Collision Theory

Cambridge publishes the marking points for collision theory almost unchanged series after series, and the temperature explanation carries two of them: collisions become more frequent, and a greater proportion have energy above the activation energy. Examiner reports confirm most candidates write only the frequency half and donate the second mark. This page is the Supplement engine room behind every rates question on Paper 4.

The theory itself

Reactions happen through particle collisions, but a collision alone is not enough. Two conditions:

1. The particles must collide.
2. The colliding particles must have energy equal to or greater than the activation energy (Ea): the minimum energy required to react.

Collisions meeting both conditions are successful (or effective) collisions. Rate of reaction is then proportional to the frequency of successful collisions (collisions per second). The word “frequency” or “per second/unit time” matters: “more collisions” describes a total, not a rate, and Cambridge mark schemes regularly flag it as insufficient. Activation energy is the same Ea drawn as the hump on Energy Level Diagrams.

Explaining each factor

The observable factor-effects are Core material on the Rate of Reaction page; the explanations below are the Extended layer.

Concentration (and pressure for gases). More particles in the same volume means particles are closer together, so collisions are more frequent, so the rate increases. The energy of the collisions is unchanged. Only frequency rises.

Surface area. Cutting a solid into smaller pieces exposes more particles at the surface. More particles are available to be hit, so collisions between the solid and the solution particles are more frequent, so the rate increases. Again: frequency only.

Temperature: the two-mark explanation. Heating gives particles more kinetic energy, so they move faster. Two consequences, each a separate marking point:

1. Collisions are more frequent (faster-moving particles collide more frequently).
2. A greater proportion of collisions have energy ≥ the activation energy, so a greater proportion of collisions are successful.

Both must appear for full marks. The second point is the bigger contributor to the rate increase, and “the particles have more energy” alone does not earn it: the energy must be compared with Ea.

Catalyst. A catalyst provides an alternative reaction pathway with a lower activation energy. The collision frequency and the particles’ energies are unchanged; what changes is the threshold. More of the existing collisions now clear the (lowered) bar, so the frequency of successful collisions rises. The catalyst is chemically unchanged and not used up.

Factor	Frequency of collisions	Energy vs Ea
↑ Concentration / pressure	Increases	Unchanged
↑ Surface area	Increases	Unchanged
↑ Temperature	Increases	Greater proportion exceed Ea
Catalyst	Unchanged	Ea lowered, so more collisions exceed it

That table is worth memorising as a unit: every explain-the-rate question is answered by one row.

Building the answer sentence

Cambridge rewards a fixed chain: change → particle consequence → collision consequence → rate consequence. For concentration: “The acid is more concentrated, so there are more acid particles per unit volume, so collisions between acid particles and magnesium are more frequent, so the rate increases.” Four links, three marks in most schemes. Skipping the middle links (“more concentrated so faster”) earns the first mark only. The same chained-reasoning skill scales up to the level-marked questions covered in the 6-mark extended response technique guide.

Worked exam question

Magnesium ribbon reacts with dilute hydrochloric acid. Explain, using collision theory, why the reaction is faster when (a) the acid is heated [3] and (b) the magnesium is powdered. [2]

Model answer: (a) The particles gain kinetic energy and move faster (1), so collisions between acid particles and magnesium are more frequent (1), and a greater proportion of collisions have energy greater than or equal to the activation energy, so more collisions are successful (1). (b) Powder has a larger surface area, so more magnesium particles are exposed (1), so collisions are more frequent and the rate increases (1).

Mark-by-mark: in (a) the third mark is the one most scripts miss: it requires the proportion-above-Ea idea, not just “more energy”. In (b) the surface-area statement and the collision-frequency consequence are separate marks; “more area so it reacts faster” compresses them into one creditable point.

The mistakes that cost marks

1. One temperature point instead of two. Frequency AND proportion of collisions exceeding Ea. Train the pair until it is automatic.
2. “More collisions” without per-second. Rate is about frequency; add “more frequent” or “more collisions per second”.
3. “The catalyst makes particles collide more.” It does not touch the collisions. It lowers the activation energy.
4. “The particles have more energy so they react.” Incomplete: the energy must be compared with the activation energy.

How examiners want it phrased

Student wording	Mark-scheme wording
”Hotter particles hit harder"	"A greater proportion of collisions have energy ≥ the activation energy"
"More particles means more hits"	"More particles per unit volume, so more frequent collisions"
"The catalyst speeds things up"	"The catalyst provides an alternative pathway with a lower activation energy"
"Powder has more sides"	"Powder has a larger surface area, so more frequent collisions at the surface”

Collision theory is a vocabulary test wearing a chemistry costume: frequency, proportion, activation energy, per second. The wider topic context is on the Chemical Reactions pillar, and if the two temperature points still refuse to stick, a free 1-hour trial lesson with a Chemistry specialist will drill them against real Paper 4 mark schemes until they do.

Test yourself

Write each answer in full sentences before you click. The chains are what get marked.

Q1 (3 marks). (Extended) Zinc granules react with dilute sulfuric acid. Explain, using collision theory, why doubling the concentration of the acid increases the rate of reaction.

Show answer

• there are more acid particles per unit volume [1] • so collisions between acid particles and zinc are more frequent [1] • so the frequency of successful collisions increases and the rate increases [1]. The energy of the collisions is unchanged.

Q2 (2 marks). (Extended) State the two conditions that must be met for a collision between particles to be successful.

Show answer

• the particles must collide [1] • the colliding particles must have energy equal to or greater than the activation energy [1]

Q3 (3 marks). (Extended) Manganese(IV) oxide increases the rate of decomposition of hydrogen peroxide. Explain, using collision theory, how it does this.

Show answer

• the catalyst provides an alternative reaction pathway [1] • with a lower activation energy [1] • so a greater proportion of collisions have energy ≥ the activation energy, increasing the frequency of successful collisions [1]. Not creditworthy: “it makes the particles collide more” (collision frequency is unchanged).