Physical Chemistry
AQA AS Chemistry — 1.7 Kinetics — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 35 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.
Lesson summary
Catalysts and reaction rate: explain the alternative pathway and show on a Maxwell–Boltzmann distribution why more collisions are successful. A catalyst increases reaction rate by providing an alternative reaction pathway with a lower activation energy, Eₐ.
- Eₐ threshold farther left
- homogeneous catalysis
- heterogeneous catalysis
- activation-energy line moves left
- Exam check
Part 1 of 4How a catalyst works
Catalyst
A catalyst increases reaction rate by providing an alternative reaction pathway with a lower activation energy, Eₐ. It participates in steps of the mechanism and is regenerated overall.
With a lower Eₐ, more collisions have enough energy to react at the same temperature. The catalyst does not need to increase the energy of every reacting particle.
Part 2 of 4Read the energy distribution
1Maxwell–Boltzmann distribution
On a Maxwell–Boltzmann distribution, put the number of molecules on the vertical axis and energy on the horizontal axis. Draw the activation energy as a vertical threshold.
2Eₐ threshold farther left
When a catalyst is present, draw a second Eₐ threshold farther left. The area under the existing curve to the right of this new threshold is larger, so a greater fraction of molecules can undergo effective collisions.
Part 3 of 4Types of catalysis
Types of catalysis
Homogeneous catalysis
In homogeneous catalysis, catalyst and reactants are in the same phase, for example all dissolved in a solution.
Heterogeneous catalysis
In heterogeneous catalysis, catalyst and reactants are in different phases, for example gaseous reactants reacting on a solid catalyst surface.
Part 4 of 4Avoid the graph mistake
1Activation-energy line moves left
At the same temperature, the molecular energy distribution itself is not shifted to higher energy by adding a catalyst. The activation-energy line moves left because the mechanism has changed.
2Exam check:
Exam check state both links in the explanation: the catalyst lowers Eₐ, then a greater fraction of particles has energy at or above Eₐ, so reaction rate increases.
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