AQA GCSE Chemistry (8462), Higher tier · 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.
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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