Section 1 of 9What activation energy is
Energy barrier
- Energy barrier The activation energy is the minimum energy that colliding particles need for a reaction to take place.
- It is needed to break existing bonds in the reactants.
- On a profile, a higher peak means a larger activation energy and a slower reaction at a given temperature.
Different reactions, different barriers
- Different reactions, different barriers A reaction with a low activation energy happens easily at room temperature;
- a high one needs heating or a spark.
Examples
- Examples A match needs friction to supply the activation energy, then keeps burning on its own because heat from combustion supplies more.
Section 2 of 9Reading a reaction profile
1
The vertical axis shows the energy of the chemicals, and the horizontal axis shows the progress of the reaction.
2
The reactants start at one energy level, drawn as a horizontal line.
3
The curve rises to a peak; the height from reactants to the peak is the activation energy.
4
It then falls to the energy level of the products.
5
The difference between reactants and products levels is the overall energy change, ΔH.
Section 3 of 9Exothermic and endothermic profiles
| Feature | Exothermic | Endothermic |
|---|---|---|
| Energy of products | Lower than reactants | Higher than reactants |
| Overall energy change | Energy given out; temperature rises | Energy taken in; temperature falls |
| Sign of ΔH | Negative | Positive |
| Examples | Combustion, neutralisation | Thermal decomposition, photosynthesis |
Section 4 of 9Catalysts and the profile
1Catalyst added
- Catalyst added Provides an alternative route for the reaction.
2Lower peak
- Lower peak The activation energy is reduced, shown by a lower curve on the profile.
3More successful collisions
- More successful collisions A greater proportion of collisions now have enough energy.
4Same products
- Same products The reactant and product energy levels are unchanged, so ΔH is the same.
Section 5 of 9Bond energy and the energy change
Why reactions have an energy change
Breaking bonds
- Breaking bonds Energy must be supplied to break bonds in the reactants, so bond breaking is endothermic.
Making bonds
- Making bonds Energy is released when new bonds form in the products, so bond making is exothermic.
Overall change
- Overall change If more energy is released making bonds than is needed to break bonds, the reaction is exothermic overall (HT only: calculate ΔH from bond energies).
Section 6 of 9Worked example: reading a profile
| Measurement | Value (kJ/mol) | How it is found |
|---|---|---|
| Energy of reactants | 100 | Read from the left-hand level |
| Energy at the peak | 180 | Highest point of the curve |
| Activation energy | 180 − 100 = 80 | Peak minus reactants |
| Energy of products | 40 | Read from the right-hand level |
| Energy change ΔH | 40 − 100 = −60 | Products minus reactants; negative so exothermic |
Section 7 of 9Everyday examples
Fuels
- Fuels Petrol and air do not react until a spark gives the first molecules enough energy;
- the energy released then keeps the reaction going.
Cold packs
- Cold packs Instant cold packs use an endothermic process, so the contents take in energy and the pack feels cold.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Activation energy | The minimum energy needed for particles to react on collision. |
| Reaction profile | A diagram showing how the energy of chemicals changes during a reaction. |
| Exothermic | A reaction that gives out energy to the surroundings. |
| Endothermic | A reaction that takes in energy from the surroundings. |
| Catalyst | A substance that speeds up a reaction by lowering the activation energy, without being used up. |
| Bond energy | The energy needed to break one mole of a particular covalent bond. |
| Energy change (ΔH) | The overall difference in energy between products and reactants. |
Section 9 of 9Exam technique
1
Measuring activation energy from the product level; measure it from the reactant level up to the peak.
2
Saying a catalyst changes the overall energy change of a reaction.
3
Labelling the energy change of an endothermic reaction as negative.
4
Forgetting that even exothermic reactions need activation energy to start.
5
Writing ΔH as the height of the peak rather than the difference between the reactant and product levels.