The effect of changing concentration (HT only) - AQA GCSE Chemistry Exam Questions

Practise 21 exam-style questions on The effect of changing concentration (HT only) for AQA GCSE Chemistry. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.

21 questions
1–6 marks each
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20 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

Iron(III) ions react with thiocyanate ions in a reversible reaction:
Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq)
Fe³⁺ ions are pale yellow, SCN⁻ ions are colourless and FeSCN²⁺ ions are blood-red.

A few drops of iron(III) chloride solution are added to a red equilibrium mixture. Predict the colour change and explain your answer.

Model answer

The solution turns a darker red. Adding iron(III) chloride increases the concentration of Fe³⁺ ions, which are a reactant, so the equilibrium shifts to the right and more red FeSCN²⁺ ions are formed.

Explanation

Increasing the concentration of a reactant makes more product until equilibrium is reached again. The colour of FeSCN²⁺ makes this shift easy to see.

Mark scheme

1 mark
the solution becomes a darker red
allow more red
1 mark
the concentration of Fe³⁺ (a reactant) increases, so the equilibrium shifts to the right and more FeSCN²⁺ forms

Question 2

2 marks

The shells of many sea creatures are made of calcium carbonate. This equilibrium is set up in seawater:
Ca²⁺(aq) + 2HCO₃⁻(aq) ⇌ CaCO₃(s) + CO₂(aq) + H₂O(l)

More carbon dioxide from the atmosphere is dissolving in the oceans. Explain why this makes it harder for sea creatures to form their shells.

Model answer

Dissolving more carbon dioxide increases the concentration of a product. The equilibrium shifts to the left to use up some of the carbon dioxide, so less calcium carbonate forms and some calcium carbonate dissolves. This makes it harder for sea creatures to build their shells.

Explanation

Increasing the concentration of a product makes the reverse reaction happen more until equilibrium is reached again. Here the reverse reaction uses up calcium carbonate, which is the material of the shells.

Mark scheme

1 mark
the concentration of carbon dioxide (a product) increases
1 mark
so the equilibrium shifts to the left, so less calcium carbonate forms / calcium carbonate dissolves

Question 3

3 marks

Carbon monoxide is toxic because it combines with haemoglobin in the blood. This equilibrium is set up in the blood:
HbO₂ + CO ⇌ HbCO + O₂
HbO₂ carries oxygen around the body. HbCO cannot carry oxygen.

A person with carbon monoxide poisoning is treated by making them breathe pure oxygen. Explain how this treatment works.

Model answer

Breathing pure oxygen increases the concentration of oxygen, which is a product of this reaction. The equilibrium shifts to the left to use up some of the oxygen. HbCO reacts with oxygen to re-form HbO₂ and release carbon monoxide, so the haemoglobin can carry oxygen around the body again.

Explanation

Increasing the concentration of a product shifts the equilibrium towards the reactants. Doctors use this idea to reverse carbon monoxide poisoning; the released carbon monoxide is then breathed out.

Mark scheme

1 mark
breathing pure oxygen increases the concentration of oxygen, which is a product
1 mark
so the equilibrium shifts to the left
1 mark
HbCO is converted back into HbO₂ / carbon monoxide is released, so the haemoglobin can carry oxygen again

Question 4

4 marks

Hydrogen is manufactured using this reaction:
CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g)
The manufacturer uses much more steam than is needed to react with all of the carbon monoxide.

Explain why using excess steam increases the yield of hydrogen and reduces the amount of carbon monoxide left in the product.

Suggest one disadvantage of using a large excess of steam.

Model answer

Using excess steam increases the concentration of one of the reactants. The equilibrium shifts to the right to use up some of the steam, so more of the carbon monoxide reacts. This forms more hydrogen and leaves less carbon monoxide in the product.
A disadvantage is that a lot of energy is needed to boil the water and heat the extra steam, which costs money.

Explanation

Adding an excess of one reactant converts more of the other reactant into products. In industry this has to be balanced against the extra cost of the excess reactant and of separating it afterwards.

Mark scheme

1 mark
excess steam increases the concentration of a reactant
1 mark
so the equilibrium shifts to the right
1 mark
more carbon monoxide reacts, so less carbon monoxide remains and more hydrogen forms
1 mark
disadvantage: energy / fuel (cost) is needed to produce and heat the extra steam
allow the excess steam has to be separated from the product / larger equipment needed

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