In a chemical reaction, both reactants and the products they form have what are called "heats of formation." Expressed by the symbol ""Hf" (delta HF), heats of formation are an important part of understanding energy transfer during chemical reactions. In order to calculate "Hf for any product or reactant, you must have on hand the total amount of heat the reaction produces ("H), as well as the "Hf value for all the other reactants and/or products, all of which your chemistry problem will provide you.
Arrange your given "Hf and "H values according to the following equation: "H = "Hf (products) - "Hf (reactants). For example, imagine that you want to know "Hf for acetylene, C2H2, for the reaction C2H2 (g) + 5/2O2 (g) --> 2CO2 (g) + H2O (g), the combustion of acetylene, whose "H is -1,256 kJ/mol. You also know that "Hf for CO2 is -394 kJ/mol and "Hf for H2O is -242 kJ/mol. Elemental reactants and products such as oxygen gas have no "heat of formation" by definition--they exist is their form naturally. Knowing this, you can express the following: "H = "Hf (products) - "Hf (reactants), or -1,256 = (2-394 + -242) - "HfC2H2, which you can rearrange as follows: "HfC2H2 = -1,256 - (2-394 + -242). Notice that you must multiply "HfCO2 by two because of the "2" coefficient in front of it in the reaction equation.
Solve your equation for "Hf, in the case of the example "HfC2H2. "HfC2H2 = -1,256 - (2*-394 + -242) = -1,256 - (-788 + -242) = -1,256 - (-1,030) = -1,256 + 1030 = -226 kJ/mol.
Adjust your "Hf value's "sign" depending on whether it's for a product or a reactant. Product "Hf values will always be negative, while those for reactants are always positive. As C2H2 is a reactant, its "Hf is positive. Therefore, "HfC2H2 = 226 kJ/mol.