In a chemical reaction, both reactants and the products they form have what are called "heats of formation." Expressed by the symbol "ΔHf" (delta HF), heats of formation are an important part of understanding energy transfer during chemical reactions. In order to calculate ΔHf for any product or reactant, you must have on hand the total amount of heat the reaction produces (ΔH), as well as the ΔHf value for all the other reactants and/or products, all of which your chemistry problem will provide you.

Arrange your given ΔHf and ΔH values according to the following equation: ΔH = ΔHf (products) - ΔHf (reactants).

For example, imagine that you want to know ΔHf for acetylene, C₂H₂, for the reaction C₂H₂ (g) + (5/2)O₂ (g) --> 2CO₂ (g) + H₂O (g), the combustion of acetylene, the ΔH of which is -1,256 kJ/mol.

You know that the ΔHf of CO₂ is -394 kJ/mol and the ΔHf of 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 all of this, you can write the following: ΔH = ΔHf (products) - ΔHf (reactants), or

-1,256 = (2 × (-394) + (-242)) - ΔHf(C₂H₂),

which you can rearrange as follows:

ΔHf(C₂H₂) = [2 ×(-394) + (-242)] +1,256.

Note that you must multiply the ΔHf of CO₂ by two because of the "2" coefficient in front of it in the reaction equation.

Solve your equation for ΔHf. In the case of the example ΔHf(C₂H₂),

ΔHf(C₂H₂) = [2 ×(-394) + (-242)] - (-1,256).

= (-1,030) + 1,256 = 226 kJ/mol.

Adjust your ΔHf value's sign depending on whether it is for a product or a reactant. Product ΔHf values will always be negative, while those for reactants are always positive. As C₂H₂ is a reactant, its ΔHf is positive. Therefore, ΔHf(C₂H₂) = 226 kJ/mol.