Each bond between two atoms in a molecule has an associated stored energy, or bond enthalpy value which is determined experimentally. This enthalpy, measured in kilojoules per mole (kj/mol) is the amount of energy required to break the bond as well as the energy released as the bond is formed. During a chemical reaction, atoms are rearranged, and bonds are broken within reactant molecules as new bonds are formed to produce product molecules. Bond enthalpy values are used calculate the enthalpy (energy) change that occurs during a chemical reaction, by subtracting the total amount of energy produced as bonds are formed from the energy used to break the bonds of the reactant molecules.
Write the chemical equation for the reaction. For example, the combustion of diatomic hydrogen and oxygen to produce water is given as H2(g) + O2(g) -> H2O(g).
Balance the chemical equation. In the above example, the balanced equation for the formation of water is H2(g) + O2(g) -> H2O(g), showing the proper ratio of hydrogen to oxygen to water molecules.
Draw the structural formula for each molecule and identify the types and number of bonds present for the products side of the reaction. For example the structural formula for H2 is H-H and for O2 is O=O. Because there are two diatomic hydrogen molecules there are two H-H bonds and one O=O bond.
Draw the structural formula for the reactants side and identify the type and number of bonds present. The structural formula for water is H-O-H and there are four total H-O bonds, because there are two water molecules in the balanced equation.
Look up and record the bond enthalpy values from a data table for each type of bond in the balanced equation. For the example, H-H = 436kJ/mol, O=O = 499kJ/mol, and H-) = 463kJ/mol.
Multiply the bond enthalpy for each type of bond by the number of that type of bond for both reactants and products. For the example, 2(436) + 499 -> 4(463)
Add together the bond enthalpies for the reactants and record this number. Add together the bond enthalpies for the products and subtract this number from the reactants total, to find the value of the bond enthalpy change for the reaction. For the example, 1371kJ/mol - 1852kJ/mol = -481kJ/mol.
Bond enthalpies are determined experimentally and average values for bonds can be found in data tables in textbooks or online. Values may vary slightly between tables as enthalpy values are averaged values.