Effect of Concentration on Rate

1. Collision Theory Recap

• Chemical reactions only happen when particles collide with enough energy.

• The rate of reaction depends on the frequency of successful collisions (collisions per second).

Graph shape explained:

• Fast at start → many reactant particles → frequent collisions.

• Slows down → fewer reactant particles left → fewer collisions.

• Stops → all reactants used → no collisions.

2. Effect of Concentration (solutions)

• Higher concentration = more particles in the same volume.

• This increases the chance of collisions.

• Therefore, rate of reaction increases.

👉 Rate ∝ Concentration

3. Effect of Pressure (gases)

• Higher pressure = gas particles are squeezed closer together.

• This increases collision frequency.

• Therefore, rate of reaction increases.

👉 Rate ∝ Pressure

4. Comparing Graphs of Product Formed vs. Time

• Higher concentration / pressure:

  • Steeper line (faster rate).

  • Higher final level of product (more reactants to start with).

• Lower concentration / pressure:

  • Shallower line (slower rate).

  • Lower final product (fewer reactants at start).

5. Summary

• Increasing concentration (solutions) or pressure (gases) → more frequent successful collisions.

• Rate of reaction is directly proportional to concentration or pressure.

• Graphs show steeper slopes and more product formed with higher concentration/pressure.