Why do some reactions warm their surroundings and others cool them, and how do bond changes explain it?
Describe exothermic and endothermic reactions in terms of temperature change and bond breaking and forming, and interpret simple energy level diagrams
A focused answer to the O-Level Combined Science outcome on energy changes. Exothermic and endothermic reactions, energy from bond breaking and forming, activation energy, and reading energy level diagrams.
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What this dot point is asking
SEAB wants you to classify reactions as exothermic or endothermic from the temperature change, to explain the energy change in terms of bond breaking and bond forming, and to read or sketch a simple energy level diagram showing reactants, products, activation energy and the overall change. The bond-energy explanation is the key understanding the examiners test.
The answer
Exothermic and endothermic reactions
- An exothermic reaction gives out energy to the surroundings, so the temperature of the surroundings rises. Combustion, neutralisation and most displacement reactions are exothermic.
- An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings falls. Thermal decomposition (such as heating a carbonate) and photosynthesis are endothermic.
Energy and bonds
Every reaction involves breaking bonds in the reactants and forming bonds in the products:
- breaking bonds takes in energy (endothermic),
- forming bonds releases energy (exothermic).
The overall energy change depends on the balance:
- if more energy is released forming bonds than is taken in breaking them, the reaction is exothermic (net energy out),
- if more energy is taken in breaking bonds than is released forming them, the reaction is endothermic (net energy in).
Activation energy
Even an exothermic reaction usually needs an initial input of energy to start, to break the first bonds. This minimum energy is the activation energy. It is why a fuel needs a spark to ignite, even though burning then releases far more energy than the spark supplied.
Energy level diagrams
An energy level diagram plots energy (vertical axis) against the progress of the reaction (horizontal axis):
- for an exothermic reaction the products are lower than the reactants (energy is lost to the surroundings),
- for an endothermic reaction the products are higher than the reactants (energy is gained from the surroundings),
- the hump between them shows the activation energy, measured from the reactants up to the top of the hump.
Examples in context
Example 1. Self-heating cans and instant cold packs. Some camping food cans use an exothermic reaction (such as calcium oxide with water) to warm the contents, while sports cold packs use an endothermic process (a salt dissolving) to cool an injury. Both are direct uses of the energy change of a reaction.
Example 2. Why fuels store usable energy. A fuel such as methane has strong bonds, but burning it forms even stronger bonds in carbon dioxide and water, releasing the difference as heat. This net release is why combustion is exothermic and why fuels are a practical energy store, linking bond energies to everyday power.
Try this
Q1. State whether neutralisation is exothermic or endothermic and what happens to the temperature of the mixture. [2 marks]
- Cue. Neutralisation is exothermic; the temperature of the mixture rises.
Q2. Explain, in terms of bonds, why a reaction is endothermic. [2 marks]
- Cue. More energy is taken in to break the bonds in the reactants than is released when the bonds in the products form, so energy is absorbed overall.
Q3. On an energy level diagram for an exothermic reaction, state where the products lie relative to the reactants and what the hump represents. [2 marks]
- Cue. The products lie lower than the reactants; the hump represents the activation energy.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksWhen a fuel burns, the temperature of the surroundings rises. Explain in terms of bond breaking and bond forming why combustion is exothermic, and state what the sign of the energy change would be.Show worked answer →
Breaking bonds in the reactants (the fuel and oxygen) takes in energy; it is endothermic. Forming the new bonds in the products (carbon dioxide and water) releases energy; it is exothermic.
In combustion, the energy released when the new bonds form is greater than the energy taken in to break the old bonds. The net result is that energy is given out to the surroundings, so the temperature rises and the reaction is exothermic.
The energy change is negative (energy is released to the surroundings).
Markers reward energy taken in to break bonds, energy released to form bonds, the energy released being greater than the energy taken in, and the negative sign for an exothermic change.
Original3 marksSketch and label an energy level diagram for an endothermic reaction, marking the reactants, the products, the activation energy and the overall energy change.Show worked answer →
The diagram should show energy on the vertical axis and progress of reaction on the horizontal axis.
The reactants are at a lower energy level than the products (because the reaction takes in energy overall).
A curve rises from the reactants over a hump (the top of the hump is the highest point) and comes down to the products. The height from the reactants up to the top of the hump is the activation energy.
The overall energy change is the difference in height between the reactants and the (higher) products, and it is positive for an endothermic reaction.
Markers reward products higher than reactants, the activation energy as the rise from reactants to the peak, and the overall energy change marked as the reactant-to-product difference (positive).
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