Particles can only react when they hit each other, and only if they hit hard enough. Every factor that speeds up a reaction does it by making successful collisions more frequent. This is the explanation examiners want. Extended content.
Extended. Particles react only when they collide with at least the activation energy. Higher concentration, higher pressure and larger surface area put more particles in contact, so the frequency of collisions increases. Higher temperature makes particles move faster: they collide more often and, more importantly, more collisions have energy at least equal to the activation energy. A catalyst provides a pathway with a lower activation energy, so more collisions are successful.
Fig. 1: a catalyst lowers the activation energy
Section 01
Successful collisions.
Extended content. Watch the particles in the simulator. Only some collisions flash as successful. Increase the concentration, then the temperature, and count the flashes.
Collision needed: particles must collide to react.
Enough energy: the collision must have at least the activation energy, Ea. Weaker collisions just bounce apart.
Rate depends on the frequency of successful collisions: the number per second, not the total number.
Factor increased
Explanation
concentration
more particles in the same volume, so more frequent collisions
pressure (gases)
particles closer together, so more frequent collisions
surface area
more particles exposed at the surface, so more frequent collisions
temperature
particles have more kinetic energy: more frequent collisions and a greater proportion of collisions with energy ≥ Ea
Examiner trap
"More collisions" is not enough for the marks: say more frequent collisions (more collisions per second). For temperature, you must also say that more particles have energy greater than or equal to the activation energy; that is the bigger effect.
Stage 1 · Learn
Check what the sim just showed you
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(Extended) According to collision theory, for particles to react they must:
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(Extended) Why does increasing the concentration of an acid increase the rate of its reaction with magnesium?
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(Extended) How does a catalyst increase the rate of reaction?
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(Extended) Why does powdered zinc react faster with acid than a lump of the same mass?
Section 02
How catalysts work.
Extended content. Compare the catalysed and uncatalysed pathways in the simulator.
Alternative pathway: a catalyst provides a different route for the reaction with a lower activation energy.
Result: a greater proportion of collisions have enough energy to react, so the frequency of successful collisions increases.
Unchanged: the catalyst is not used up, and the energy change of the reaction, ΔH, is the same.
Industry: catalysts let reactions run fast at lower temperatures, saving energy and money: iron in the Haber process, vanadium(V) oxide in the Contact process. Enzymes are biological catalysts.
Stage 2 · Exam
Exam-style questions
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(Extended) Which gives the full explanation of why increasing the temperature increases the rate of reaction?
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(Extended) Increasing the pressure of a gas-phase reaction increases its rate because:
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(Extended) A catalyst is added to a reaction. What happens to the activation energy and to the enthalpy change?
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(Extended) Why do foods go off more slowly in a fridge?
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