5.6.2.5The effect of changing concentration (HT only)
AQA GCSE Combined Science (8464), Higher tier · Chemistry › The rate and extent of chemical change › Reversible reactions and dynamic equilibrium
Practise The effect of changing concentration (HT only). 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.
Higher tier only. Changing the concentration of a reactant or product means the system is no longer at equilibrium, and the concentrations of all the substances change until a new equilibrium is reached. You need to predict the effect, often using colour changes or data given in the question.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
5
Predict the effect of adding more reactantMore products are formed until equilibrium is reached again.
6
Predict the effect of removing a productMore reactants react until equilibrium is reached again, so more product forms.
6
Predict the effect of adding more productThe position shifts to the left, so more reactants form.
7
Explain shifts using Le Chatelier's principleThe system counteracts the change by using up an added substance or replacing a removed one.
8
Interpret colour changes and given dataUse the colours or concentrations in the question to decide which way the position moved.
8
Describe all concentration changes after a disturbanceAn added substance falls a little but stays higher than before; the other concentrations change until constant again.
Notes
The rules
If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium.
The concentrations of all the substances then change until equilibrium is reached again.
Increase the concentration of a reactant: more products are formed until equilibrium is reached again (the position shifts right).
Decrease the concentration of a product (for example, by removing it): more reactants react until equilibrium is reached again (the position shifts right).
Increase the concentration of a product, or decrease the concentration of a reactant: the position shifts left, so more reactants form.
Why it happens
Adding a substance: the system counteracts the change by using some of it up.
Removing a substance: the system counteracts the change by making more of it.
Removing a product as it forms keeps the forward reaction going, so more of the reactants are turned into products.
After extra reactant is added, its concentration falls a little as it reacts, but at the new equilibrium it is still higher than before the change. grade 8+
Example: a colour change
Yellow chromate ions and orange dichromate ions are in equilibrium in solution: 2CrO42−(aq) + 2H+(aq) ⇌ Cr2O72−(aq) + H2O(l)
Adding acid increases the concentration of H+ ions, so the position shifts right and the solution turns orange.
Adding alkali removes H+ ions (they react to form water), so the position shifts left and the solution turns yellow.
You do not need to learn this example: in the exam you are given the equation and the colours.
Cheatsheet
Add more reactant → more product forms (shift right)
Remove product → more reactants react (shift right)
Add more product → more reactants form (shift left)
Remove reactant → shift left
All the concentrations change until equilibrium is reached again
The system uses up what you add and replaces what you remove
How to answer each type of question
Predict the effect of changing a concentration
1 to 2 marks5
Find the substance whose concentration changes, and which side of the equation it is on.
Added: the position shifts away from it. Removed: the position shifts towards it.
State what happens to the substance the question asks about.
Example. Hydrogen and iodine react in a sealed container and reach equilibrium. H2(g) + I2(g) ⇌ 2HI(g) More iodine is added to the container. Predict what happens to the amount of hydrogen iodide. Explain your answer.
Show the model answer
The amount of hydrogen iodide increases (1). The position of equilibrium shifts to the right to use up some of the added iodine (1).
Explain why removing a product gives more product
2 marks6
Say that removing the product lowers its concentration.
Say the position shifts to the right to replace it, so more reactant reacts.
Example. Calcium carbonate decomposes when it is heated in a reversible reaction. CaCO3(s) ⇌ CaO(s) + CO2(g) In a lime kiln, a flow of air carries the carbon dioxide away. Explain why removing the carbon dioxide means more calcium oxide is produced.
Show the model answer
Removing the carbon dioxide lowers its concentration (1), so the position of equilibrium shifts to the right to replace it and more calcium carbonate decomposes (1).
Predict and explain a colour change from given information
3 marks7
Identify which substance's concentration changes, and whether it goes up or down.
Say which way the position shifts to counteract this.
Link to the colour of the substance that increases.
Example. In a solution, a pink substance and chloride ions are in equilibrium with a blue substance: pink substance + chloride ions ⇌ blue substance + water A student adds concentrated hydrochloric acid, which contains chloride ions, to the pink equilibrium mixture. Predict and explain the colour change.
Show the model answer
The solution turns (more) blue (1). The concentration of chloride ions increases (1), so the position of equilibrium shifts to the right to use up chloride ions, forming more of the blue substance (1).
Describe how all the concentrations change after a disturbance
3 to 4 marks8
The added substance: a sudden rise, then a small fall as it reacts, levelling off higher than before.
The other reactant: falls, then levels off lower than before.
The product: rises, then levels off higher than before.
Example. A and B react to form C in a sealed container. A(g) + B(g) ⇌ C(g) The mixture is at equilibrium. At 10 minutes, extra B is added. Describe how the concentrations of A, B and C change after 10 minutes until equilibrium is reached again.
Show the model answer
The concentration of B rises suddenly, then decreases (1) but levels off higher than it was before 10 minutes (1). The concentration of A decreases and levels off lower than before (1). The concentration of C increases and levels off higher than before (1).
Shortcuts and memory tricks
You add it, it uses it; you take it, it makes it.
Adding a reactant and removing a product both push the position to the right, towards more product.
Colour questions: find the colour of the side the position shifts towards.
Where marks are lost
Saying the added substance ends up back at its original concentration. It falls a little but stays higher than before.
Saying only the substance you changed has a different concentration. All of them change.
Getting the direction backwards: adding a reactant shifts the position right, not left.
Describing a colour change without linking it to the substance in the equation that has that colour.
Exam technique
Say what happens to the concentration first (e.g. 'the concentration of H+ ions increases'), then the shift, then the result.
'Predict' questions only need the outcome; 'explain' questions also need the reason (the system counteracts the change).
Look for hidden concentration changes: adding an alkali removes H+ ions, and a substance that reacts with a product removes that product.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
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)