AQA GCSE Combined Science (8464), Higher tier · Chemistry › The rate and extent of chemical change › Reversible reactions and dynamic equilibrium
Practise Energy changes and reversible reactions. 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.
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.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Recall what exothermic and endothermic meanExothermic reactions transfer energy to the surroundings; endothermic reactions take in energy from the surroundings.
4
Know the reverse has the opposite energy changeIf the forward reaction is exothermic, the reverse reaction is endothermic, and the other way round.
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.
5
Describe the copper sulfate reversible reactionHeating blue hydrated copper sulfate gives white anhydrous copper sulfate and water; adding water reverses it.
6
Link the colour changes to energy changesBlue to white takes in energy (endothermic); white to blue gives out energy (exothermic).
6
Give the energy change for the reverse reactionSame amount, opposite direction: e.g. forward takes in 50 kJ, reverse gives out 50 kJ.
Notes
Energy in both directions
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.
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.
Cheatsheet
Exothermic in one direction ⇌ endothermic in the other direction
The same amount of energy is transferred in each 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 For the amounts shown in the equation, the forward reaction transfers 57 kJ of energy to the surroundings. Give the energy change for the reverse reaction, for the same amounts.
Show the model answer
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 marks5
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.
Show the model answer
(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)
Describe and explain the copper sulfate reaction
3 to 4 marks6
Give the observations: the colour change, and the temperature change when water is added.
Say which direction is endothermic (heating, blue to white) and which is exothermic (adding water, white to blue).
Say that the same amount of energy is transferred in each direction.
Example. A student heated blue hydrated copper sulfate crystals in a test tube until they turned white. After letting the tube cool, the student added a few drops of water to the white solid. Describe what the student would observe when the water was added, and explain the energy changes in both steps.
Show the model answer
The white solid turns blue (1) and gets hot / the temperature rises (1). Heating hydrated copper sulfate is endothermic, because energy is taken in to remove the water (1). Adding water is the reverse reaction, which is exothermic and gives out the same amount of energy (1).
Shortcuts and 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.
Where marks are lost
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.
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.
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 student added water to 0.010 mol of white anhydrous copper sulfate. The solid turned blue and the temperature increased. anhydrous copper sulfate + water ⇌ hydrated copper sulfate The forward reaction releases 78 kJ per mole of anhydrous copper sulfate.
(a) Calculate the energy released in this experiment. Give your answer in joules.[2]
(b) The student then heated the blue solid until it turned white again. How much energy must be taken in by the solid to reverse the reaction completely?[1]
(c) Explain why the blue solid must be heated for the reverse reaction to happen.[2]
(d) In practice, the student had to supply much more energy than the value in your previous answer. Suggest why.[1]
Show the answer and mark scheme
(a)Answer: 780 J
0.010 × 78 = 0.78 (kJ)
780 (J)
(b)Answer: 780 J
780 (J) / the same amount of energy
(c)Answer: The reverse reaction is endothermic, so energy must be supplied.
the reverse reaction (blue to white) is endothermic
so energy must be supplied / taken in from the surroundings
(d)Answer: Energy is also transferred to the surroundings and the test tube.
energy is also transferred to the surroundings / used to heat the test tube / used to evaporate the water
Question 3Hard7 marks
Some energy storage systems use reversible reactions. calcium hydroxide ⇌ calcium oxide + water Ca(OH)2(s) ⇌ CaO(s) + H2O(g) The forward reaction is endothermic. It takes in 109 kJ for each mole of calcium hydroxide that decomposes.
(a) Explain how this reaction could be used to store energy from the Sun in summer and release it in winter.[3]
(b) Calculate the energy released when 1.00 kg of calcium oxide reacts completely with water. Relative formula mass (Mr): CaO = 56[3]
(c) Suggest why the calcium oxide must be kept dry until the energy is needed.[1]
Show the answer and mark scheme
(a)Answer: In summer, sunlight heats the calcium hydroxide and the endothermic forward reaction takes in energy. The products are stored separately. In winter, water is added to the calcium oxide and the exothermic reverse reaction releases the same energy.
in summer, energy from the Sun heats the calcium hydroxide and the (endothermic) forward reaction takes in energy
the calcium oxide and water are stored separately
in winter, water is added to the calcium oxide and the (exothermic) reverse reaction releases the energy (109 kJ per mole)
(b)Answer: 1950 kJ (1.95 MJ) kJ
1000 ÷ 56 = 17.9 (mol)
17.9 × 109
1950 (kJ)
(c)Answer: Otherwise it would react with water and release the energy too early.
it would react with water (vapour) and release the energy too early