International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What is meant by the term activation energy?

1. The energy needed to start a chemical reaction

2. The total energy released by a reaction

3. The energy absorbed during a reaction

4. The energy required for successful catalysis

The correct answer is: The energy needed to start a chemical reaction

Activation energy is the minimum amount of energy required for a chemical reaction to occur. This energy is essential to break the bonds of the reactants so that they can rearrange to form new products. The concept is crucial in understanding why some reactions proceed quickly while others take longer or may not occur at all under certain conditions. In many chemical reactions, reactants need to overcome an energy barrier to enter into the transition state, where they can then transform into products. This activation energy can be thought of as the energy 'hump' that must be overcome for reactants to react. Factors such as temperature and the presence of a catalyst can influence the level of activation energy required. Although other concepts such as total energy released or energy absorbed might also be relevant in the broader context of a chemical reaction, they do not accurately capture the specific idea of activation energy, which focuses solely on the energy needed to initiate the reaction. Additionally, while catalysts lower the activation energy needed, the term itself does not imply anything about catalysis specifically. Therefore, the definition focusing on the energy needed to start a chemical reaction is the most appropriate understanding of activation energy.