Which of the following is the best definition of an exothermic reaction? (A) A reaction that absorbs heat. (B) A reaction that is spontaneous. (C) A reaction that is accelerated by cooling. (D) A reaction that is accelerated by heating. Or (E) a reaction that gives out heat.
In this question, we need to identify the best definition of an exothermic reaction. In any chemical reaction, the reactants at the beginning of the reaction transform to produce the products at the end of the reaction. The reactants and products have different chemical arrangements, and they also have different energy levels. We can visualize the energy levels of reactants and products with a diagram like this one that shows energy on the 𝑦-axis and reaction progress along the 𝑥-axis.
In the example we’ve drawn here, the reactants on the left have a relatively low energy level compared to the products on the right, which are higher up on the 𝑦-axis. Another possibility would be if the products ended up having a lower energy level than the reactants. Another way of saying that the energy decreases over the course of the reaction is to say that the enthalpy change or the Δ𝐻 of the reaction is less than zero. A negative enthalpy change is a defining feature of an exothermic reaction.
We can say that during an exothermic reaction, energy is released as the reactants transform into the products. When that energy is released from the system to its surroundings, it often takes the form of heat, although the energy is usually some combination of thermal energy and at least one other type of energy. The answer choice that best fits this definition is choice (E), a reaction that gives out heat. This answer choice matches the meaning of the word “exothermic,” exo- meaning out and “thermic” meaning heat. The heat goes out of the system.
However, we need to be careful about this simplified definition that deals with heat. As we have stated before, by definition, an exothermic reaction will release energy, which may or may not take the form of heat. So which of the following is the best definition of an exothermic reaction? That’s choice (E), a reaction that gives out heat.