Collision Theory and Activation Energy
MOD 6 • RATE • COLLISION THEORY
The mechanism behind every rate factor. Why "per second" matters, what activation energy actually is, and the single rule that explains every Paper 2 rate question.
Why Collision Theory Matters
Lesson 6.02 told you which factors change rate and in which direction. This lesson tells you why — using the single underlying mechanism that AQA expects in every "explain why" rate question.
Collision Theory (spec 4.6.1.3)
"Chemical reactions can occur only when reacting particles collide with each other AND with sufficient energy." Both conditions must be met.
What This Unlocks
Every rate factor changes either how often particles collide, how energetic those collisions are, or both. That's it. If you can identify which one a factor changes, you can explain its effect on rate from first principles.
Two Requirements for a Reaction
For two reactant particles to actually react with each other, two conditions must be met at the same moment:
Requirement 1
They must COLLIDE
Particles that never meet can never react. They have to physically come into contact.
Requirement 2
They must have enough ENERGY
The collision must have at least the activation energy (Eₐ). A weak collision just bounces off — no reaction.
Both Required
If particles collide but lack enough energy → they bounce off, no reaction. If particles have enough energy but never meet → no reaction. Only collisions with both features lead to a reaction.
Activation Energy (Eₐ)
Activation Energy (Mark-Scheme Definition)
The minimum amount of energy that particles must have for a reaction to occur.
You can picture Eₐ as a hill the reactants must climb before they can roll down the other side as products. Particles with less than Eₐ don't make it over.
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Note — Reaction Profile Geometry
This shows a typical exothermic profile (products at lower energy than reactants — overall energy is released). Eₐ is measured from the reactants level up to the peak, regardless of where the products end up. The catalyst version (with a lower second peak) is in Lesson 6.04.
Most Collisions Don't Work
Particles in a reaction mixture collide millions of times per second — but most collisions don't produce a reaction because they're below the activation energy.
Unsuccessful — Bounce
Particles collide, but the collision energy is less than Eₐ. They bounce off unchanged — no reaction.
Reaction — Energy ≥ Eₐ
Particles collide AND have energy ≥ Eₐ. The collision succeeds and a reaction occurs.
Examiner Alert — P2 Jun24 Q09.5 MS Note
The mark scheme explicitly says "ignore successful". Writing "more successful collisions" on its own does not score the mark. Be specific: write "greater proportion of collisions have enough energy to react" or "more collisions have energy ≥ Eₐ".
Concentration & Pressure
These two factors do exactly the same thing — pack more particles into the same volume. Concentration applies to solutions, pressure applies to gases.
Mechanism (1 mark per step)
Higher concentration / pressure means more particles per unit volume.
Particles encounter each other more often, so the frequency of collisions increases (more collisions per second).
More collisions per second → more reactions per second → faster rate.
Concentration / Pressure — Frequency Only
These factors only change collision frequency. They do not change the energy of individual collisions — particles aren't moving faster, just more crowded. Energy distribution per particle is unchanged.
Surface Area
Surface area applies to solid reactants. Inside a lump of solid, atoms are buried — they can't collide with the other reactant until the surface above has reacted.
Mechanism — Spec Language Required
The spec (4.6.1.3) explicitly requires explaining surface-area effects "in terms of surface area to volume ratio".
Smaller pieces have a larger surface area to volume ratio (SA : V) for the same mass.
More reactant atoms are exposed at the surface — available to collide with the other reactant.
More exposed atoms means more collisions per second — and so a faster rate.
Frequency Only — Same as Concentration
Like concentration, surface area only changes collision frequency — not the energy of individual collisions. The energy distribution of particles is identical for a lump and a powder of the same substance.
Temperature — the Special One
Temperature is the only factor that does both things. The spec calls this out verbatim: heating "increases the frequency of collisions AND makes the collisions more energetic".
The Cause (1 mark)
Heating gives particles more (kinetic) energy — they move faster. (Both phrasings score the same mark.)
Two Consequences — Either Scores 1 Mark on Q09.5
Frequency of collisions increases — faster particles encounter each other more often, so more collisions per second.
A greater proportion of collisions have enough energy to react — the spec's "more energetic" wording. More particles now have energy ≥ Eₐ, so a higher fraction of collisions succeed.
Examiner Alert — P2 Jun24 Q09.5 / P2 Jun22 Q07.6
For Q09.5 (3 marks), the MS awards M3 for either consequence — but other temperature questions across years (e.g. Jun22 Q07.6 indicative content lists both "more frequent collisions" AND "more particles have the activation energy" as separate bullet points). Top-marks habit: always include both consequences in temperature explanations. It's never penalised and protects you against MS variation.
Which Factor Does What
This single table summarises the entire spec. Memorising it gives you the structure for every "explain why" rate question.
FactorIncreases collision frequency?Increases proportion with E ≥ Eₐ?Lowers Eₐ?
Concentration✓ YesNoNo
Pressure (gas)✓ YesNoNo
Surface area✓ YesNoNo
Temperature✓ Yes✓ YesNo
CatalystNo(via lower Eₐ)✓ Yes
The Pattern
Concentration, pressure, surface area = frequency only. Temperature = frequency AND energy. Catalyst = energy only (via lowered Eₐ). Full catalyst mechanism in Lesson 6.04.
The "Per Second" Rule
This is the single biggest source of avoidable lost marks on rate questions. Examiners flag it every year.
Wrong — 0 marks
"More collisions"
Insufficient — describes a total, not a rate.
Mark-Scheme
"More collisions per second" or "frequency of collisions increases" or "greater collision frequency"
Examiner Alert — P2 Jun23 Q07.1 / P2 Jun24 Q09.5
"More collisions" without a time component is not a mark-scoring phrase. Rate is, by definition, events per unit time. You must always include either "per second" or "frequency" in any rate explanation.
Worked Example — Temperature
3 MARKS
Question (P2 Jun24 Q09.5 verbatim): Explain how increasing the temperature of a reaction mixture increases the rate of reaction.
M1
Increasing the temperature increases the rate of reaction.
M2
The particles have more (kinetic) energy (allow: particles move faster).
M3
Either alternative scores: (a) the frequency of collisions increases, or(b) a greater proportion of collisions have enough energy to react.
Mark scheme exactly. Top-marks habit: include both consequences for safety. Never write "successful" alone — write "have enough energy to react" or "have energy ≥ Eₐ".
Worked Example — Catalyst Mechanism
2 MARKS
Question (P2 Jun23 Q07.1 verbatim): Explain how a manganese dioxide catalyst increases the rate of decomposition of hydrogen peroxide.
M1
A catalyst provides a different pathway for the reaction.
M2
This pathway has a lower activation energy.
Mark scheme exactly (P2 Jun23 Q07.1). Use "different pathway" — that's the verified verbatim. ("Alternative pathway" is widely used in textbooks and likely accepted, but only "different" appears in the published MS.) The full mechanism (more particles have energy ≥ Eₐ → more successful collisions per second) is in Lesson 6.04. Avoid: "catalyst reduces collisions" (wrong — it reduces the energy threshold, not the frequency); "catalyst starts the reaction" (wrong — the reaction occurs without it, just slower).
Phrases That Lose Marks
The same wrong phrases appear in examiner reports year after year. Recognising them in advance is the easiest way to avoid the trap.
WrongRightWhy
"More collisions""More collisions per second" / "frequency increases"Rate is per-time; total is irrelevant
"More successful collisions""More collisions have energy ≥ Eₐ" / "...have enough energy to react"MS explicitly ignores "successful" alone
"Catalyst starts the reaction""Catalyst provides a different pathway with lower Eₐ"Reaction occurs without catalyst, just slower
"Catalyst reduces the number of collisions""Catalyst lowers the activation energy"Catalyst doesn't change frequency, only the threshold
"Concentration decreases" (for slowing rate)"Concentration of reactants decreases"Must specify reactants (not products)
Practice 1 — Concentration
3 MARKS
Question: Explain, in terms of particles, why increasing the concentration of an acid increases the rate of reaction with magnesium ribbon.
M1
A higher concentration of acid means more reactant particles per unit volume.
M2
Particles encounter each other more often — the frequency of collisions increases (more collisions per second).
M3
More collisions per second between acid particles and Mg → more reactions per second → faster rate.
Standard 3-mark structure: particles per volume → collision frequency → rate. Note: the energy of individual collisions is unchanged — concentration only affects frequency, not energy.
Practice 2 — Surface Area
3 MARKS
Question: A student crushes a piece of marble (CaCO₃) into powder before adding it to hydrochloric acid. Explain, using collision theory, why the powder reacts faster than the lump — even though the same total mass is used.
M1
Crushing increases the surface area to volume ratio (SA : V) of the marble.
M2
More CaCO₃ particles are exposed at the surface and available to collide with the acid.
M3
The frequency of collisions increases (more collisions per second between acid particles and exposed surface) → faster rate.
Spec language required: the spec explicitly asks for "surface area to volume ratio". Saying just "more surface area" can lose a mark — write SA : V or "surface area to volume ratio".
Strategy — How to Read a Rate Question
Every rate-mechanism question on Paper 2 follows the same template. Identify the factor, identify what it does, write the answer.
Three-Step Approach
Identify the factor — concentration, pressure, surface area, temperature, or catalyst.
Decide what it changes — frequency only? Frequency + energy? Lower Eₐ? (Use the table from slide 9.)
Write the chain — cause → effect on collisions → effect on rate. Always include "per second" or "frequency".
Spec Skill — Simple Proportionality
Spec 4.6.1.3 requires you to "use simple ideas about proportionality". For factors that affect collision frequency only:
Doubling concentration ≈ doubles initial rate (twice as many particles per volume → twice as many collisions per second).
Halving concentration ≈ halves initial rate.
Tested directly in Paper 2 sketch questions (Lesson 6.02 covers the curve-sketching technique).
Marks-per-Mark Mapping
Most rate-mechanism questions are 2 or 3 marks. 2 marks = cause + frequency/energy effect. 3 marks = cause + collision change + rate consequence. For temperature, including both consequences (frequency AND proportion) maximises marks. For catalyst, "different pathway" + "lower Eₐ" is the standard 2-mark pair.
Lesson Recap
You should now be able to:
State the two requirements for a reaction: particles must collide AND have energy ≥ activation energy.
Define activation energy as the minimum energy particles must have for a reaction to occur.
Explain concentration / pressure / surface area effects in terms of frequency of collisions only.
Explain the surface-area effect using "surface area to volume ratio" (spec language).
Explain temperature effects using both consequences: frequency increases AND greater proportion have energy ≥ Eₐ.
Always include "per second" or "frequency" in rate explanations — never write "more collisions" alone.
Avoid the word "successful" on its own — write "have enough energy to react" or "have energy ≥ Eₐ".
Use the comparison table to identify which mechanism applies to any factor.
Top-Marks Habit
Build every rate explanation as a three-link chain: cause (what changes) → collision effect (frequency / energy / Eₐ) → rate consequence (faster reaction). Include "per second". The structure protects against partial-credit losses on every Paper 2 rate question.
UP NEXT • LESSON 6.04
Catalysts
The full catalyst mechanism with reaction profiles. Industrial examples from the spec. And the critical "catalyst doesn't change yield" point that's tested every year on Paper 2 equilibrium questions.
Next: Why Collision Theory Matters