2.6.1Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Collision Theory — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 37 of 53 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.
Section 1 of 4The Core Ideas
For a reaction to happen, particles must collide, and the collision must be energetic enough and correctly oriented.
Collision
- Collision Reacting particles must collide with each other before any bonds can break or form.
Activation energy
- Activation energy Eₐ is the minimum energy colliding particles need for a collision to result in a reaction.
Successful collision
- Successful collision Only collisions with energy at or above Eₐ, and a suitable orientation, produce products.
Rate
- Rate The reaction rate depends on how many successful collisions occur per unit time.
Two factors
- Two factors Rate can be raised by increasing collision frequency or by increasing the fraction of collisions that have enough energy.
Orientation
- Orientation Colliding particles must also meet in the right orientation so that the reacting parts of the molecules touch.
Section 2 of 4Changing Collision Frequency
Changing Collision Frequency
Concentration
- Concentration More particles in the same volume collide more often, so a higher concentration of reactant gives a faster rate.
Pressure
- Pressure Raising the pressure of a gas pushes particles closer together, increasing collision frequency in the same way.
Surface area
- Surface area Powdered solids expose more surface than lumps, so more particles can be hit at once and the rate is faster.
Temperature
- Temperature Particles move faster at higher temperature, so they collide more frequently.
Catalyst
- Catalyst A catalyst raises the rate by giving a route with lower activation energy, not by changing the collision frequency.
Section 3 of 4Effect of Temperature on Energy
Temperature has a much larger effect on rate through the energy of collisions than through their frequency alone.
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Heating increases the average kinetic energy of the particles, so they travel faster.
2
A larger fraction of particles now has energy equal to or greater than the activation energy.
3
Collisions therefore succeed more often even if collision frequency is only slightly higher.
4
The rate of many reactions roughly doubles for a 10 °C rise in temperature near room temperature.
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A catalyst can also help by providing a different pathway with lower activation energy, covered in a later topic.
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Collision theory therefore explains both why higher concentration speeds a reaction up and why a modest temperature rise has an even greater effect.
Section 4 of 4Exam Checklist & Tips
2.6.1 – Key Points for the Exam
- Activation energy The minimum energy that colliding particles need to react.
- Collision frequency The number of collisions per unit time.
- Successful collision A collision with enough energy and the right orientation to cause a reaction.
- Rate of reaction The change in concentration of a reactant or product per unit time.
Common Mistakes
- Saying all collisions lead to a reaction; most collisions are unsuccessful.
- Explaining a faster rate at higher temperature only by more frequent collisions.
- Claiming that a higher concentration lowers the activation energy.
- Describing activation energy as the energy released by the reaction.
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