Bond energy calculator: energy change of a reaction

H₂ + Cl₂ → 2HCl, with bond energies H–H 436, Cl–Cl 243 and H–Cl 432 kJ/mol (values given in the question).

• Bonds broken: $436 + 243 = 679$ kJ/mol.
• Bonds made: $2 \times 432 = 864$ kJ/mol.
• Energy change $= 679 - 864 = -185$ kJ/mol: exothermic.

CH₄ + 2O₂ → CO₂ + 2H₂O, with C–H 413, O=O 498, C=O 805 and O–H 464 kJ/mol.

• Broken: $4 \times 413 + 2 \times 498 = 2648$ kJ/mol.
• Made: $2 \times 805 + 4 \times 464 = 3466$ kJ/mol.
• Energy change $= 2648 - 3466 = -818$ kJ/mol.

Draw out each molecule to count the bonds: methane has four C–H bonds, and each water molecule has two O–H bonds.

Multiply each molecule's bonds by the number in front of it in the balanced equation. 2O₂ means two O=O double bonds; 2H₂O means four O–H bonds. Double bonds have their own (larger) energy, so C=O isn't twice C–O.

On a reaction profile (energy level diagram), an exothermic reaction ends lower than it starts, and an endothermic reaction ends higher. The hump between them is the activation energy, the minimum energy needed for particles to react. Catalysts lower the activation energy without changing the overall energy change.

• Doing made − broken (the sign comes out backwards).
• Missing the coefficients from the balanced equation.
• Treating a double bond as two single bonds.

Practise with our bond energy questions.

• Draw the displayed formula of every molecule so you can count each bond.
• Set out "bonds broken" and "bonds made" as two separate lists before subtracting.
• Use only the bond energies given in the question: values in different data books vary slightly.
• State whether the reaction is exothermic or endothermic from the sign of your answer.

Bond energies in data tables are averages: a C–H bond in methane doesn't have exactly the same energy as a C–H bond in ethanol. So an energy change worked out from bond energies is an estimate, and it can differ from the value measured in a calorimetry experiment. The calculation also assumes everything is a gas. For exam questions, use the values given and don't worry about the difference unless the question asks you to explain it.