Section 1 of 7Two opposite processes
Breaking bonds
- Breaking bonds Energy must be supplied to pull atoms apart, so breaking bonds is an endothermic process.
- Every reaction begins with the bonds in the reactants being broken.
Making bonds
- Making bonds Energy is released to the surroundings when atoms join together, so making bonds is an exothermic process.
- Every reaction finishes with new bonds forming in the products.
Which wins?
- Which wins? The overall change depends on the balance between the energy taken in and the energy given out.
Section 2 of 7Linking the balance to the type of reaction
| Reaction type | Bond energy comparison | Overall energy change |
|---|---|---|
| Exothermic | Energy released making bonds is greater than energy needed to break bonds | Negative |
| Endothermic | Energy needed to break bonds is greater than energy released making bonds | Positive |
Section 3 of 7Calculating the energy change
1
Write out the balanced equation and draw the displayed formulae so that every bond can be counted.
2
Add up the bond energies of all the bonds in the reactants. This is the energy taken in.
3
Add up the bond energies of all the bonds in the products. This is the energy given out.
4
Use: energy change = energy taken in − energy given out, with the answer in kJ/mol.
5
A negative answer means exothermic; a positive answer means endothermic.
Section 4 of 7Worked example
1Reaction
- Reaction H2 + Br2 → 2HBr with bond energies H–H 436, Br–Br 193 and H–Br 366 kJ/mol.
2Energy in
- Energy in Bonds broken: 436 + 193 = 629 kJ.
3Energy out
- Energy out Bonds made: 2 × 366 = 732 kJ.
4Energy change
- Energy change 629 − 732 = −103 kJ.
- The sign is negative, so the reaction is exothermic because more energy is released than is absorbed.
Section 5 of 7Linking to energy level diagrams
Bond energy and the diagram
Climbing the peak
- Climbing the peak The rise to the top of the curve relates to energy taken in to break the bonds in the reactants.
Falling down
- Falling down The fall afterwards relates to energy released as new bonds form.
Net difference
- Net difference If the fall is bigger than the rise, the products finish lower than the reactants and the reaction is exothermic.
Ammonia example
- Ammonia example In making ammonia from nitrogen and hydrogen, the bonds broken need 2253 kJ and the bonds made release 2346 kJ,
- so the overall change is −93 kJ.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Bond energy | The energy needed to break one mole of a particular covalent bond, which is also the energy released when it forms. |
| Energy taken in | The total of the bond energies of all the bonds broken in the reactants. |
| Energy given out | The total of the bond energies of all the bonds formed in the products. |
| Exothermic | A reaction in which more energy is released making bonds than is used breaking bonds. |
| Endothermic | A reaction in which more energy is used breaking bonds than is released making bonds. |
Section 7 of 7Exam technique
1
Reversing the formula and calculating energy out minus energy in, which flips the sign of the answer.
2
Forgetting to multiply a bond energy by the number of such bonds, such as two H–Br bonds in 2HBr.
3
Saying that bond breaking releases energy; breaking bonds always needs energy to be supplied.
4
Not counting every bond in a molecule, for example missing that CH4 contains four C–H bonds.
5
Giving a final answer without units or without a sign, and then not stating whether the reaction is exothermic or endothermic.