In the field of chemistry, delta H literally represents a change in system enthalpy. The delta symbol represents the actual change while the H is for enthalpy. Enthalpy in its most basic is the change in the amount or presence of heat in a given system. Considering that change is happening to a chemical reaction for example, the heat generated will also change because of the alterations in both pressure and volume. When there is such a change in the presence of heat, this is considered a Delta H reaction or occurrence.

When heat is added or taken away for example, enthalpy will result. For this important chemical event, Delta H is used to signify such change. As the heat increases or decreases in a given system, the volume and pressure also changes causing the process of enthalpy. In a chemical reaction, all changes involving the substance or compound involved are part of the actual enthalpy or change in the heat of this particular system. Changes in the molecules, atoms, and ions are all part of the enthalpy of reaction. The reactants meanwhile are excluded from the process. They are needed only to start the change or chemical reaction but the processes and changes they go through are not computed as part of the enthalpy of reaction.

The enthalpy of reaction may either be positive or negative. Positive enthalpy simply means that the sum of the changes involving the substance or compound products used is greater than that of the change that involved the reactants. The opposite effect will then be labelled as negative enthalpy wherein the change in the reactants resulted to a greater sum compared to the actual substance or products. In a positive enthalpy, heat is generated within and this is why it is also referred to as endothermic. On the other hand, negative enthalpy results to heat being lost from the system making it an exothermic type of enthalpy.