Advanced Placement (AP) Chemistry Practice Exam

What characterizes an exothermic reaction?

1. Heat is absorbed, leading to increased temperature

2. Products have stronger bonds than the reactants, resulting in heat release

3. The overall energy change is positive

4. Reactants and products are at the same energy level

The correct answer is: Products have stronger bonds than the reactants, resulting in heat release

An exothermic reaction is characterized by the release of energy, usually in the form of heat, into the surroundings. This occurs because the total energy of the products is lower than that of the reactants. In such a scenario, when products have stronger bonds than the reactants, the energy required to break the reactant bonds is less than the energy released when forming the bonds in the products. As a result, the excess energy is released, often causing an increase in the temperature of the surroundings. In the context of the other options, they do not accurately reflect the nature of exothermic reactions. For example, some processes do absorb heat, which is indicative of an endothermic reaction. Furthermore, in exothermic reactions, the overall energy change is negative since energy is released, countering the claim of a positive change. Lastly, while reactants and products can exist at different energy levels, stating they are at the same level does not reflect the distinct energy levels that define exothermic processes. Thus, the correct choice is supported by the fundamental principles of thermodynamics regarding bond energies and energy transfer.