Enthalpy is expressed as Delta H, which is the amount of heat content used or released in a system at constant pressure. Delta H equals change in volume multiplied by the constant pressure of the system plus the change in internal energy.

At constant pressure, the heat flow for the process is also equal to the change in enthalpy. Heat flow is expressed as "q." The relationship between Delta H and q can be determined based on whether the reaction is exothermic or endothermic. If the reaction absorbs heat from the surroundings, it is endothermic, and q is positive. If q is positive, then Delta H is also positive. If the reaction releases heat to its surroundings, it is exothermic, and q is negative. If q is negative, then Delta H is also negative.