5.1.3Kinetics
Factors Affecting Reaction Rate — AP CHEMISTRY Revision Notes & Topical Questions
5 min read 4 parts Lesson 15 of 16 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4The Collision Model
A reaction needs particles to meet, and rate depends on how many of those meetings succeed.
1
Reactant particles must collide before any bonds can be broken or formed, so collisions are the starting point for every reaction.
2
Only collisions with kinetic energy at least equal to the activation energy can break the necessary bonds.
3
The particles must also meet with a suitable orientation, so many energetic collisions still fail to produce products.
4
The rate of reaction is therefore proportional to the number of successful collisions per unit time.
5
Any factor that raises the frequency of collisions, or the fraction that are successful, increases the rate.
Part 2 of 4Five Ways to Change the Rate
Each factor acts on collision frequency, collision energy, or the energy barrier itself.
Concentration
- Concentration More particles per unit volume means collisions occur more often, so the rate generally rises for solutions.
Pressure
- Pressure For gases, raising pressure squeezes the same particles into less volume, which acts like an increase in concentration.
Temperature
- Temperature Particles move faster and a larger fraction have energy of at least the activation energy, giving a large rise in rate.
Surface area
- Surface area Crushing a solid exposes more particles to the other reactant, so more collisions can occur at the surface.
Catalyst
- Catalyst It provides a different pathway with lower activation energy, so a greater fraction of collisions succeed at the same temperature.
| Factor changed | Collision frequency | Fraction with enough energy |
|---|---|---|
| Concentration up | Increases | Unchanged |
| Temperature up | Slightly increases | Increases strongly |
| Surface area up | Increases | Unchanged |
| Catalyst added | Unchanged | Increases (lower Eₐ) |
Part 3 of 4What Does Not Change
Rate factors affect how fast equilibrium or completion is reached, not how far the reaction goes.
What Does Not Change
Yield
- Yield Changing rate alone does not alter the amount of product that a given quantity of limiting reactant can form.
Catalyst use
- Catalyst use A catalyst is not consumed overall, so it does not change the overall enthalpy change of the reaction.
Nature of reactants
- Nature of reactants Reactions between ions in solution are often faster than those needing covalent bonds to break,
- because less rearrangement is needed.
Heterogeneous reactions
- Heterogeneous reactions Reactions involving solids and gases occur only at the surface, so exposed area matters more than quantity.
Part 4 of 4Exam Checklist & Tips
5.1.3 – Key Points for the Exam
- Reaction rate The change in concentration of a reactant or product per unit time.
- Activation energy The minimum energy that colliding particles need for a reaction to occur.
- Successful collision A collision with sufficient energy and suitable orientation to form products.
- Catalyst A substance that speeds up a reaction by lowering activation energy and is regenerated.
- Surface area The exposed area of a solid where collisions with other reactants can happen.
Common Mistakes
- Saying that a higher temperature lowers the activation energy;
- it only raises the fraction of particles able to exceed it.
- Claiming a catalyst increases the yield or alters the equilibrium position.
- Explaining the effect of concentration as particles moving faster, rather than colliding more frequently.
- Treating a larger lump of solid as having more surface area than the same mass crushed to powder.
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