GCSE Chemistry revision

Energy transfer during exothermic and endothermic reactions

Exothermic and endothermic reactions

AQA 4.5.1.1 Foundation & Higher

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Different molecules store different amounts of energy in their chemical bonds, meaning reactants and products have different total energy levels.
The key comparison in any reaction is the total energy of all the reactants versus the total energy of all the products.

In an exothermic reaction, the products have less energy than the reactants, so energy is released to the surroundings.
Energy is most commonly transferred to the surroundings in the form of heat, causing a measurable temperature increase.
Common examples of exothermic reactions include combustion, neutralisation reactions between acids and bases, and most oxidation reactions.

2. What is an Exothermic Reaction? AQA

In an exothermic reaction, the products have less energy than the reactants, so energy is released to the surroundings.
Energy is most commonly transferred to the surroundings in the form of heat, causing a measurable temperature increase.
Common examples of exothermic reactions include combustion, neutralisation reactions between acids and bases, and most oxidation reactions.

In an endothermic reaction, the products have more energy than the reactants, so energy is absorbed from the surroundings.
Endothermic reactions require a continuous input of heat energy to proceed, such as heating calcium carbonate to decompose it into calcium oxide and carbon dioxide.
Because energy is taken in from the surroundings, endothermic reactions cause a measurable temperature decrease in the surroundings.

3. What is an Endothermic Reaction? AQA

In an endothermic reaction, the products have more energy than the reactants, so energy is absorbed from the surroundings.
Endothermic reactions require a continuous input of heat energy to proceed, such as heating calcium carbonate to decompose it into calcium oxide and carbon dioxide.
Because energy is taken in from the surroundings, endothermic reactions cause a measurable temperature decrease in the surroundings.

Exothermic reactions release energy to the surroundings, resulting in a temperature increase, whilst endothermic reactions absorb energy, resulting in a temperature decrease. On a reaction profile, exothermic reactions show products at a lower energy level than reactants, whereas endothermic reactions show products at a higher energy level. Both types of reaction still require activation energy to begin, as shown by the peak of the curve on their respective reaction profiles.

4. Key Differences: Exothermic vs Endothermic at a Glance AQA

Exothermic reactions release energy to the surroundings, resulting in a temperature increase, whilst endothermic reactions absorb energy, resulting in a temperature decrease. On a reaction profile, exothermic reactions show products at a lower energy level than reactants, whereas endothermic reactions show products at a higher energy level. Both types of reaction still require activation energy to begin, as shown by the peak of the curve on their respective reaction profiles.

Energy cannot be created or destroyed; it can only be transferred from one place to another — this is the law of conservation of energy.
In exothermic reactions, the energy lost by the chemicals is equal to the energy released to the surroundings.
In endothermic reactions, the energy gained by the chemicals is equal to the energy absorbed from the surroundings.

5. Conservation of Energy AQA

Energy cannot be created or destroyed; it can only be transferred from one place to another — this is the law of conservation of energy.
In exothermic reactions, the energy lost by the chemicals is equal to the energy released to the surroundings.
In endothermic reactions, the energy gained by the chemicals is equal to the energy absorbed from the surroundings.