Bond Energy Calculations
A calculation that puts a number on whether a reaction is exothermic or endothermic. One equation, one sign convention, and one habit that prevents most errors: count every bond.
1. The principle
BREAKING bonds REQUIRES energy — ENDOTHERMIC. MAKING bonds RELEASES energy — EXOTHERMIC.
This is the foundation, and it was recorded wrong twice. You must put energy in to pull atoms apart; energy comes out when they join.
Bond energy is the energy needed to break one mole of a particular bond, in kJ/mol.
2. The equation
ΔH = (total energy of bonds BROKEN) − (total energy of bonds MADE) ΔH = energy IN − energy OUT
Interpreting the sign:
| ΔH | Meaning |
|---|---|
| NEGATIVE (−) | EXOTHERMIC — more energy released than absorbed |
| POSITIVE (+) | ENDOTHERMIC — more energy absorbed than released |
Bonds broken are the REACTANTS; bonds made are the PRODUCTS. Getting them the wrong way round flips the sign and the conclusion.
A negative ΔH means EXOTHERMIC — energy has left the system. This sign convention was recorded as a confusion.
3. The method
1. Draw or list ALL the bonds in the REACTANTS. 2. Draw or list ALL the bonds in the PRODUCTS. 3. Add up the bond energies for each side. 4. ΔH = broken − made. 5. State exothermic or endothermic from the SIGN.
Worked example — the combustion of methane:
CH₄ + 2O₂ → CO₂ + 2H₂O
Bond energies (kJ/mol): C–H 413, O=O 498, C=O 805, O–H 464
Bonds BROKEN (reactants):
- 4 × C–H = 4 × 413 = 1652
- 2 × O=O = 2 × 498 = 996
- Total in = 2648 kJ/mol
Bonds MADE (products):
- 2 × C=O (in CO₂) = 2 × 805 = 1610
- 4 × O–H (2 water molecules × 2 bonds) = 4 × 464 = 1856
- Total out = 3466 kJ/mol
ΔH = 2648 − 3466 = −818 kJ/mol → EXOTHERMIC
Count the bonds carefully, including the coefficients. Two O₂ molecules means 2 O=O bonds; two H₂O molecules means 4 O–H bonds, not 2. This is where most marks are lost.
Each water molecule has TWO O–H bonds, and CO₂ has TWO C=O bonds — count the bonds in the molecule, then multiply by the coefficient.
4. Practical tips
Draw the displayed formulae of the reactants and products if you are unsure — then simply count the lines. A double bond counts as one bond with its own bond energy, not two single bonds.
A double bond has its own value — C=O is 805, not twice the C–O value. Look up the right entry.
Set your working out in two columns — “broken” and “made” — with a subtotal for each. It makes the arithmetic checkable and earns method marks.
Check the answer against expectation: combustion reactions are always exothermic, so a positive ΔH there means an error.
5. Relating to energy level diagrams
Exothermic (negative ΔH): products LOWER than reactants; ΔH arrow points down. Endothermic (positive ΔH): products HIGHER than reactants; ΔH arrow points up.
The activation energy is the height of the hump from the reactants — a different quantity from ΔH, which is measured between reactants and products.
6. Mistakes that cost marks
Saying bond breaking releases energy.
Reversing the equation — made minus broken.
Getting the sign convention backwards.
Miscounting bonds, especially with coefficients.
Forgetting water has two O–H bonds.
Treating a double bond as two single bonds.
Omitting the sign or the units (kJ/mol).
Not stating exothermic or endothermic at the end.
Frequently asked questions
Is bond breaking exothermic or endothermic? Endothermic — it requires energy.
Is bond making exothermic or endothermic? Exothermic — it releases energy.
What is the equation for ΔH? Bonds broken − bonds made (energy in − energy out).
What does a negative ΔH mean? The reaction is exothermic.
What does a positive ΔH mean? Endothermic.
How many O–H bonds are in 2H₂O? Four — two per molecule.
How do I treat a double bond? Use its own bond energy value — don’t double the single bond value.
What are the units? kJ/mol.
How do I check my answer? Combustion should be exothermic (negative). If it isn’t, recheck the counting.
What’s the difference between ΔH and activation energy? ΔH is between reactants and products; Ea is up to the top of the hump.
Quick revision checklist
- I know breaking absorbs, making releases
- I know ΔH = broken − made
- I know negative means exothermic
- I list bonds for reactants and products separately
- I multiply by the coefficients
- I know H₂O has two O–H bonds and CO₂ has two C=O bonds
- I use the correct value for double bonds
- I set my working in two columns
- I give the sign and units
- I state exothermic or endothermic at the end
- I can link the sign to an energy level diagram
These notes cover bond energy calculations in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons. This was one of the less-covered subtopics in that set, so the page follows the syllabus closely rather than being padded. Always check the current syllabus and data booklet for your own exam series.
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