Factors Affecting Rate of Reaction

This lesson builds on effective collisions and activation energy. Revise Collision Theory first if those ideas are unfamiliar.

1. Definition

Factors affecting rate of reaction are conditions that change how fast reactants are used up or products are formed by changing the number of effective collisions per unit time.

2. Key Ideas

• Factors that increase rate do one (or both) of these:
  • increase collision frequency (more collisions per unit time), and/or
  • increase the proportion of collisions that are effective (energy ≥ Eₐ).
• These are the syllabus factors:
  • Concentration (solutions) and pressure (gases): more particles per unit volume → more collisions per unit time.
  • Particle size (solids): smaller particles give a larger total surface area → more collisions at the surface per unit time.
  • Temperature: particles move faster and more particles have energy ≥ Eₐ.
• Catalysts act through a different mechanism: see catalysts and enzymes.

3. Detailed Explanations

A. Concentration (solutions)

For a reacting solute, increasing concentration means more reactant particles per unit volume. In the collision model, the rate increases because:

• there are more reactant particles per unit volume,
• particles are closer together,
• so there are more frequent collisions per unit time,
• so there are more frequent effective collisions per unit time (faster rate).

Acid concentration and gas-production rate

Two illustrative cumulative gas-volume curves. Both reach 40 cm³ and remain there through 120 s. The higher-concentration trial reaches its plateau sooner.

Scroll across the graph to read all labels.

Open full-size graph

View figure data

Values for Acid concentration and gas-production rate

Time (s)	Lower concentration	Higher concentration
0	0	0
10	6	12
20	12	22
30	18	29
40	24	34
50	29	37
60	33	39
70	36	40
80	38	40
90	40	40
120	40	40

Values for Acid concentration and gas-production rate

Time (s)	Lower concentration	Higher concentration
0	0	0
10	6	12
20	12	22
30	18	29
40	24	34
50	29	37
60	33	39
70	36	40
80	38	40
90	40	40
120	40	40

These constructed curves compare equal marble portions with acid at different concentrations. Acid is in excess in both trials; marble mass and particle size, acid volume, temperature and gas-collection conditions are held fixed. The more concentrated acid gives a greater initial rate, but the same marble amount gives the same final gas volume. Changing concentration does not always preserve final amount: check which reactant limits the reaction.

B. Pressure (gases)

For a fixed amount of reacting gas at constant temperature, compressing it into a smaller volume raises pressure and usually increases the rate in the collision model because:

• gas particles are forced closer together (same number of particles in a smaller volume),
• so there are more collisions per unit time,
• so there are more frequent effective collisions per unit time.

C. Surface area (solids)

For reactions involving a solid, the syllabus factor is particle size; the collision-theory explanation uses the resulting change in total surface area.

For the same mass of solid, decreasing particle size (for example, powdering it) increases rate because:

• total surface area increases,
• more solid particles are exposed to the other reactant,
• so there are more collisions at the surface per unit time,
• so there are more frequent effective collisions per unit time.

Compare equal masses of large chips and powder while keeping acid concentration and temperature fixed.

In this ideal model, one cube of edge 2a has six faces with total area 6(2a)² = 24a². Eight separate cubes of edge a expose 8 × 6a² = 48a². Their combined volume is still 8a³, so the same solid has twice the exposed surface. The new cut faces matter only when the other reactant can reach them; pieces pressed tightly together do not expose every face.

D. Temperature

For the reactions described by this model, increasing temperature usually increases rate because:

• particles have more kinetic energy (energy of motion) and move faster,
• so there are more collisions per unit time, and
• a larger proportion of particles have energy ≥ Eₐ, so a larger proportion of collisions are effective,
• so there are more frequent effective collisions per unit time.

E. Catalysts

Catalysts change the reaction pathway rather than reactant concentration, particle size or temperature. Continue with Catalysts and Enzymes for lower activation energy, industrial examples and biological catalysts.

4. Common Mistakes

• Writing “rate increases because concentration/pressure is higher” with no particle explanation.
• Treating total gas pressure as sufficient evidence: check that the concentration of reacting gas particles has increased.
• Using “particle size” without linking it to surface area and collisions at the surface.
• Explaining temperature using only “more collisions” and forgetting “larger proportion have energy ≥ Eₐ”.
• Saying “a catalyst is used up” or “a catalyst gives particles energy” (both false).
• Confusing “faster rate” with “more products”. Rate describes how quickly change occurs; any yield conclusion must come from the reaction information or supplied data.

5. Exam Tips

• In a fair test, change the intended factor and hold the others fixed. For a concentration comparison, use different concentrations but the same acid volume, marble mass and particle size, temperature and apparatus.
• For temperature, distinguish greater particle energy from a changed activation-energy barrier.

6. Worked Examples

Catalyst: what changes and what stays the same

This exercise now belongs with the pathway explanation: Catalyst: what changes and what stays the same.

7. Mind Stretchers

8. Practise and check