1.7.4Physical Chemistry
Effect of Temperature on Reaction Rate — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 43 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.
Part 1 of 4Effect on the Distribution
Shift
- Shift At a higher temperature the peak of the curve moves to the right, because the average energy of the molecules has increased.
Lower, broader peak
- Lower, broader peak The peak becomes lower and the curve is broader, but the total area under the curve stays the same.
Ea unchanged
- Ea unchanged The activation energy does not change with temperature, so the vertical Ea line stays in the same place.
Larger fraction
- Larger fraction A greater proportion of molecules now has energy greater than or equal to Ea, which is the important change.
Small rise, big effect
- Small rise, big effect A small rise in temperature can increase the fraction above Ea by a large amount because the tail is steep.
Part 2 of 4Two Contributing Factors
Two Contributing Factors
Collision frequency
- Collision frequency Faster-moving molecules collide more often, which slightly increases the rate.
Energy of collisions
- Energy of collisions Many more collisions have energy of at least Ea, which is the main reason for the increased rate.
Combined result
- Combined result The frequency of successful collisions rises, so the rate of reaction increases.
Reaction type
- Reaction type Both exothermic and endothermic reactions go faster at higher temperature.
| Quantity | Higher temperature | Lower temperature |
|---|---|---|
| Peak position | Further right | Further left |
| Peak height | Lower | Higher |
| Fraction with E ≥ Ea | Larger | Smaller |
Part 3 of 4Model Answer Structure
1
State that raising the temperature increases the average kinetic energy of the molecules.
2
Say that a greater proportion of molecules have energy equal to or greater than the activation energy.
3
Add that molecules also move faster so collisions are more frequent, though this is the smaller effect.
4
Conclude that the frequency of successful collisions increases.
5
Therefore the rate of reaction increases.
Part 4 of 4Exam Checklist & Tips
1.7.4 – Key Points for the Exam
- Kinetic energy The energy a particle has due to its motion.
- Distribution The way in which particle energies are spread across a range of values.
- Fraction The proportion of molecules with a particular property, such as E ≥ Ea.
- Successful collision A collision with energy of at least Ea and the correct orientation.
Common Mistakes
- Saying that more molecules have energy greater than the activation energy because Ea has become lower.
- Claiming that the faster speed alone explains the large increase in rate.
- Drawing the higher temperature curve with a different total area.
- Omitting the words "proportion of" or "fraction" when describing molecules with enough energy.
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