Advanced Placement (AP) Chemistry Practice Exam

What does it mean when a reaction is described as being "exothermic"?

1. The reactants and products have the same energy

2. The products are less stable than the reactants

3. Heat is released during the reaction

4. The reaction proceeds without a change in temperature

The correct answer is: Heat is released during the reaction

When a reaction is described as "exothermic," it specifically refers to the process where heat is released into the surroundings during the course of the reaction. This occurs because the total energy of the products is lower than that of the reactants. As a result of this energy difference, energy in the form of heat is emitted, which can lead to an increase in the temperature of the surroundings. In an exothermic reaction, the flow of energy is from the system (the reactants and products) to the surroundings, signifying a loss of energy in the form of heat. Common examples of exothermic reactions include combustion, such as burning fuels, and some types of neutralization reactions, where an acid and a base react to form water and a salt, releasing heat in the process. Understanding that exothermic reactions release heat helps in predicting how a reaction might affect the temperature and behavior of the system and its surroundings.