in pressure have no effect on the position of equilibrium? Tick (✓) one box.[1]
2SO2(g) + O2(g) ⇌ 2SO3(g)
N2(g) + 3H2(g) ⇌ 2NH3(g)
H2(g) + I2(g) ⇌ 2HI(g)
N2O4(g) ⇌ 2NO2(g)
(b) Explain your answer.[1]
Show the answer and mark scheme
(a)Answer: H2(g) + I2(g) ⇌ 2HI(g)
(b)Answer: This reaction has the same number of gas molecules (2) on each side of the equation.
it has the same number of gas molecules on each side of the equation (2 and 2)
Question 3Hard7 marks
Ethanol can be made by reacting ethene with steam: C2H4(g) + H2O(g) ⇌ C2H5OH(g) The reaction is carried out at 300 °C and a pressure of 60 to 70 atmospheres, using a catalyst.
(a) Explain the effect of increasing the pressure on the yield of ethanol.[3]
(b) Increasing the pressure also increases the rate of the reaction. Explain why.[2]
(c) At pressures above about 70 atmospheres, some of the ethene polymerises to form poly(ethene). Give two reasons why a pressure much higher than 70 atmospheres is not used.[2]
Show the answer and mark scheme
(a)Answer: There are 2 molecules of gas on the left and 1 on the right; a higher pressure shifts the equilibrium towards fewer molecules, so the yield of ethanol increases.
there are 2 molecules (of gas) on the left and 1 on the right
increasing the pressure shifts the equilibrium towards the side with fewer molecules
so the yield of ethanol increases
(b)Answer: There are more gas particles in the same volume, so collisions are more frequent.
there are more gas particles in the same volume
so collisions are more frequent
(c)Answer: Some ethene would be wasted by forming poly(ethene); higher pressures are more expensive and more dangerous.
some ethene would be wasted by forming an unwanted product (poly(ethene))
high pressures are expensive (stronger equipment / more energy for compressors)