Explaining Rates Using Collision Theory
Section: 6. Chemical Reactions | Syllabus: Cambridge AS Level Physics 9702
Using Collision Theory to Explain Rate Changes Collision theory can be used to explain WHY changing conditions affects the rate of reaction. The key is to explain the effect on collision frequency and/or the proportion of successful collisions .
For exam answers, you should always: State how the factor affects the particles (e.g., "particles move faster") Explain the effect on collisions (e.g., "more frequent collisions") Link to successful collisions (e.g., "more successful collisions per second") State the effect on rate (e.g., "rate increases") Explaining Effect of TEMPERATURE Increasing temperature INCREASES rate of reaction Full collision theory explanation: When temperature increases: Particles have more kinetic energy and move faster This leads to more frequent collisions between particles More importantly, a greater proportion of particles now have energy ≥ activation energy (Ea) Therefore, there are more successful collisions per second So the rate of reaction increases Key point: The increase in the proportion of particles with E ≥ Ea has a MUCH BIGGER effect than the increase in collision frequency.
This is why a 10°C rise can double the rate! Explaining Effect of CONCENTRATION (Solutions) Increasing concentration INCREASES rate of reaction Full collision theory explanation: When concentration increases: There are more particles (of reactant) in the same volume Particles are closer together This leads to more frequent collisions Therefore, there are more successful collisions per second So the rate of reaction increases Note: Concentration affects collision FREQUENCY only.
It does NOT change the proportion of particles with E ≥ Ea (that stays the same). Explaining Effect of PRESSURE (Gases) Increasing pressure INCREASES rate of reaction (for gases) Full collision theory explanation: When pressure increases: The same number of gas particles are forced into a smaller volume Gas particles are closer together / more concentrated This leads to more frequent collisions Therefore, there are more successful collisions per second So the rate of reaction increases Note: Pressure (for gases) works in exactly the same way as concentration (for solutions) - both increase collision frequency by bringing particles closer together.
Explaining Effect of SURFACE AREA (Solids) Increasing surface area INCREASES rate of reaction (for solids) Full collision theory explanation: When surface area increases (e.g., using powder instead of lumps): There is more surface exposed for particles to collide with More reactant particles are available at the surface for collisions This leads to more frequent collisions Therefore, there are more successful collisions per second So the rate of reaction increases Example: Powdered calcium carbonate has much more surface area than a single lump of the same mass.
Many more acid particles can collide with carbonate particles at once. Explaining Effect of CATALYST Adding a catalyst INCREASES rate of reaction Full collision theory explanation: When a catalyst is added: The catalyst provides an alternative reaction pathway This pathway has a lower activation energy (Ea) A greater proportion of particles now have energy ≥ the new, lower Ea Therefore, there are more successful collisions per second So the rate of reaction increases The catalyst is not used up and can be reused Important: Catalysts do NOT affect collision frequency.
They ONLY increase the proportion of successful collisions by lowering Ea. Comparing All Five Factors - Collision Theory Explanations Factor Effect on Particles Effect on Collisions Why Rate Increases Temperature ↑ Particles move faster, more kinetic energy More frequent + more energetic collisions More particles have E ≥ Ea → more successful collisions/sec Concentration ↑ More particles in same volume More frequent collisions More collisions overall → more successful collisions/sec Pressure ↑ Same particles in smaller volume More frequent collisions More collisions overall → more successful collisions/sec Surface area ↑ More surface exposed More frequent collisions (at surface) More collisions possible → more successful collisions/sec Catalyst added Provides lower Ea pathway Same collision frequency, but more successful Greater proportion have E ≥ Ea → more successful collisions/sec What Changes and What Doesn't Factor Changed Collision Frequency % Particles with E ≥ Ea Temperature INCREASES (slightly) INCREASES (significantly) Concentration INCREASES No change Pressure INCREASES No change Surface area INCREASES No change Catalyst No change INCREASES (Ea lowered) Common Exam Question Types Q: Explain why increasing temperature increases rate of reaction.
Good answer: When temperature increases, particles have more kinetic energy and move faster. This causes more frequent collisions. More importantly, a greater proportion of particles now have energy greater than or equal to the activation energy.
Therefore, there are more successful collisions per second, so t…
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