The method used in calculating enthalpy is Hess's Law of Heat Summation. The standard state enthalpy of a reaction is equal to the sum of the enthalpies of formation of the products minus the sum of the enthalpies of formation of the reactant.
Change in enthalpy is also known as heat transfer, or delta H. Hess's Law states that if a reaction is divided or multiplied by some factor, then delta H must also be divided or multiplied by that same factor. The law also states that if the reaction is reversed, the sign of delta H must also be reversed.
There are three factors that affect the enthalpy of a reaction: the partial pressure of any gases involved, the concentrations of the reactants and products, and the temperature of the system. The effects of the changes in these factors are relative to the standard state enthalpy of the reaction, which is the change in enthalpy that begins and ends under standard state conditions. Standard state conditions indicate that the concentration of all aqueous solutions is 1 molar and that the partial pressure of all gases involved in the reaction is 0.1 megapascals. These measurements are generally taken at a temperature of 298 Kelvin, or 25 degrees Celsius.