Advanced Placement (AP) Chemistry Practice Exam

What is a defining feature of an endothermic reaction?

1. Heat is released, causing temperature to rise

2. Reactants have stronger bonds than the products

3. The total enthalpy change is negative

4. Energy is not required to break bonds

The correct answer is: Reactants have stronger bonds than the products

In an endothermic reaction, energy is absorbed from the surroundings, leading to a decrease in the temperature of the surroundings. A defining feature of such reactions is that the products typically have higher energy than the reactants. This often results in the products having weaker bonds than those in the reactants, as the energy absorbed during the reaction is used to break the bonds in the reactants and form new bonds in the products. Although option B states that reactants have stronger bonds than the products, this characterization fits endothermic reactions. In contrast, exothermic reactions, which release energy, often show the opposite trend where products generally have stronger bonds than the reactants due to the energy released in forming those bonds. This characteristic of endothermic reactions underscores that energy input is essential for proceeding with the reaction, which leads to the products possessing less stability compared to the reactants. Thus, the statement reflects a true aspect of the energy dynamics involved in endothermic processes.