Additivity of Reaction Heats

Chemical equations and DH values may be manipulated algebraically in order to find the DH of formation. This principle is known as Hess's Law of Heat Summation. This principle states that, when a reaction can be expressed as the algebraic sum of two or more other reactions, the heat of the reaction is the algebraic sum of the heats of these reactions. This is based upon the First Law of Thermodynamics, which, simply stated, says that the total energy of the universe is constant and cannot be created or destroyed.

This allows calculations of DH's that cannot be easily determined experimentally. An example is the determination of the DH of CO from the final DH of CO2.

C + O2 ---> CO2 (final DH = -94.0 kcal)

CO + 1/2O2 ---> CO2 (final DH = -67.6 kcal)

The equation wanted is

C + 1/2O2 -->CO

To get this, we reverse the second equation and add it to the first:

C + O2 ---> CO2 (final DH = -94.0 kcal)

CO2 ---> 1/2O2 + CO (final DH = +67.6 kcal)

Addition Yields

C + 1/2O2 -->CO (final DH = -26.4 kcal)

If you have table of standard heats of formation handy, an alternative and easier method of calculating enthalpies is based on the concept that initial DH is equal to the difference between the total enthalpy of the reactants and that of the products. This can be expressed as follows:

DH reaction = [S (sum of) DH of the products] - [S (sum of) DH of the reactants]