Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019

Exothermic and Endothermic Processes


· To consider the direction of energy flow as heat.

In this section we will consider the energy changes that accompany chemical reactions. To explore this idea, let’s consider the striking and burning of a match. Energy is clearly released through heat as the match burns. To discuss this reaction, we divide the universe into two parts: the system and the surroundings. The system is the part of the universe on which we wish to focus attention; the surroundings include everything else in the universe. In this case we define the system as the reactants and products of the reaction. The surroundings consist of the air in the room and anything else other than the reactants and products.

When a process results in the evolution of heat, it is said to be exothermic (exo- is a prefix meaning “out of”); that is, energy flows out of the system. For example, in the combustion of a match, energy flows out of the system as heat. Processes that absorb energy from the surroundings are said to be endothermic . When the heat flow moves into a system, the process is endothermic. Boiling water to form steam is a common endothermic process.

Where does the energy, released as heat, come from in an exothermic reaction? The answer lies in the difference in potential energies between the products and the reactants. Which has lower potential energy, the reactants or the products? We know that total energy is conserved and that energy flows from the system into the surroundings in an exothermic reaction. Thus the energy gained by the surroundings must be equal to the energy lost by the system. In the combustion of a match, the burned match has lost potential energy (in this case potential energy stored in the bonds of the reactants), which was transferred through heat to the surroundings (Fig. 10.5). The heat flow into the surroundings results from a lowering of the potential energy of the reaction system. In any exothermic reaction, some of the potential energy stored in the chemical bonds is converted to thermal energy (random kinetic energy) via heat.

Figure 10.5.An illustration shows the difference in potential energies of reactants and products, indicated by an increasing potential energy scale. Reactants have a higher potential energy as compared to products, and the difference in the energies, denoted by delta PE, is released to the surroundings as heat.

The energy changes accompanying the burning of a match.