A Level Chemistry OCR Practice Exam

An exothermic reaction is characterized by the fact that the enthalpy of the products is less than that of the reactants. This occurs because during an exothermic reaction, energy is released to the surroundings as heat, which leads to a decrease in the overall energy content of the products compared to the reactants.

In a chemical reaction, if the products have a lower enthalpy than the reactants, this indicates that energy was released during the transformation. As the reaction proceeds, the energy difference manifests as heat, which can increase the temperature of the surroundings. This understanding is key in distinguishing exothermic reactions from endothermic reactions, where the reverse situation occurs.

In summary, an exothermic reaction features a decrease in enthalpy, signifying that the energy released manifests as an increase in temperature in the surrounding environment, thereby influencing the reaction's thermal characteristics.