An endothermic chemical reaction is a type of reaction in which energy is transferred from the surroundings to the products, e.g. photosynthesis.

An exothermic chemical reaction is a type of reaction in which energy is transferred from the reactants to the surroundings, e.g. combustion.

The minimum energy that the particles must have in order to react is known as the activation energy.

The energy required to break a covalent bond, or released when making a covalent bond. is known as the bond energy.

Breaking and making bonds: using bond energies

Combined science

In the Rate of Reaction unit, pupils will build on their understanding of energy changes to learn about the factors that influence the rate of chemical reactions.

In the Fuels and Energetics unit, pupils learned about energy changes during chemical reactions, including exothermic and endothermic reactions.

Energy changes in reactions

negative, because the energy released is greater than the energy absorbed.

positive, because the energy absorbed is greater than the energy released.

The overall energy change for an exothermic reaction is...

Given the following data: reactants energy = 200 kJ, products energy = 300 kJ, what is the overall energy change? Is the reaction endothermic or exothermic?

Given the following data: reactants energy = 400 kJ, products energy = 250 kJ, what is the overall energy change? Is the reaction endothermic or exothermic?

+200 kJ, indicating an endothermic reaction

-200 kJ, indicating an endothermic reaction

+200 kJ, indicating an exothermic reaction

-200 kJ, indicating an exothermic reaction

During the combustion of methane, the reactants have an energy of 500 kJ and the products have an energy of 300 kJ. What is the overall energy change? Is the reaction endothermic or exothermic?

How many C-H bonds are there in the reactants and in the products of this reaction?

What is the overall energy change when ethanol (C₂H₅OH) reacts to form ethene (C₂H₄) and water (H₂O)? Give your answer in kJ/mol.

the energy changes during a chemical reaction

the temperature changes during a reaction

What does an energy profile diagram show?

Atoms are rearranged and chemical bonds are broken/formed.

a reaction where energy is transferred from the reactants to the surrounding

a reaction that absorbs energy from the surroundings

a reaction that does not involve energy change

a reaction in which energy is transferred from the surroundings to the products

a reaction that releases energy to the surroundings

the minimum energy that the particles must have in order to react

In which energy profile does the blue arrow show the energy change of an exothermic reaction?

This lesson will be removed by end of Summer Term 2025.

Breaking and making bonds: using bond energies

Lesson outcome

This lesson will be removed by end of Summer Term 2025.