AQA GCSE Chemistry (8462), Higher tier · Energy changes › Exothermic and endothermic reactions
Practise Reaction profiles. 20 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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Calculate the overall energy change for this reaction.
−75 kJ/mol
What happens to the overall energy change of the reaction when the catalyst is added?
It stays the same.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) Which statement correctly describes the activation energy of a reaction? Tick (✓) one box.[1]
The overall energy change of the reaction
The minimum energy that particles must have to react when they collide
The energy released when new bonds form
The energy of the products
(b) On the reaction profile for a different reaction, the reactants are at an energy of 240 kJ/mol and the products are at an energy of 165 kJ/mol. Is this reaction exothermic or endothermic? Give a reason for your answer.[2]
(c) Calculate the overall energy change for this reaction.[1]
Show the answer and mark scheme
(a)Answer: The minimum energy that particles must have to react when they collide
(b)Answer: Exothermic: the products have less energy than the reactants.
exothermic
the products are at a lower energy than the reactants / energy is transferred to the surroundings
(c)Answer: −75 kJ/mol
(165 − 240 =) −75 (kJ/mol)
Question 2Medium6 marks
Calcium carbonate decomposes when it is heated strongly. CaCO3(s) → CaO(s) + CO2(g) The reaction is endothermic.
[object Object]
(a) Complete the reaction profile in {fig} for this reaction. Label the products, the activation energy and the overall energy change.[4]
(b) Calcium carbonate must be heated all the time for the reaction to keep going. Explain why.[2]
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(a)Answer: Product line above the reactant line, a curve rising to a peak above both, activation energy from the reactants to the peak, energy change from the reactants up to the products.
product line drawn above the reactant line and labelled CaO + CO2
curve that rises from the reactants to a maximum above the product level and then falls to the products
activation energy shown from the reactant level to the top of the curve
overall energy change shown from the reactant level to the product level
(b)Answer: The reaction is endothermic, so it takes in energy; energy must keep being supplied for particles to have the activation energy.
the reaction is endothermic / takes in energy from the surroundings
so energy must keep being supplied for particles to reach the activation energy / the reaction does not release energy to keep itself going
Question 3Hard6 marks
In a gas cooker, methane and air can be mixed at room temperature without reacting. When a spark is made, the methane starts to burn, and it then keeps burning without any more sparks. CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) The reaction is exothermic.
Explain these observations, using ideas about the reaction profile for the reaction. You may include a sketch of the reaction profile in your answer.[6]
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Answer: Particles react only if they collide with at least the activation energy; at room temperature very few do. The spark gives some particles this energy. The reaction profile is exothermic: the products are lower in energy than the reactants, and the curve rises to a peak (the activation energy) before falling. The energy released is more than the activation energy and heats nearby methane and oxygen, so the reaction keeps itself going.
particles react only if they collide with at least a minimum amount of energy, the activation energy
at room temperature very few methane and oxygen particles have enough energy (the activation energy), so the mixture does not react
the spark transfers energy to some particles, so that they have at least the activation energy, and they react
the reaction profile shows the products at a lower energy than the reactants because the reaction is exothermic; the difference is the energy released (the overall energy change)
the curve rises from the reactants to a peak before falling to the products; the height of the peak above the reactants is the activation energy
the energy released by the reaction is greater than the activation energy, and it is transferred to nearby unreacted methane and oxygen, giving those particles enough energy to react
so once started, the reaction provides its own activation energy and keeps going without more sparks (until the methane or oxygen runs out)
Marked with levels of response: the full level descriptors are in the app.