Advanced Placement (AP) Chemistry Practice Exam

What is activation energy?

1. The maximum energy needed for a reaction

2. The energy required to break bonds in a reactant

3. The minimum amount of energy required to start a chemical reaction

4. The energy released by the products

The correct answer is: The minimum amount of energy required to start a chemical reaction

Activation energy is defined as the minimum amount of energy required to initiate a chemical reaction. This energy is critical because it helps to overcome the energy barrier that exists between the reactants and the transition state, which is the high-energy state that occurs during a reaction before products are formed. This concept is fundamental in understanding reaction kinetics. The activation energy determines the rate at which a reaction will proceed; reactions with lower activation energy tend to occur more rapidly, whereas those with higher activation energy are slower. The other choices do not accurately describe activation energy. For example, the maximum energy needed for a reaction refers to the total energy of the transition state rather than the initial requirement to start the reaction. The energy required to break bonds in a reactant focuses on bond dissociation energy rather than the energy required for the reaction overall. Similarly, the energy released by the products relates to the enthalpy of the reaction and is not pertinent to the concept of activation energy. Therefore, the correct understanding of activation energy encompasses its role as the threshold energy required to initiate a reaction, making the identification of it as the minimum energy required to start a chemical reaction appropriate.