1.7.5Physical Chemistry
Effect of Concentration and Pressure — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 44 of 102
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.
Section 1 of 4Concentration
More particles
- More particles A higher concentration means more reactant particles in the same volume of solution.
Closer together
- Closer together The particles are closer together, so they collide more frequently.
Successful collisions
- Successful collisions A greater frequency of collisions means a greater frequency of successful collisions, so the rate increases.
Distribution unchanged
- Distribution unchanged The proportion of molecules with energy of at least Ea stays the same, as temperature has not changed.
Rate falls over time
- Rate falls over time As a reaction proceeds the concentration of reactants falls, so the rate of reaction falls.
Section 2 of 4Pressure and Surface Area
Pressure and Surface Area
Gas pressure
- Gas pressure Increasing the pressure of a gas squeezes the same number of molecules into a smaller volume,
- which is the same as raising the concentration.
Only gases
- Only gases Changing pressure affects reaction rates only when reactants are gases, not solids or liquids.
Surface area
- Surface area Breaking a solid into smaller pieces exposes more surface, so more particles are available to collide with the other reactant.
Fine powder
- Fine powder A powder reacts faster than a lump of the same mass because many more particles are at the surface.
| Change | Effect on number of particles per unit volume | Effect on rate |
|---|---|---|
| Double concentration | Doubled | Increases |
| Double gas pressure | Doubled | Increases |
| Powder instead of lump | Surface exposed increases | Increases |
Section 3 of 4Explaining It
1
State that there are more particles in a given volume when concentration or pressure is raised.
2
Say that the particles collide more often because they are closer together.
3
Link this to a higher frequency of successful collisions without changing the energy of the particles.
4
Conclude that the rate of reaction increases.
5
Note that a Maxwell–Boltzmann curve for the new conditions would have the same shape, with the same fraction above Ea.
Section 4 of 4Exam Checklist & Tips
1.7.5 – Key Points for the Exam
- Concentration The amount of solute per unit volume of solution, in mol dm⁻³.
- Pressure The force exerted by gas particles per unit area on the walls of their container.
- Surface area The total area of a solid that is exposed and available for reaction.
- Frequency of collisions The number of collisions per second between reacting particles.
Common Mistakes
- Saying the particles have more energy at higher concentration; the energy of each particle is unchanged.
- Claiming that more collisions are successful because the fraction above Ea is greater.
- Applying pressure arguments to reactions with only solids or liquids.
- Writing "the particles collide more" without mentioning frequency or time.
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