AQA GCSE Combined Science (8464), Higher tier · Chemistry › 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.
A reaction profile is a simple energy diagram showing how energy changes as reactants turn into products. You need to draw and label profiles for exothermic and endothermic reactions, use them to decide which type a reaction is, and explain what the activation energy is.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
State what activation energy isThe minimum amount of energy that particles must have to react when they collide.
4
Label the parts of a reaction profileIdentify the reactants, products, activation energy and overall energy change on a given profile.
4
Identify exothermic or endothermic from a profileProducts lower than reactants means exothermic; products higher than reactants means endothermic.
5
Read energy values from a profileActivation energy = peak − reactants; overall energy change = products − reactants.
6
Draw a reaction profileShow the reactants and products at the correct relative heights, a curve with a peak above both, and labelled arrows.
7
Explain why some reactions need heating to startParticles must collide with at least the activation energy, so energy (e.g. a spark or flame) must be supplied at first.
8
Explain why exothermic reactions keep going once startedThe energy released gives more particles at least the activation energy, so the reaction continues without further heating.
Notes
Activation energy
Chemical reactions can only happen when reacting particles collide with enough energy.
The activation energy is the minimum amount of energy that particles must have to react.
Reactions with a high activation energy need energy to be supplied before they start, e.g. a spark or a flame to light a fuel.
Once an exothermic reaction starts, the energy it releases gives more particles the activation energy, so the reaction keeps going. This is why a fuel keeps burning after the match is taken away. grade 8+
Drawing a reaction profile
Label the axes: energy (vertical) and progress of reaction (horizontal).
Draw a short horizontal line for the reactants on the left and one for the products on the right, at the correct relative heights. Label each line with the names or formulae.
Join the two lines with a smooth curve that rises to a peak above both, then falls to the products.
Activation energy: an arrow from the reactants line up to the top of the curve.
Overall energy change: an arrow from the reactants line to the products line (down for exothermic, up for endothermic).
Exothermic or endothermic?
Exothermic: the products are lower in energy than the reactants. The difference is the energy released to the surroundings.
Endothermic: the products are higher in energy than the reactants. The difference is the energy taken in from the surroundings.
Both types have a peak: every reaction has an activation energy, even an exothermic one.
In an endothermic reaction the peak must be above the products, so the activation energy is always bigger than the overall energy change. grade 7+
Cheatsheet
Activation energy = minimum energy particles must have to react (when they collide)
Axes: energy (vertical), progress of reaction (horizontal)
Exothermic profile: products below reactants
Endothermic profile: products above reactants
Activation energy = energy at the peak − energy of the reactants
Overall energy change = energy of products − energy of reactants (negative for exothermic)
Both arrows start at the reactants line
How to answer each type of question
Identify the parts of a given profile
1 to 4 marks4
Find the reactants (left line) and the products (right line).
The arrow from the reactants up to the peak is the activation energy.
The arrow between the reactants line and the products line is the overall energy change.
Products lower means exothermic; products higher means endothermic.
Example. A reaction profile has three labelled arrows. Arrow A goes from the reactants line up to the top of the curve. Arrow B goes from the reactants line down to the products line. Arrow C goes from the products line up to the top of the curve. (a) Which arrow shows the activation energy? (1) (b) Which arrow shows the overall energy change? (1) (c) Is the reaction exothermic or endothermic? Give a reason for your answer. (2)
Show the model answer
(a) A (1) (b) B (1) (c) Exothermic (1) because the products have less energy than the reactants / arrow B points down (1)
Calculate values from a reaction profile
2 to 3 marks5
Read the energy of the reactants, the peak and the products from the axis.
Activation energy = peak − reactants.
Overall energy change = products − reactants. A negative answer means energy is released (exothermic).
Example. On a reaction profile, the reactants are at 150 kJ/mol, the top of the curve is at 240 kJ/mol and the products are at 40 kJ/mol. (a) Calculate the activation energy. (1) (b) Calculate the overall energy change and state whether the reaction is exothermic or endothermic. (2)
Show the model answer
(a) 240 − 150 = 90 kJ/mol (1) (b) 40 − 150 = −110 kJ/mol (1); exothermic, because the products have less energy than the reactants (1)
Draw a reaction profile
3 to 4 marks6
Draw and label the axes: energy (vertical) and progress of reaction (horizontal).
Put the reactants and products lines at the correct relative heights and label them.
Draw a smooth curve with a peak higher than both lines.
Add labelled arrows for the activation energy and the overall energy change, both starting at the reactants line.
Example. The thermal decomposition of calcium carbonate is endothermic. CaCO3 → CaO + CO2 Draw a reaction profile for this reaction. Label the reactants, the products, the activation energy and the overall energy change. (4)
Show the model answer
Products line (CaO + CO2) drawn higher than the reactants line (CaCO3), both labelled (1). A curve rising from the reactants to a peak above the products line, then falling to the products (1). Activation energy: arrow from the reactants line to the peak (1). Overall energy change: arrow from the reactants line up to the products line (1).
Explain the role of activation energy
2 to 3 marks7
Define activation energy: the minimum energy particles need to react.
Link it to collisions: particles must collide with at least this much energy.
Say what supplying energy (a spark, flame or heating) does at the start.
For a reaction that keeps going, say that the energy released provides the activation energy for more particles.
Example. A mixture of methane and air does not react at room temperature, but once it is lit with a spark the methane keeps burning. Explain why. (3)
Show the model answer
Particles must collide with at least the activation energy to react; at room temperature the particles do not have enough energy (1). The spark supplies the activation energy, so some particles react (1). Combustion is exothermic, so the energy released gives more particles enough energy to react and the reaction keeps going (1).
Shortcuts and memory tricks
Exothermic profiles go downhill overall (energy exits); endothermic profiles go uphill overall.
Every arrow starts at the reactants line.
The hump is the activation energy; the step between the two lines is the overall energy change.
Sense check an endothermic profile: the peak must be higher than the products line, so the activation energy arrow is always longer than the energy change arrow.
Where marks are lost
Drawing the activation energy arrow from the products line or from the axis, instead of from the reactants line up to the peak.
Drawing the overall energy change arrow from the peak down to the products. It goes from the reactants line to the products line.
Leaving out the hump on an exothermic profile. Every reaction has an activation energy.
Drawing the peak lower than the products line on an endothermic profile.
Writing that activation energy is 'the energy given out' or 'the energy of the products'. It is the minimum energy the particles need to react.
Exam technique
Use a ruler for the arrows and make them start and finish exactly on the levels they measure.
Label with the exact words the question uses: reactants, products, activation energy and overall energy change.
If a question adds a catalyst, draw a second, lower peak between the same reactants and products lines: a catalyst lowers the activation energy but does not change the overall energy change (see the rates of reaction notes).
In 'explain' questions about activation energy, mention both collisions and energy: particles must collide with at least the activation energy to react.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
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]
Show the answer and mark scheme
(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]
Show the answer and mark scheme
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.