Every chemical reaction involves energy. Some reactions give out heat, like a campfire. Others take in heat and feel cold, like an instant cold pack.
This matters because we use these reactions every day. They warm our hands, cool sports injuries and heat our homes. Chemists also need to predict how much energy a reaction will transfer.
By the end you will be able to name the two types of reaction and sketch a reaction profile. You will explain energy changes using bonds. If you are on the Higher tier, you will also calculate an energy change from bond energies.
Exothermic or endothermic?
Energy cannot be made or destroyed. It can only be moved from one place to another. In a reaction, energy moves between the chemicals and the surroundings. The surroundings are everything around the reaction, such as the air and the container.
An exothermic reaction transfers energy to the surroundings, so the temperature goes up. Think of a hot water bottle giving out heat. Combustion and neutralisation are exothermic.
An endothermic reaction takes in energy from the surroundings, so the temperature goes down. Thermal decomposition, which means heat breaking a compound down, is endothermic. So is photosynthesis.
Where you use them
Exothermic reactions are useful when you want heat. Hand warmers and self-heating cans use them. Some hand warmers use iron reacting with oxygen in the air. That reaction is called oxidation.
Endothermic reactions are useful when you want cold. An instant cold pack for a sports injury uses one. You squeeze the pack and it quickly gets cold.
To measure an energy change, react the chemicals in a polystyrene cup with a lid. Polystyrene is a good insulator, so less energy escapes. It is like putting a lid on a hot drink. Record the temperature before and after.
Reading a reaction profile
A reaction profile is a graph of the energy of the chemicals during a reaction. The horizontal axis shows the progress of the reaction. The vertical axis shows energy. The reactants start on the left and the products finish on the right.
In an exothermic reaction, the products end lower than the reactants. In an endothermic reaction, they end higher. The gap between the two levels is the overall energy change. Think of stairs: going down ends lower, going up ends higher.
On the way, the curve rises to a peak. The height of the peak above the reactants is the activation energy.
Bars showing the energy of reactants at 60 and of products at 20 in an exothermic reaction, so the products have less energy.
Activation energy and catalysts
Activation energy is the minimum energy that particles need to react. Even exothermic reactions need a push to start. A match must be struck before a candle will burn. Think of pushing a ball over a hill before it can roll down the other side.
To draw a profile, start with a flat line for the reactants. Draw a curve up to a peak and down to the products. Label the peak height as the activation energy. Label the gap between the two flat lines as the overall energy change.
A catalyst lowers the activation energy, so the peak is lower. It does not change the energy of the reactants or products.
Breaking and making bonds
A bond is the link that holds atoms together. Breaking a bond takes in energy, so it is endothermic. Making a bond gives out energy, so it is exothermic. Think of two magnets: pulling them apart takes effort, and letting them snap together releases energy.
In a reaction, old bonds in the reactants break and new bonds form in the products. Both things happen at the same time.
If more energy is released making bonds than is taken in breaking them, the reaction is exothermic. If it is the other way round, the reaction is endothermic.
Calculating an energy change
Higher tier. Bond energy is the energy needed to break a bond. It is measured in kJ/mol. The energy change is the energy to break bonds minus the energy released making bonds.
Take H₂ + Cl₂ → 2HCl. The bond energies are H–H 436, Cl–Cl 243 and H–Cl 432. Bonds broken: 436 + 243 = 679. Bonds made: 2 × 432 = 864. The energy change is 679 − 864 = −185 kJ/mol.
A negative answer means the reaction is exothermic. Count every bond in every molecule. Methane, CH₄, has four C–H bonds, not one.
Recap
Exothermic reactions give energy to the surroundings. Endothermic reactions take it in.
On a profile, the peak is the activation energy and the gap between the levels is the energy change.
Breaking bonds takes in energy. Making bonds releases it.
Higher tier: energy change = bonds broken − bonds made. A negative answer means exothermic.
Key points
Exothermic: energy to surroundings, temperature rises. Endothermic: energy from surroundings, temperature falls.
Reaction profile: the peak is the activation energy and the gap is the overall energy change.
Breaking bonds is endothermic. Making bonds is exothermic.
Energy change = energy to break bonds − energy released making bonds.
Worked examples
Example 1
Name the type of reaction that makes the temperature of the surroundings rise.
The temperature rises when energy is transferred to the surroundings.
That is what exothermic means.
The answer is exothermic.
Answer: exothermic
Common slips
Slip: The answer is endothermic. Endothermic reactions take in energy, so the temperature falls.
Slip: The answer is neutral. This is not a type of energy change.
Slip: The temperature rises because energy is destroyed. Energy is never destroyed. It is transferred.
Example 2
Draw or describe a reaction profile for an exothermic reaction. Label the activation energy.
Start with the reactants at a higher energy than the products.
Draw a peak above the reactants. The height from the reactants to the peak is the activation energy.
The curve finishes at the products, lower than the reactants. The gap is the energy released.
Answer: Reactants high, a peak above them for the activation energy, and products lower for the energy released.
Common slips
Slip: The products are above the reactants. In an exothermic reaction, energy is released, so the products are lower.
Slip: The activation energy is the gap between reactants and products. That gap is the overall energy change. The activation energy is the height of the peak.
Slip: There is no peak. Every reaction needs activation energy to break the first bonds.
Example 3
Calculate the energy change for N₂ + 3H₂ → 2NH₃. Bond energies in kJ/mol: N≡N 945, H–H 436, N–H 391.
Bonds broken: 945 + 3 × 436 = 945 + 1308 = 2253.
Bonds made: 2 NH₃ has 6 N–H bonds, so 6 × 391 = 2346.
Energy change = 2253 − 2346 = −93 kJ/mol.
Answer: −93 kJ/mol
Common slips
Slip: Bonds broken: 945 + 436. There are 3 H₂ molecules, so 3 × 436.
Slip: Bonds made: 2 × 391. Each NH₃ has three N–H bonds, so 2 molecules have 6.
Slip: Energy change = 2346 − 2253 = +93 kJ/mol. Energy change is bonds broken minus bonds made, which is −93.
Questions and answers
Exothermic and endothermic reactions practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
Question 1 Easy
What happens to the temperature in an endothermic reaction?
Show answer
Answer: it falls
Energy is taken in from the surroundings.
So the temperature falls.
Question 2 Easy
Is combustion exothermic or endothermic?
Show answer
Answer: exothermic
Burning releases heat.
So it is exothermic.
Question 3 Easy
Is thermal decomposition exothermic or endothermic?
Show answer
Answer: endothermic
Heat must be supplied.
So it is endothermic.
Question 4 Easy
What name is given to the minimum energy needed for a reaction to start?
Show answer
Answer: activation energy
The particles must collide with enough energy.
It is the activation energy.
Question 5 Easy
Is neutralisation exothermic or endothermic?
Show answer
Answer: exothermic
Neutralisation transfers energy to the surroundings.
So it is exothermic.
Question 6 Easy
Is breaking bonds exothermic or endothermic?
Show answer
Answer: endothermic
Energy is needed to pull atoms apart.
So breaking bonds is endothermic.
Question 7 Medium
Bonds broken need 679 kJ/mol and bonds made release 864 kJ/mol. Calculate the energy change in kJ/mol.
Show answer
Answer: -185 kJ/mol
Energy change = 679 − 864.
= −185 kJ/mol.
Question 8 Medium
Bonds broken need 1000 kJ/mol and bonds made release 700 kJ/mol. Is the reaction exothermic or endothermic?
Show answer
Answer: endothermic
More energy is needed to break the bonds than is released.
So the energy change is +300 and it is endothermic.
Exam-style questions
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Exam-style question 14 marks
Hydrochloric acid reacts with sodium hydroxide in a polystyrene cup. The temperature rises from 20 °C to 27 °C.
(a) State the type of reaction. [1 mark]
(b) Give one reason for using a polystyrene cup. [1 mark]
(c) Sketch or describe the reaction profile, labelling the activation energy. [2 marks]
Show answer and marking points
Answer: (a) exothermic (b) It reduces energy lost to the surroundings. (c) The products are lower than the reactants and a peak above the reactants shows the activation energy.
(a) exothermic (1)
(b) insulator, so less energy is lost (1)
(c) products lower than the reactants (1)
(c) activation energy labelled as the peak above the reactants (1)
Exam-style question 25 marks
Hydrogen reacts with oxygen: 2H₂ + O₂ → 2H₂O. Bond energies in kJ/mol: H–H 436, O=O 498, O–H 464.
(a) Calculate the energy needed to break all the bonds in the reactants. [2 marks]
(b) Calculate the energy released when the bonds in the products form. [1 mark]
(c) Calculate the overall energy change and state whether the reaction is exothermic or endothermic. [2 marks]