Collision Theory
Collision theory: explain effective collisions, activation energy and how particle behaviour changes reaction rate.
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Collision theory replaces vague statements such as “the reactants want to react” with a particle explanation based on collisions, energy and orientation.
If the definition of rate still feels uncertain, revise Speed of Reaction before using the particle model here.
1. Definition
A. Collision Theory
Collision theory explains reactions by particle collisions: reactant particles must collide for a reaction to happen.
B. Effective Collision
An effective collision is a collision that produces products.
C. Conditions for an Effective Collision
In the collision model used here, reacting particles must meet with:
- enough energy (collision energy ≥ Eₐ), and
- a suitable orientation for the required bonds to rearrange.
State both conditions: enough energy to overcome the barrier, and an orientation that allows the required bonds to rearrange. Merely saying “the particles collide” does not explain why products form.
2. Key Ideas
- Not all collisions give products → rate depends on effective collisions, not total collisions.
- Rate increases when there are more frequent effective collisions per unit time.
- Raising temperature does two things: particles collide more often and a larger fraction has kinetic energy (energy of motion) ≥ Eₐ.
- A catalyst increases rate by lowering Eₐ (alternative pathway), not by “giving particles more energy”.
3. Detailed Explanations
- Collision frequency: how many collisions occur per unit time.
- Successful fraction: the proportion of collisions that form products.
- Effective-collision frequency: how many successful collisions occur per unit time.
A rate comparison needs the last of these. More collisions can mean more products, but only if enough of those collisions are effective.
A. Correct Orientation (Why some collisions fail)
When bonds must break and form, particles need to meet in a suitable orientation. In the schematic below, C must approach the A end of AB to form AC. Meeting the B end does not form the required bond, even if the collision has enough energy. The letters represent atoms in a simplified model, not a specific chemical reaction.
B. Activation Energy (What it actually means)
The activation energy, Eₐ, is the energy barrier for a reaction pathway. In this collision model, a collision needs enough energy to overcome that barrier before products can form. Having enough energy is necessary; orientation can still make the collision unsuccessful.
At a fixed temperature, some collisions have energy below Eₐ and cannot produce products. Increasing temperature raises the fraction of collisions with energy ≥ Eₐ. A catalyst provides an alternative pathway with a lower Eₐ, so a larger fraction of collisions can be effective.
- Observation: gas collects more quickly, so the volume gained each second is greater.
- Particle explanation: successful collisions occur more frequently.
- Symbols: a larger Δ V/Δ t represents a greater gas-production rate; Eₐ labels the energy barrier, not the measured rate.
Eₐ is not the “energy released” and it is not “heat added”. It is the minimum collision energy needed for reaction. A catalyst lowers Eₐ but does not increase particle energy.
C. Why higher rate = more effective collisions
Rate is controlled by how many collisions produce products each second:
In this particle model, which leaves out orientation, count the collisions each second and see which of them have enough energy to react as you change the conditions.
B as a gas, 1.00 mol/dm³ of A at 25 °C, without a catalyst (Eₐ = 10 kJ/mol). Press play to start the particles moving.
- A–B collisions
- — /s
- Successful collisions
- — /s
- Fraction successful
- — %
- AB formed
- 0
- Exposed B particles
- 0
Try this
0 of 4 doneRun for 30 s at two concentrations, with everything else the same. (not done yet)
More particles in the same space collide more often, so there are more successful collisions each second. The fraction that succeeds does not change.
Run for 30 s, then raise the temperature by at least 40 °C and run again. (not done yet)
Collisions become only a little more frequent, but a larger fraction of them have energy ≥ Eₐ: the shaded area grows. That is the main reason the rate rises.
Run for 30 s without the catalyst and 30 s with it, at the same temperature. (not done yet)
The catalyst gives a pathway with a lower Eₐ. The particles have the same energies, but more of their collisions now have enough energy to react.
Compare one lump of B with the powder, running each for 30 s. (not done yet)
Only particles on the surface can be hit. The powder exposes more of the same solid, so there are more collisions, and more successful ones, each second.
D. Use the model in the next lesson
Apply this model to concentration, gas compression, solid particle size and temperature in Factors Affecting Rate of Reaction. Then study the pathway mechanism in Catalysts and Enzymes.
4. Common Mistakes
- Writing “higher temperature means more collisions, so rate is higher” and stopping there (incomplete): you must also mention more particles have energy ≥ Eₐ.
- Saying “a catalyst gives particles more energy” (false): it lowers Eₐ.
- Confusing total collisions with effective collisions.
- Defining activation energy as “energy released” (wrong term).
5. Exam Tips
Explain what changes and how it affects successful collisions. Greater concentration or exposed surface mainly increases collision frequency. Higher temperature also increases the fraction of collisions above the barrier. A catalyst lowers the barrier without giving particles extra kinetic energy.
Do not apply “more collisions per second” as the whole catalyst explanation. Its key effect is that a larger fraction of collisions can succeed.
- If the question asks for “effective collisions”, you must mention both: energy ≥ Eₐ and correct orientation.
- If the reaction involves a solid, the keyword is surface area (not “particle size” by itself).
6. Worked Examples
Modelled example 1
Identify the missing condition
Problem
Study the worked solution
Reject collision alone
Method
Distinguish all collisions from effective collisions.Reason
Many colliding particles separate without forming products.Working
Only effective collisions result in reaction.Supply both conditions
Method
Require sufficient energy and correct orientation.Reason
Particles must overcome the activation-energy threshold and meet so the necessary bonds can rearrange.Working
Collision energy ≥ Eₐ and correct orientation.
Guided practice 2
Explain both temperature effects
Problem
Build both causal branches
Hints
Hint 1: two effects
Hint 2: finish with rate
View solution step by step
Increase collision frequency
Method
State that particles gain kinetic energy and move faster.Reason
Faster motion makes particles collide more frequently per unit time.Working
Higher temperature → greater collision frequency.Increase the successful fraction
Method
State that a larger proportion has energy at least Eₐ.Reason
More collisions can overcome the activation-energy threshold.Working
Larger fraction with energy ≥ Eₐ.Conclude the rate effect
Method
Combine the two changes.Reason
More collisions occur and a larger fraction of them can react.Working
More frequent effective collisions per unit time, so rate increases.
Common misconception 3
Catalyst trap
Learner claim
Separate frequency from effectiveness
View solution step by step
Reject the claimed mechanism
Method
Do not claim that a catalyst necessarily makes particles collide more often.Reason
Collision frequency depends mainly on particle spacing and speed, not simply on catalyst presence.Working
The catalyst does not give reactant particles extra kinetic energy.State the correct pathway effect
Method
Lower the activation energy through an alternative pathway.Reason
At the same temperature, a larger proportion of collisions now has enough energy to react.Working
Lower Eₐ → more effective collisions → higher rate.
Guided practice 4
Surface area vs mass (fair test)
Compare two forms of the same solid
Connect the controlled variable to collision frequency
Hints
Hint 1: solid interface
View solution step by step
Compare exposed surface
Method
Give the powder the larger total surface area.Reason
Breaking the same mass into smaller pieces exposes more marble particles to the acid.Working
Powder: more exposed reacting sites.Translate to collision rate
Method
Increase acid–marble collisions at the surface per second.Reason
More exposed sites allow more frequent effective collisions per unit time.Working
Larger surface area → higher reaction rate.Justify the fair test
Method
Keep marble mass and acid conditions constant.Reason
Otherwise a different reactant amount or acid condition could also change the result.Working
The intended independent variable is marble surface area.
7. Mind Stretchers
Mind stretcher 1: Same temperature, different ratesExtension
Question: Two illustrative models describe the same type of reaction in equal volumes, with the same product amount formed per successful collision. Model A has 1000 collisions per second, of which 2% are effective. Model B has 600 collisions per second, of which 5% are effective. Which predicts the greater product-formation rate? Explain why total collision frequency alone gives the wrong answer.
Show Answer
A predicts 1000 × 0.02 = 20 effective collisions per second; B predicts 600 × 0.05 = 30. B therefore predicts the greater product-formation rate despite fewer total collisions. The successful fraction matters as well as collision frequency. These model counts are not measured chemical rate constants.
Mind stretcher 2: Make the explanation preciseExtension
Question: A student explains a concentration increase by saying, “The particles have more energy, so the activation energy is lower.” Correct both claims when temperature and the reaction pathway stay the same.
Show Answer
At the same temperature, particle kinetic energies do not increase simply because concentration is greater. The same pathway also has the same activation-energy barrier. Higher concentration puts more reacting particles per unit volume, so collisions occur more frequently; under otherwise unchanged conditions, effective collisions also occur more frequently.
8. Practise and check
Use the topic check to practise effective collisions and distinguish particle energy from the reaction barrier.
Practise and check reaction ratesSyllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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