Advanced Placement (AP) Chemistry Practice Exam

In terms of energy, what does an endothermic reaction require?

1. Energy to be absorbed

2. Energy to be released

3. No energy change

4. High pressure to proceed

The correct answer is: Energy to be absorbed

An endothermic reaction is characterized by the absorption of energy from the surroundings. This means that, during the reaction, the system takes in energy, typically in the form of heat. As a result, the temperature of the surroundings decreases, which can be observed in some reactions where a cooling effect is noticeable. In essence, the requirement for energy absorption allows the reactants to reach a transition state or to break bonds, ultimately forming the products. This concept of energy absorption is fundamental in understanding how reactions are driven and how they interact with their environment. The other options do not accurately represent the nature of an endothermic reaction. Energy release is associated with exothermic reactions, a condition of no energy change implies a reaction at equilibrium, and specifying high pressure to proceed does not universally apply to endothermic processes, as not all of them are affected by pressure in the same way.