International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Which of the following best describes an exothermic reaction?

1. A reaction in which heat energy is absorbed

2. A reaction in which heat energy is given out

3. A reaction that requires electricity

4. A reaction that occurs rapidly without a heat change

The correct answer is: A reaction in which heat energy is given out

An exothermic reaction is defined as a chemical process that releases heat energy to its surroundings. This release of energy often leads to an increase in temperature in the environment around the reaction. In an exothermic reaction, the energy of the products is lower than that of the reactants, indicating that energy has been released during the formation of products. This characteristic is commonly observed in reactions such as combustion, where fuels react with oxygen to produce carbon dioxide, water, and heat. Other examples include the reaction between acids and bases and the process of respiration in living organisms. In contrast, a reaction that absorbs heat energy would be classified as endothermic, not exothermic. Reactions requiring electricity typically indicate electrochemical processes and do not specifically define the heat exchange properties of a reaction. Likewise, a reaction that occurs rapidly without a heat change does not accurately capture the essential nature of exothermicity, as exothermic reactions typically involve notable thermal changes. Thus, the best definition of an exothermic reaction is one in which heat energy is given out.