5.2.3Additional lessons
Collision Theory — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 98 of 172
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 4Requirements for Reaction
Most collisions are unsuccessful, which is why reactions are usually not instantaneous.
Collision
- Collision Particles must collide, so a reaction rate depends on the frequency of collisions.
Activation energy
- Activation energy Colliding particles must have kinetic energy at least equal to the activation energy, Ea, to break bonds.
Orientation
- Orientation The molecules must collide with a suitable orientation so that the reactive parts meet.
Successful collisions
- Successful collisions Only collisions that satisfy both energy and orientation requirements result in reaction.
Section 2 of 4Factors Affecting Rate
1
Higher concentration or pressure puts more particles in a given volume, so collision frequency rises.
2
A larger surface area for a solid exposes more particles to collisions with the other reactant.
3
Higher temperature increases the average kinetic energy, so a larger fraction of molecules has energy of at least Ea.
4
Higher temperature also raises collision frequency slightly, but the greater fraction above Ea is the main reason the rate increases.
5
A catalyst provides an alternative route with lower activation energy, so more collisions are successful at the same temperature.
Section 3 of 4Maxwell-Boltzmann Distribution
Maxwell-Boltzmann Distribution
Shape
- Shape The curve shows the number of molecules at each kinetic energy;
- it starts at the origin and has a long tail.
Area
- Area The total area under the curve represents the total number of molecules, which does not change with temperature.
Higher temperature
- Higher temperature The peak shifts right and becomes lower, and the area beyond Ea becomes larger.
Catalyst
- Catalyst Ea is shifted to the left on the energy axis, so more of the same distribution lies beyond it.
Section 4 of 4Exam Checklist & Tips
5.2.3 – Key Points for the Exam
- Collision theory A model stating that reactions occur through successful collisions between particles.
- Activation energy The minimum kinetic energy needed for a collision to lead to reaction.
- Catalyst A substance that speeds up a reaction without being used up by providing a lower-Ea pathway.
- Maxwell-Boltzmann distribution A graph showing how molecular kinetic energies are distributed in a sample.
Common Mistakes
- Saying that a rise in temperature works only by making molecules collide more often.
- Stating that catalysts increase the energy of molecules.
- Showing the Maxwell-Boltzmann curve touching the energy axis at high energies.
- Describing activation energy as the energy released in the reaction.
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