AQA GCSE Chemistry (8462), Higher tier · The rate and extent of chemical change › Reversible reactions and dynamic equilibrium
Practise Effect of changing concentration. 12 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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What happens to the amount of ethyl ethanoate at equilibrium if more ethanol is added?
It increases.
Predict the effect on the amount of C at equilibrium if extra B is added.
It increases.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Ethanol reacts with ethanoic acid to form ethyl ethanoate and water. The reaction is reversible and reaches equilibrium. ethanol + ethanoic acid ⇌ ethyl ethanoate + water
(a) What happens to the amount of ethyl ethanoate at equilibrium if more ethanol is added?[1]
(b) What happens to the amount of ethyl ethanoate at equilibrium if water is removed from the mixture as it forms?[1]
(c) What happens to the amount of ethyl ethanoate at equilibrium if more water is added?[1]
(d) Complete the sentence. If the concentration of a reactant is increased, more ________ will be formed until equilibrium is reached again.[1]
Show the answer and mark scheme
(a)Answer: It increases.
it increases
(b)Answer: It increases.
it increases
(c)Answer: It decreases.
it decreases
(d)Answer: products
products
Question 2Medium6 marks
In aqueous solution, chromate ions and dichromate ions are in equilibrium: 2CrO42−(aq) + 2H+(aq) ⇌ Cr2O72−(aq) + H2O(l) Chromate ions, CrO42−, are yellow. Dichromate ions, Cr2O72−, are orange.
(a) A few drops of dilute acid are added to a yellow solution containing chromate ions. Predict and explain the colour change.[3]
(b) Sodium hydroxide solution is then added. Hydroxide ions react with hydrogen ions to form water. Predict and explain the colour change.[2]
(c) Explain why changing the pressure has no effect on the position of this equilibrium.[1]
Show the answer and mark scheme
(a)Answer: It turns orange: the concentration of H+ increases, so the equilibrium shifts to the right, forming more dichromate ions.
the solution turns orange
the concentration of H+ ions increases
so the equilibrium shifts to the right (to reduce the H+ concentration), forming more dichromate ions
(b)Answer: It turns yellow again: H+ ions are removed, so the equilibrium shifts to the left, forming more chromate ions.
the solution turns yellow (again)
H+ ions are removed, so the equilibrium shifts to the left to replace them, forming more chromate ions
(c)Answer: There are no gases involved.
there are no gases in the reaction
Question 3Hard7 marks
Iron(III) ions react with thiocyanate ions to form an ion that is blood-red, in a reversible reaction: Fe3+(aq) + SCN−(aq) ⇌ FeSCN2+(aq) Solutions of Fe3+ ions are pale yellow and solutions of SCN− ions are colourless. A student made an equilibrium mixture that was orange-red.
(a) The student added a few drops of concentrated iron(III) chloride solution to the mixture. Predict and explain what the student would see.[3]
(b) The student then added sodium hydroxide solution. Hydroxide ions react with Fe3+ ions to form a precipitate of iron(III) hydroxide. Predict and explain what the student would see.[3]
(c) After the iron(III) chloride was added and equilibrium was reached again, the concentration of SCN− ions was lower than before. Explain why.[1]
Show the answer and mark scheme
(a)Answer: The mixture becomes a deeper red: the Fe3+ concentration increases, so the equilibrium shifts to the right and more FeSCN2+ forms.
the mixture becomes a darker red
the concentration of Fe3+ ions increases, so the equilibrium shifts to the right (to reduce it)
more FeSCN2+ ions form
(b)Answer: A brown precipitate forms and the red colour becomes paler, because Fe3+ ions are removed and the equilibrium shifts to the left.
a brown precipitate forms
the red colour becomes paler
Fe3+ ions are removed so the equilibrium shifts to the left / FeSCN2+ ions break down to replace them
(c)Answer: SCN− ions reacted with the extra Fe3+ ions as the equilibrium shifted to the right.
SCN− ions reacted with the extra Fe3+ ions (when the equilibrium shifted to the right)