AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › The rate and extent of chemical change › Reversible reactions and dynamic equilibrium
Practise Energy changes and reversible reactions. 4 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
If a reversible reaction is exothermic in one direction, it is endothermic in the other, and the same amount of energy is transferred each way. Questions often use the hydrated copper sulfate example and ask you to state the energy change for the reverse reaction.
Key facts
Exothermic in one direction ⇌ endothermic in the other direction
The same amount of energy is transferred in each direction
Recall: an exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings rises. An endothermic reaction takes in energy from the surroundings, so the temperature falls.
If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction.
The same amount of energy is transferred in each direction.
Example: if the forward reaction takes in 30 kJ, then for the same amounts of substances the reverse reaction gives out 30 kJ.
'Hydrated' means containing water; 'anhydrous' means without water.
Forward reaction (endothermic): heating blue hydrated copper sulfate drives off the water, leaving white anhydrous copper sulfate. Energy must be supplied.
Reverse reaction (exothermic): adding water to white anhydrous copper sulfate turns it blue again, and the mixture gets hot.
The energy taken in to remove the water is the same as the energy given out when the water is added back.
The same amount of energy is taken in one way and given out the other way.
Example: ammonium chloride
Decomposing ammonium chloride into ammonia and hydrogen chloride is endothermic: it only happens while you keep heating.
So the reverse reaction, where ammonia and hydrogen chloride form ammonium chloride, is exothermic.
How to answer each type of question
State the energy change for the reverse reaction
2 marksGrade 5
Find the energy change given for one direction.
Swap it: exothermic becomes endothermic, and endothermic becomes exothermic.
Keep the same amount of energy, and say whether it is taken in or given out.
Example. Nitrogen dioxide forms dinitrogen tetroxide in a reversible reaction. 2NO2 ⇌ N2O4 The forward reaction transfers 57 kJ of energy to the surroundings. Give the energy change for the reverse reaction.
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57 kJ (1) is taken in from the surroundings / the reverse reaction is endothermic (1).
Deduce the energy changes from information about a reaction
3 marksGrade 5
If energy must be supplied (heating) to make a direction happen, that direction is endothermic.
The opposite direction is then exothermic.
It transfers the same amount of energy.
Example. When solid X is heated, it decomposes into gases Y and Z. When the gases cool, they react to form X again. X ⇌ Y + Z (a) Is the forward reaction exothermic or endothermic? Give a reason. (b) Describe the energy change of the reverse reaction.
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(a) Endothermic (1), because energy (heat) has to be supplied to decompose X (1) (b) It is exothermic and transfers the same amount of energy to the surroundings (1)
Don’t lose marks
Saying the reverse reaction transfers a different amount of energy. It is exactly the same amount.
Mixing up the colours: hydrated copper sulfate is blue, anhydrous copper sulfate is white.
Saying both directions are exothermic because 'heat is involved' in both.
Describing only the colour change when asked what you would observe on adding water. Also say it gets hot.
More tips
Memory tricks
Flip the reaction, flip the energy change: exo becomes endo, and the amount stays the same.
Blue to white needs heat; white to blue gives out heat.
If you have to keep heating to make it go, that direction is endothermic.
Exam technique
Use the words 'exothermic' and 'endothermic' and say which direction each one applies to.
When giving the energy change for the reverse reaction, state both the amount and whether energy is taken in or given out.
Use 'hydrated' and 'anhydrous' correctly: examiners expect the right names with the right colours.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Recall what exothermic and endothermic meanExothermic reactions transfer energy to the surroundings; endothermic reactions take in energy from the surroundings.
Grade 4
Know the reverse has the opposite energy changeIf the forward reaction is exothermic, the reverse reaction is endothermic, and the other way round.
Grade 5
State that equal energy is transferred each wayThe energy taken in one way is exactly the same as the energy given out the other way.
Grade 5
Describe the copper sulfate reversible reactionHeating blue hydrated copper sulfate gives white anhydrous copper sulfate and water; adding water reverses it.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
When blue hydrated copper sulfate is heated, it forms white anhydrous copper sulfate and water. hydrated copper sulfate ⇌ anhydrous copper sulfate + water The forward reaction is endothermic. What type of reaction is the reverse reaction?
Exothermic
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
When blue hydrated copper sulfate is heated, it forms white anhydrous copper sulfate and water. hydrated copper sulfate ⇌ anhydrous copper sulfate + water The forward reaction is endothermic.
(a) What type of reaction is the reverse reaction?[1]
(b) Give two observations when water is added to white anhydrous copper sulfate.[2]
(c) When 1 mole of hydrated copper sulfate is heated, 78 kJ of energy is taken in. How much energy is released when 1 mole of anhydrous copper sulfate reacts with water?[1]
Show the answer and mark scheme
(a)Answer: Exothermic
exothermic
(b)Answer: The solid turns blue; it gets hot.
the solid turns blue
it gets hot / the temperature increases
(c)Answer: 78 kJ
78 (kJ)
Question 2Medium6 marks
A teacher heated some white solid ammonium chloride at the bottom of a long test tube, loosely plugged with mineral wool. NH4Cl(s) ⇌ NH3(g) + HCl(g) The forward reaction is endothermic. After a few minutes the solid at the bottom of the tube had disappeared, and a white solid had formed on the cooler glass near the top of the tube.
(a) Explain why the ammonium chloride had to be heated for it to decompose.[2]
(b) Explain why a white solid formed near the top of the tube.[2]
(c) Compare the energy transferred when a sample of ammonium chloride decomposes with the energy transferred when the same mass of ammonium chloride re-forms.[2]
Show the answer and mark scheme
(a)Answer: The forward reaction is endothermic, so it takes in energy; the energy has to be supplied by heating.
the forward (decomposition) reaction is endothermic / takes in energy
so energy must be supplied (continuously) by heating for it to happen
(b)Answer: The ammonia and hydrogen chloride gases cooled near the top of the tube, where the exothermic reverse reaction happened, re-forming solid ammonium chloride.
the ammonia and hydrogen chloride gases rise and cool near the top of the tube
at the lower temperature the reverse reaction happens (it is exothermic), re-forming (white solid) ammonium chloride
(c)Answer: The same amount of energy is transferred: it is taken in when the ammonium chloride decomposes and released when it re-forms.
the same amount of energy is transferred in each direction
energy is taken in by the decomposition but released when the ammonium chloride re-forms