Enthalpy (ΔH) is the heat change in a chemical reaction and can be shown in two ways: either written with the equation or included as a reactant or product. A negative ΔH signifies an exothermic reaction that releases heat, which can be shown as a product in the equation. A positive ΔH signifies an endothermic reaction that absorbs heat, which can be shown as a reactant. In stoichiometry problems, the ΔH value acts as a conversion factor relating moles of a substance to the amount of heat released or absorbed, allowing you to calculate the heat for a given amount of reactant or vice versa. The most important rule to remember is to always convert to moles before using ΔH in your calculations.