1.6.4Physical Chemistry
Effect of Concentration and Pressure
5 min read 6 parts Lesson 33 of 63
Section 1 of 6Concentration
Effect
- Increasing the concentration of a reactant in solution increases the rate of reaction.
Explanation
- There are more particles in each unit volume, so they collide more often.
Successful collisions
- The frequency of successful collisions rises in proportion, although the proportion with energy ≥ Ea is unchanged.
Change with time
- As reactants are used up their concentration falls, so the rate gradually decreases.
Section 2 of 6Pressure of Gases
Pressure of Gases
Effect
- Increasing the pressure of a gaseous reaction mixture increases the rate.
Explanation
- The same number of molecules occupies a smaller volume, so the concentration is higher.
Collisions
- Molecules are closer together and collide more often per second.
Not for solids and liquids
- Pressure has almost no effect on the rate of reactions involving only solids and liquids.
Section 3 of 6Surface Area
Heterogeneous reactions
- When a solid reacts with a liquid or gas, reaction occurs only at the surface of the solid.
Smaller pieces
- Dividing a solid into smaller pieces exposes more surface for a given mass.
Effect
- A larger surface area gives more collisions per second with the other reactant and so a faster reaction.
Example
- Powdered marble reacts with acid faster than the same mass of marble chips.
Section 4 of 6Maxwell–Boltzmann Link
1
Changing concentration or pressure does not change the shape of the Maxwell–Boltzmann curve at constant temperature.
2
The proportion of molecules with energy of at least Ea therefore stays the same.
3
The number of molecules per unit volume increases, so the number of successful collisions per second rises.
4
The rate of reaction therefore increases because the collision frequency has increased.
5
State clearly that the activation energy is not changed by these factors.
Section 5 of 6Comparing the Factors
A summary of how each factor affects reaction rate helps in extended answers.
Concentration and pressure
- Both raise the number of particles in a given volume and so the collision frequency.
Surface area
- This helps only for solids, as it increases the area exposed to the other reactant.
Temperature
- This affects both the frequency of collisions and the proportion with sufficient energy.
Catalyst
- This changes the energy needed rather than the number of collisions.
| Factor | Effect on collisions | Effect on Ea |
|---|---|---|
| Concentration up | More frequent | None |
| Gas pressure up | More frequent | None |
| Surface area up | More frequent at surface | None |
| Temperature up | More frequent and more energetic | None |
| Catalyst added | More successful | Lowered route |
Section 6 of 6Exam Checklist & Tips
1.6.4 – Key Points for the Exam
- Concentration The amount of a substance per unit volume, such as mol dm⁻³.
- Collision frequency The number of collisions between particles per second.
- Surface area The total area of a solid that is exposed to reaction.
- Heterogeneous reaction A reaction in which the reactants are in different phases.
Common Mistakes
- Saying that more concentrated solutions have particles with more energy.
- Saying that increasing pressure changes the activation energy.
- Writing that there are 'more collisions' without stating 'per unit time' or 'per second'.
- Applying the effect of pressure to a reaction of solids and liquids.