5.1.3Energy from Chemicals
Bond Breaking and Bond Forming — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 3 of 3 Unit quiz
Part 1 of 5Two opposite processes (HT only)
Breaking bonds
- Breaking bonds Energy must be supplied to pull atoms apart, so bond breaking is always endothermic.
- The energy is taken in from the surroundings.
Making bonds
- Making bonds When atoms join and a new bond forms, energy is given out to the surroundings, so bond making is always exothermic.
Every reaction does both
- Every reaction does both In any reaction the bonds in the reactants are broken and new bonds are made in the products,
- so the overall result depends on which of the two energy totals is bigger.
Part 2 of 5Deciding the type of reaction (HT only)
1Add up energy in
- Add up energy in Total the energy needed to break every bond in the reactants.
2Add up energy out
- Add up energy out Total the energy released by making every bond in the products.
3Compare
- Compare Subtract: ΔH = energy to break bonds − energy released on making bonds.
4Conclude
- Conclude If the result is negative the reaction is exothermic. If it is positive the reaction is endothermic.
Part 3 of 5Exothermic and endothermic compared (HT only)
| Feature | Exothermic | Endothermic |
|---|---|---|
| Energy comparison | More released making bonds than taken in breaking bonds | More taken in breaking bonds than released making bonds |
| Products compared with reactants | Lower energy | Higher energy |
| ΔH | Negative | Positive |
| Temperature of surroundings | Rises | Falls |
Part 4 of 5Worked example: hydrogen and chlorine (HT only)
1
Equation: H2 + Cl2 → 2HCl. Use bond energies of H–H = 436, Cl–Cl = 243 and H–Cl = 432 kJ/mol.
2
Energy taken in to break bonds: 436 + 243 = 679 kJ.
3
Energy released making two H–Cl bonds: 2 × 432 = 864 kJ.
4
ΔH = 679 − 864 = −185 kJ/mol.
5
The sign is negative, so the reaction is exothermic: more energy is released than was needed to break the bonds.
Part 5 of 5Linking ideas
Link to pathway diagrams
- Link to pathway diagrams The activation energy relates to the energy needed to start breaking bonds,
- while ΔH is the overall difference between breaking and making.
Heat felt in the surroundings
- Heat felt in the surroundings In an exothermic change the extra energy released on forming bonds is transferred to the surroundings as heat,
- which is why a flask feels warm.
Key terms
| Term | Meaning |
|---|---|
| Bond energy | The energy needed to break one mole of a particular covalent bond; the same amount is released when it forms. |
| Endothermic | Taking in energy from the surroundings; ΔH is positive. |
| Exothermic | Releasing energy to the surroundings; ΔH is negative. |
| ΔH | The overall enthalpy change of a reaction, in kJ/mol. |
| Surroundings | Everything outside the reacting chemicals, including the air and the container. |
Avoid these mistakes
1
Saying bond breaking releases energy; breaking bonds always needs energy to be supplied.
2
Subtracting in the wrong order and getting the wrong sign: use (energy to break) − (energy released on making).
3
Forgetting to multiply the bond energy by the number of bonds, for example two H–Cl bonds in the product.
4
Writing that energy is 'made' or 'destroyed' in a reaction; it is only transferred.
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