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SAChemistrySyllabus dot point

What factors control how fast a reaction proceeds, and why?

Use collision theory to explain the effect of concentration, surface area, temperature and pressure on reaction rate.

How collision theory explains reaction rate, and how concentration, surface area, temperature and pressure affect the frequency and energy of effective collisions.

Generated by Claude Opus 4.77 min answer

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  1. What this dot point is asking
  2. Collision theory
  3. Concentration
  4. Surface area
  5. Temperature
  6. Pressure (gases)

What this dot point is asking

You must use collision theory to explain the effect of each factor on rate, distinguishing effects on collision frequency from effects on collision energy.

Collision theory

For a reaction to happen, particles must collide. But not every collision reacts.

The rate of reaction depends on how many successful collisions occur per second.

Concentration

Increasing the concentration of a reactant packs more particles into the same volume, so collisions happen more often. More collisions per second means more successful collisions per second, so the rate increases.

Surface area

For a solid reactant, only the surface particles can collide with the other reactant. Breaking the solid into smaller pieces (or a powder) exposes more surface area, increasing collision frequency and rate.

Temperature

Raising the temperature has two effects:

  1. Particles move faster, so they collide more often.
  2. More importantly, a larger fraction of particles have energy ≥Ea\geq E_a, so a greater proportion of collisions are successful.

The second effect dominates, which is why even a small temperature rise (around 10 ∘C10\ ^\circ\text{C}) can roughly double the rate.

Pressure (gases)

Increasing the pressure of a gas (by reducing the volume) pushes the particles closer together, increasing concentration and therefore collision frequency. The rate increases. Pressure has no effect on rate for reactions of solids or solutions.

Exam-style practice questions

Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

2023 SACE Stage 23 marksMicrobes synthesise GABA by fermentation. In terms of collision theory, explain the effect of increasing the reaction temperature from 10 degrees C to 35 degrees C.
Show worked answer →

Increasing temperature increases the rate, and collision theory explains why.

  1. At a higher temperature the reactant particles have greater average kinetic energy and move faster, so they collide more frequently.

  2. More importantly, a greater proportion of particles have energy equal to or greater than the activation energy.

  3. Therefore a greater fraction of collisions are successful (effective), and the overall rate of reaction increases (more GABA is produced in a given time). One mark each. (Note: above an optimum temperature the enzyme/microbe may be denatured, which is why this only holds up to about 35 degrees C.)

2024 SACE Stage 23 marksExplain why the concentration of NO produced is decreased when the temperature inside the engine is lower.
Show worked answer →

NO forms from N2 and O2 (N2 + O2 -> 2NO), a reaction with a very high activation energy.

  1. At a lower temperature, the reactant particles (N2 and O2) have lower average kinetic energy.

  2. A smaller proportion of colliding particles possess the high activation energy needed to react, and collisions are less frequent.

  3. Therefore fewer successful collisions occur per second, the rate of NO formation falls, and less NO is produced. One mark per linked point.

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