A Level Chemistry OCR Practice Exam

What is activation energy?

1. The energy required to sustain a reaction

2. The minimum energy required to initiate a reaction

3. The energy released during a chemical reaction

4. Any energy input into a reaction system

The correct answer is: The minimum energy required to initiate a reaction

Activation energy is defined as the minimum energy required to initiate a chemical reaction. This concept is fundamental in understanding reaction kinetics, as it represents the threshold that must be overcome for reactants to transform into products. When molecules collide, they must possess sufficient energy to break the bonds of the reactants and facilitate the formation of new bonds in the products. If the energy of the colliding molecules is below this threshold, the reaction will not occur, regardless of how often the molecules collide. This energy barrier is significant because it explains why some reactions occur slowly at room temperature while others occur rapidly. For example, adding a catalyst lowers the activation energy of a reaction, thereby increasing the reaction rate without affecting the overall energy changes of the reaction itself. The other options describe different aspects of chemical reactions but do not accurately define activation energy. For instance, sustaining a reaction involves other factors such as the availability of reactants and the conditions under which the reaction takes place, while the energy released during a reaction pertains to the overall change in energy, rather than the energy required to start that reaction. Similarly, the concept of any energy input into a reaction system is broader and includes energy contributions that do not directly relate to initiating reactions.