To calculate bond energy in a chemical reaction, you inspect the reaction equation, and add up the energies in the bonds of the molecules for both products and reactants. The calculation reveals whether the reaction is exothermic (releases heat) or endothermic (absorbs heat).
TL;DR (Too Long; Didn't Read)
To calculate bond energy in a chemical reaction, inspect the reaction equation, and add up the energies in the bonds of the molecules for both products and reactants.
Making and Breaking Bonds
Atoms are more stable and occupy lower energy states when bonded together. As you calculate bond energies, remember that the energy on the left side of the equation goes into breaking the bonds of the reactant molecules, and energy on the right side comes from energy released by making bonds in the product molecules.
Find the Bond Energy
To calculate bond energy, examine the equation for the chemical reaction you’re interested in. Note the elements involved in the reaction, and the bonds that hold them together. Look up the bonds in a table of bond energies and note down each one on both sides of the equation. Also note if the bond is single, double or triple. For example, this is the equation for the combustion of methane:
CH4 + 2O2 → 2H2O + CO2
On the left side of the equation you have 4 hydrogen-carbon (single) bonds and 2 oxygen-oxygen (double) bonds. The right side has 4 oxygen-hydrogen (single) bonds and 2 carbon-oxygen (double) bonds. From a table, you can find that each hydrogen-carbon bond accounts for 413 KJ/Mol, oxygen-oxygen is 495 KJ/Mol, oxygen-hydrogen is 467 KJ/Mol, and carbon-oxygen is 358 KJ/Mol.
Calculate Bond Energy
For your reaction equation, multiply the number of bonds by the bond energies:
(4) Carbon-hydrogen bonds at 413 KJ/Mol = 1,652 KJ/Mol.
(2) Oxygen-oxygen bonds (double bonds) at 495 KJ/Mol = 990 KJ/Mol.
(4) Oxygen-hydrogen bonds at 467 KJ/Mol = 1,868 KJ/Mol.
(2) Carbon-oxygen bonds (double bonds) at 799 KJ/Mol = 1,598 KJ/Mol.
Endothermic or Exothermic?
To see if the reaction is endothermic or exothermic, add up the energies for both sides, and compare them. The left side has 1,652 KJ/Mol + 990 KJ/Mol = 2,642 KJ/Mol. The right side has 1,868 KJ/Mol + 1,598 KJ/Mol = 3,466 KJ/Mol. The energy needed to break the bonds on the left side of the equation is less than the energy given off by making the bonds on the right side. By convention, 2,642 KJ/Mol - 3,466 KJ/Mol = -824 KJ/Mol. The negative number means energy is leaving the reaction as heat. Since the total is a negative number, the reaction is exothermic. If the number was positive, the reaction would be endothermic.