Collision theory and activation-barrier changes
Explain each rate factor through collision frequency, collision energy or a lower-barrier route.
Section 6.2 · Rate of reaction
Extended only (Supplement) · Chemistry 0620, 2026–2028.
Before you begin
- Describe the Core rate-factor trends.
- Define activation energy and distinguish it from the overall enthalpy change.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What must a collision overcome for a reaction to occur?
What you will learn
- Explain concentration, gas pressure, surface area and temperature effects using collision theory.
- Explain catalysts through a lower activation-energy route, including enzymes as biological catalysts.
More encounters and more successful encounters are different
Particles must collide to react, and colliding particles need sufficient energy to overcome the activation barrier and a suitable arrangement. More reactant particles per unit volume usually means more collisions per unit time. Increasing solution concentration raises the number of reactant particles per volume; compressing a reacting gas at fixed temperature increases its particle density. Both can increase collision frequency without requiring a greater mean kinetic energy at the unchanged temperature.
Breaking a solid into smaller pieces exposes more surface particles to contact with another reactant, so collisions at the reacting surface can occur more frequently. Increasing temperature makes particles move faster and increases their average kinetic energy. Collision frequency can increase, and a larger fraction of collisions has enough energy to overcome the barrier; do not describe temperature solely as adding more particles to the same volume.
A catalyst offers a different route
A suitable catalyst provides a reaction route with lower activation energy. At the same temperature, a greater fraction of colliding particles can now react, increasing the rate. The catalyst is regenerated overall and does not increase the reactants' mean kinetic energy, supply extra product atoms or change the overall reactant-to-product enthalpy difference.
Enzymes are biological catalysts and can also lower a reaction's barrier by providing a suitable route. Their structures matter to their function; outside an appropriate temperature range, structural changes can reduce their activity. Thus a temperature trend for an enzyme-catalysed process must stay within the stated conditions. Adding or removing a catalyst changes the available route, whereas concentration, pressure and surface changes primarily affect encounters in the comparisons described.
Pause and explain
At the same temperature, how does a suitable catalyst increase rate?
Worked example
Explain why increasing the pressure of reacting gases by compression at unchanged temperature increases rate, and contrast this with raising temperature.
Show the worked solution
- Compression puts more reactant particles in each unit volume, increasing collision frequency.
- At unchanged temperature, mean kinetic energy is not increased by the compression comparison.
- Raising temperature increases mean kinetic energy and the fraction of collisions exceeding the activation barrier, as well as affecting collision frequency.
Answer Pressure changes particle density; temperature changes collision energy and the successful fraction.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain why a higher solution reactant concentration and a higher reacting-gas pressure at fixed temperature usually increase rate.
Give me a hint
In each case, consider reactant particles per unit volume.
Compare my reasoning
- Higher concentration places more solution reactant particles in each unit volume.
- Higher reacting-gas pressure through compression also increases particle density at the unchanged temperature.
- More frequent reactant collisions increase the number of successful collisions per unit time, without needing a change in mean kinetic energy.
Greater particle density increases collision frequency; mean kinetic energy stays tied to the unchanged temperature.
Look for these in your work
- Both factors are connected with density and encounters.
- Collision frequency and energy are not confused.
Compare the explanations for smaller solid pieces and a higher temperature in a suitable solid–solution reaction.
Give me a hint
One exposes more reacting surface; the other changes motion and collision energies.
Compare my reasoning
- Smaller pieces expose more surface sites for collisions with the solution reactant.
- Higher temperature increases average kinetic energy and collision frequency.
- A larger fraction of collisions can overcome Eₐ at the higher temperature, so more successful collisions occur per unit time.
Surface area exposes more reaction sites; temperature increases kinetic energy and the fraction of sufficiently energetic collisions.
Look for these in your work
- The two mechanisms are distinguished.
- The energy-threshold effect is included.
Two routes have the same reactant and product energy levels, but a catalyst lowers the barrier from 70 to 45 kJ/mol. Explain the rate change, give the barrier decrease and correct a claim that the catalyst makes the reaction 25 kJ/mol more exothermic.
Give me a hint
Only the peak changes; the endpoint energy difference remains the same.
Compare my reasoning
- Barrier decrease is 70−45=25 kJ/mol.
- More collisions can overcome the smaller barrier at the same temperature, increasing rate.
- ΔH is fixed by the unchanged endpoints, so it does not become 25 kJ/mol more negative.
Eₐ decreases by 25 kJ/mol, increasing the successful fraction; ΔH is unchanged.
Look for these in your work
- The barrier decrease is calculated.
- Rate and net energy transfer remain separate.
Common mistakes
- Saying compression at fixed temperature raises average kinetic energy.
- Saying a catalyst heats the particles or lowers ΔH.
- Explaining increased temperature only as more particles per volume.
- Assuming every collision reacts, regardless of energy or arrangement.
What can you explain now?
A catalyst lowers Eₐ from 70 to 45 kJ/mol at unchanged temperature. What changes about the successful fraction and the overall ΔH?
Compare with the explanation
More collisions can react; ΔH is unchanged.
The barrier is lower by 25 kJ/mol, so more collisions have sufficient energy at the same temperature. The reactant and product energy levels do not change.
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Make it stick
Explain all five rate factors using density, encounters, kinetic energy or a new route. For each explanation, name what changes and avoid adding a second mechanism without evidence.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.