Types of Reactions (Exothermic Versus Endothermic)
When physical or chemical changes occur, energy changes are involved. Change
of heat content can be designated as ΔH. The heat content (H) is sometimes
referred to as the enthalpy. Every system has a certain amount of heat. This
changes during the course of a physical or chemical change. The change in heat
content, ΔH, is the difference between the heat content of the products and that of
the reactants. The equation is:
If the heat content of the products is greater than the heat content of the
reactants, ΔH is a positive quantity (ΔH > 0) and the reaction is endothermic. If,
however, the heat content of the products is less than the heat content of the
reactants, ΔH is a negative quantity (ΔH < 0) and the reaction is exothermic.
This relationship is shown graphically in Figures 1 and 2 in Chapter 1. This topic
is developed in detail in Chapter 8.
Conservation of Energy
Experiments have shown that energy is neither gained nor lost in physical or
chemical changes. This principle is known as the Law of Conservation of
Energy and is often stated as follows: Energy is neither created nor destroyed in
ordinary physical and chemical changes. If the system under study loses energy,