Calculating bond energies

Use the same three lines every time

1. BREAK: add the energies of all reactant bonds.
2. FORM: add the energies of all product bonds.
3. SUBTRACT: overall energy change = breaking − forming.
A negative answer means exothermic. A positive answer means endothermic. Use the supplied bond energies; you do not need to memorise them.

Worked example: hydrogen chloride

For H₂ + Cl₂ → 2HCl, use H–H = 436, Cl–Cl = 243 and H–Cl = 431 kJ/mol. The result below is for one mole of reaction as written: one mole H₂ reacts with one mole Cl₂ to make two moles HCl.

Watch the numbers in front

For CH₄ + 2O₂ → CO₂ + 2H₂O: break 4 C–H bonds and 2 O=O bonds; form 2 C=O bonds and 4 O–H bonds. An O=O is one double bond: use its supplied double-bond energy once, not twice. A water molecule has two O–H bonds, so 2H₂O has four.

Break: 4 × 413 + 2 × 498 = 2648 kJ/mol.
Form: 2 × 805 + 4 × 464 = 3466 kJ/mol.
Overall: 2648 − 3466 = −818 kJ/mol, exothermic. Bond-energy calculations give estimates using the supplied values.