Bond Energy Calculations
What Are Bond Energies?
Bond energy is the amount of energy needed to break one mole of a particular covalent bond (in a gaseous molecule). It is measured in kJ/mol.
The same amount of energy is released when the same bond is formed.
- Breaking bonds requires energy — it is endothermic
- Making bonds releases energy — it is exothermic
This is a fundamental principle: energy must always be put in to pull atoms apart, and energy is always released when atoms come together.
How Bond Energies Determine Reaction Type
The overall energy change of a reaction depends on the balance between energy taken in (breaking bonds) and energy given out (forming bonds):
- If more energy is released forming bonds than is taken in breaking bonds → the reaction is exothermic (overall energy change is negative)
- If more energy is needed to break bonds than is released forming bonds → the reaction is endothermic (overall energy change is positive)
Calculating Energy Changes from Bond Energies
Method
1. Draw out the structural formulae of all reactants and products (showing every bond)
2. List all the bonds broken in the reactants
3. List all the bonds formed in the products
4. Look up the bond energy for each bond
5. Calculate: energy change = total energy to break bonds − total energy released forming bonds
If the answer is negative → exothermic
If the answer is positive → endothermic
Common Bond Energies
| Bond | Bond energy (kJ/mol) |
|---|---|
| C−H | 413 |
| C−C | 347 |
| C=C | 614 |
| C−O | 358 |
| C=O | 805 |
| O−H | 464 |
| O=O | 498 |
| H−H | 436 |
| N−H | 391 |
| N≡N | 945 |
| C−Cl | 346 |
| H−Cl | 432 |
(Exam papers provide bond energy values in a table — you do not need to memorise them.)
Worked Example: Combustion of Methane
CH₄ + 2O₂ → CO₂ + 2H₂O
Step 1: Bonds Broken (Reactants)
- CH₄: 4 × C−H bonds = 4 × 413 = 1652 kJ
- 2O₂: 2 × O=O bonds = 2 × 498 = 996 kJ
Total energy in (breaking) = 1652 + 996 = 2648 kJ
Step 2: Bonds Formed (Products)
- CO₂: 2 × C=O bonds = 2 × 805 = 1610 kJ
- 2H₂O: 4 × O−H bonds = 4 × 464 = 1856 kJ
Total energy out (forming) = 1610 + 1856 = 3466 kJ
Step 3: Overall Energy Change
Energy change = 2648 − 3466 = −818 kJ/mol
The answer is negative, so the reaction is exothermic. This makes sense — combustion always releases energy.
Worked Example: Breaking Down Hydrogen Chloride
H₂ + Cl₂ → 2HCl
Bonds broken: 1 × H−H (436) + 1 × Cl−Cl (243) = 679 kJ
Bonds formed: 2 × H−Cl (432) = 864 kJ
Energy change = 679 − 864 = −185 kJ/mol (exothermic)
Important Notes
- Bond energies are averages — the actual energy of a bond varies slightly depending on the molecule it is in
- Bond energy calculations work best for gaseous reactions — they are less accurate when liquids or solids are involved because additional energy is needed for changes of state
- At GCSE, you will always be given the bond energies in the question
Linking to Energy Profiles
- In an exothermic reaction (negative energy change), the energy profile shows products lower than reactants — consistent with more energy being released in forming bonds than taken in to break them
- In an endothermic reaction (positive energy change), the products are higher — more energy was needed to break bonds than was released in forming them
Exam Tips
- Always write out the full structural formulae showing every bond — do not miss any
- Count bonds carefully: CH₄ has 4 C−H bonds, not 1; 2H₂O has 4 O−H bonds
- Remember: breaking = endothermic (energy IN), making = exothermic (energy OUT)
- The formula is: energy change = bonds broken − bonds made
- Negative = exothermic, positive = endothermic
- Show all working — marks are awarded for method even if the arithmetic goes wrong