1. Collision Theory and Activation Energy
The change in concentration of a reactant or product per unit time. Its units are usually mol dm⁻³ s⁻¹.
For a chemical reaction to occur between reacting particles, they must satisfy two essential conditions:
- Particles must collide with kinetic energy equal to or greater than the activation energy (\(E \ge E_a\)).
- Particles must collide with the correct steric orientation.
The minimum kinetic energy required for colliding reactant particles to result in a successful reaction.
Why Most Collisions Do Not Lead to a Reaction
- Most colliding particles have energy less than \(E_a\). They bounce apart unchanged.
- Some collisions have enough energy, but the particles are not correctly oriented. The bonds that need to break are not lined up.
- Only collisions with \(E \ge E_a\) and the correct orientation are successful.
2. The Maxwell-Boltzmann Distribution
In any gas or liquid sample, molecular speeds vary across a broad spectrum due to random collisions. The energy distribution is described by the Maxwell-Boltzmann curve:
Mathematical Characteristics
- Starts at the origin (0,0): No molecules have zero kinetic energy.
- Peak of the curve: Corresponds to the most probable energy (\(E_{mp}\)).
- Mean energy: Located slightly to the right of the peak because of the extended high-energy tail.
- Asymptotic tail: The curve never touches the x-axis at high energy because there is no theoretical upper limit to molecular velocity.
- Area under the curve: Represents the total number of particles in the sample.
3. The Effect of Temperature on Reaction Rate
When the temperature of a reaction mixture increases from \(T_1\) to \(T_2\):
- Molecules gain kinetic energy and move faster.
- The distribution curve flattens and broadens; the peak shifts to the right and downwards.
- The total area under the curve remains identical (total particle count is constant).
- The fraction of molecules possessing energy equal to or greater than the activation energy (\(E \ge E_a\)) increases significantly (shaded area expands dramatically).
- Consequently, the frequency of successful collisions increases markedly, producing a sharp rise in reaction rate.
- A small rise in temperature gives a large increase in the number of particles with \(E \ge E_a\). This is the main effect.
- Particles also move faster, so collisions become more frequent. This effect is much smaller.
4. The Effect of Concentration and Pressure
- A higher concentration means more particles in the same volume.
- So collisions between reactant particles are more frequent.
- So there are more collisions with \(E \ge E_a\) per unit time, and the rate increases.
- For gases, a higher pressure has the same effect. The same number of gas particles is squeezed into a smaller volume.
The temperature does not change, so the fraction of particles with \(E \ge E_a\) stays the same. Only the collision frequency increases.
| Change | Collision frequency | Fraction of particles with \(E \ge E_a\) |
|---|---|---|
| Higher concentration | Increases | No change |
| Higher pressure (gases) | Increases | No change |
| Higher temperature | Increases a little | Increases a lot |
| Add a catalyst | No change | Increases (lower \(E_a\)) |
5. Catalysts and Reaction Pathways
A substance that increases the rate of a chemical reaction without undergoing permanent chemical change or being consumed.
Mechanism of Action
A catalyst provides an alternative reaction pathway with a lower activation energy (\(E_{cat} < E_a\)).
On the Maxwell-Boltzmann distribution, the threshold line shifts to the left from \(E_a\) to \(E_{cat}\). A much larger proportion of colliding particles now possess sufficient energy to react successfully upon collision.
A Catalyst on One Maxwell-Boltzmann Curve
A catalyst does not change the temperature, so the curve stays the same. Draw one curve with two \(E_a\) lines:
- The curve does not change shape, because the temperature is the same.
- The catalyst lowers \(E_a\), so the \(E_a\) line moves to the left.
- The area to the right of the new line is larger. More particles have \(E \ge E_a\), so there are more successful collisions per unit time.
Go deeper
These UK AQA pages cover the same ideas in more detail. They may include content that is not in 9620.
6. Practice Questions
Show Answer & Mark Scheme
Mark Scheme:
- At higher temperature, average kinetic energy increases, shifting the Maxwell-Boltzmann distribution peak right and down. (1 mark)
- A much larger fraction of particles possess kinetic energy equal to or greater than the activation energy (\(E \ge E_a\)). (1 mark)
- Therefore, the frequency of successful collisions increases significantly, far outweighing the small increase in total collision rate. (1 mark)
Show Answer & Mark Scheme
Mark Scheme:
- The change in concentration of a reactant or product per unit time. (1 mark)
- mol dm⁻³ s⁻¹. (1 mark)
Show Answer & Mark Scheme
Mark Scheme:
- There are more particles in the same volume, so collisions are more frequent. (1 mark)
- So there are more collisions with \(E \ge E_a\) per unit time. (1 mark)
Show Answer & Mark Scheme
Mark Scheme:
- The catalyst provides an alternative route with a lower activation energy. (1 mark)
- The curve does not change, but more particles now have \(E \ge E_a\) (a larger area to the right of the new \(E_a\) line). (1 mark)
- So there are more successful collisions per unit time. (1 mark)