More frequent collisions (more collisions per second).
MORE IMPORTANTLY: a greater PROPORTION of particles have energy ≥ the activation energy.
More successful collisions → faster rate.
Rule of thumb: increasing temperature by 10°C approximately DOUBLES the rate of many reactions.
DECREASING TEMPERATURE slows the reaction rate — used in food preservation (refrigeration slows bacterial growth and chemical spoilage).
Temperature has a BIGGER effect than just increasing collision frequency — it mainly increases the proportion of successful (high-energy) collisions.
Concentration and Pressure
INCREASING CONCENTRATION of a solution increases reaction rate.
Why:
More particles in the same volume.
Particles are closer together.
More frequent collisions per unit time.
More successful collisions → faster rate.
Example: doubling concentration approximately doubles the number of collisions.
INCREASING PRESSURE of GASES increases reaction rate:
Gas particles are compressed into a smaller volume.
Particles are closer together.
More frequent collisions → faster rate.
Same principle as increasing concentration — more particles per unit volume.
Note: pressure only affects GASEOUS reactions — has no significant effect on reactions in solution.
Surface Area
INCREASING SURFACE AREA (by using smaller pieces/powder) increases reaction rate.
Why:
More surface area exposed to reactant particles.
More collisions possible per unit time.
More of the reactant is accessible → faster rate.
This is why POWDERS react faster than LUMPS of the same mass:
A 1 g marble chip: small surface area → slow reaction with acid.
The same 1 g crushed to powder: massive increase in surface area → much faster reaction with acid.
EXAMPLES:
Powdered coal dust is EXPLOSIVE — high surface area. Lump coal burns slowly.
Flour dust in mills can cause explosions.
Coal fires — smaller pieces of coal burn faster.
Surface area investigation: marble chips vs powder in HCl — compare gas produced over time.
⚠️ Common Mistake
All four factors (temperature, concentration, surface area, pressure for gases) increase rate by increasing the NUMBER OF SUCCESSFUL COLLISIONS. Temperature is special — it also increases the PROPORTION of particles with sufficient energy (≥ activation energy), making it particularly effective. Always explain in terms of collisions.
📌 Key Note
Higher temperature → faster particles, more collisions, more above activation energy. Higher concentration → more particles per volume, more collisions. Larger surface area → more exposed particles, more collisions. Higher pressure (gases) → more particles per volume, more collisions. All increase rate by increasing successful collisions.
🎯 Matching Activity — Factor → Effect on Rate
Match each factor change to its effect on reaction rate and why. — drag the symbols on the right to match the component names on the left.
Increase temperature
Drop here
Increase concentration
Drop here
Use powder instead of lumps
Drop here
Increase pressure (gases)
Drop here
Decrease temperature
Drop here
Rate decreases — fewer particles have enough energy to react successfully
Rate increases — more particles per unit volume, more frequent collisions
Rate increases — more particles exceed activation energy, more frequent and more energetic collisions
Rate increases — greater surface area exposes more particles to collisions
Rate increases — gas particles compressed into smaller volume, more frequent collisions
🧪 Required Practical
🔬 RP6 (Chemistry) — Investigate effect of concentration on rate (Na₂S₂O₃ + HCl). Can also investigate surface area (marble chips vs powder with HCl) or temperature effects.
Know the method, variables, equipment and how to analyse results.
🎯 Test Yourself
Question 1 of 2
1. Why does increasing temperature increase reaction rate more than just increasing the frequency of collisions?
2. Marble chips (CaCO₃) react with HCl. The experiment is repeated with the same mass of powdered marble. How does the rate change?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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