3.2.2Chemical Reactions
Calculating Energy Changes — OCR Gateway GCSE Combined Science A J250 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 11 of 20
Part 1 of 4Breaking and making bonds (HT only)
Bond energy
Bond breaking
- Bond breaking Energy must be supplied to break bonds.
- It is taken in from the surroundings, so bond breaking is endothermic.
Bond making
- Bond making Energy is released when new bonds form, so bond making is exothermic.
Both happen together
- Both happen together Every reaction involves breaking the bonds in the reactants and making the bonds in the products, for example N2 + 3H2 → 2NH3.
Bond energy
- Bond energy Each type of bond has a mean bond energy, measured in kJ/mol, which is the energy needed to break it or released when it forms.
Part 2 of 4Overall energy change (HT only)
| Type of reaction | Comparison | Energy change |
|---|---|---|
| Exothermic | Energy released making bonds is greater than energy taken in breaking bonds | Negative |
| Endothermic | Energy taken in breaking bonds is greater than energy released making bonds | Positive |
Part 3 of 4Calculation method (HT only)
1
Draw out the molecules so that every bond in the reactants and the products is visible.
2
Add up the bond energies of all the bonds broken in the reactants. This is the energy in.
3
Add up the bond energies of all the bonds made in the products. This is the energy out.
4
Calculate: energy change = energy in − energy out.
5
A negative answer means exothermic, a positive answer means endothermic. Give the unit kJ/mol.
Part 4 of 4Worked example (HT only)
Question
- Question H2 + Cl2 → 2HCl. Bond energies in kJ/mol: H–H 436, Cl–Cl 243, H–Cl 432.
Energy in
- Energy in Bonds broken: one H–H and one Cl–Cl, so 436 + 243 = 679 kJ/mol.
Energy out
- Energy out Bonds made: two H–Cl, so 2 × 432 = 864 kJ/mol.
Answer
- Answer Energy change = 679 − 864 = −185 kJ/mol.
- The value is negative, so the reaction is exothermic: more energy is released making bonds than is needed to break them.
Key terms
| Term | Meaning |
|---|---|
| Exothermic | A reaction that releases energy to the surroundings. |
| Endothermic | A reaction that takes in energy from the surroundings. |
| Bond energy | The mean energy needed to break one mole of a particular bond. |
| Energy in | The total energy needed to break all the bonds in the reactants. |
| Energy out | The total energy released forming all the bonds in the products. |
Avoid these mistakes
1
Mixing up which process is exothermic. Breaking bonds takes in energy (endothermic) and making bonds releases it (exothermic).
2
Subtracting the wrong way round. It is energy in minus energy out.
3
Counting each bond type only once instead of multiplying by the number of bonds, for example forgetting 3 H–H bonds in 3H2.
4
Leaving out the sign or the unit of the answer, so the type of reaction is unclear.
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