5.2.5Additional lessons
Activation Energy — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 70 of 98
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 6The Energy Barrier
The activation energy is the barrier that reactants must climb.
Definition
- Definition Activation energy (Eₐ) is the minimum energy that colliding particles must have in order to react.
Why needed
- Why needed Existing bonds must begin to break before new bonds can form, so energy is needed at the start even for exothermic reactions.
Units
- Units Activation energy is quoted in kJ mol⁻¹, as the amount per mole of reacting particles.
Size
- Size A large Eₐ means few collisions are successful at a given temperature, so the reaction is slow.
Part 2 of 6Effects on Rate
Effects on Rate
High Eₐ
- High Eₐ Reactions such as the burning of fuels do not proceed at room temperature until a spark supplies enough energy.
Low Eₐ
- Low Eₐ Reactions such as acid–base neutralisation in solution are very fast because almost every collision has enough energy.
Temperature
- Temperature At a higher temperature a greater fraction of particles exceeds Eₐ, so more collisions succeed.
Catalyst
- Catalyst Reduces Eₐ without changing the temperature, again increasing the fraction of successful collisions.
Part 3 of 6Reading Eₐ
1
On an energy profile, find the reactant level and the top of the peak.
2
Measure the vertical distance between them to find the forward activation energy.
3
For the reverse reaction, measure from the product level to the top of the peak.
4
Notice that Eₐ for the reverse reaction equals the forward Eₐ minus ΔH.
| Reaction type | Relation between forward and reverse Eₐ |
|---|---|
| Exothermic | Forward Eₐ is smaller than reverse Eₐ |
| Endothermic | Forward Eₐ is larger than reverse Eₐ |
Part 4 of 6Misconceptions about Eₐ
Misconceptions about Eₐ
Not the same as ΔH
- Not the same as ΔH ΔH is the net energy difference between products and reactants, whereas Eₐ is the barrier at the start.
Not released
- Not released Eₐ is energy that must be supplied, not energy given out.
Still needed
- Still needed Exothermic reactions also need to overcome Eₐ.
Heating
- Heating Heating supplies the energy to increase the number of particles reaching Eₐ.
Part 5 of 6Worked Example
Question
- A reaction has ΔH = +60 kJ mol⁻¹ and a forward activation energy of 100 kJ mol⁻¹.
- Find the activation energy of the reverse reaction.
- Because ΔH is positive, the products lie 60 kJ mol⁻¹ above the reactants.
- The peak is 100 kJ mol⁻¹ above the reactants, so it is 100 − 60 = 40 kJ mol⁻¹ above the products.
- The reverse reaction starts at the products, so its activation energy is 40 kJ mol⁻¹.
Answer Reverse Eₐ = 40 kJ mol⁻¹
Part 6 of 6Exam Checklist & Tips
5.2.5 – Key Points for the Exam
- Activation energy The minimum energy colliding particles need to react.
- Energy barrier The peak of an energy profile, which reactants must climb to form products.
- Fraction of successful collisions The proportion of collisions that have energy equal to or greater than Eₐ.
- Energy profile A diagram of enthalpy against reaction progress.
Common Mistakes
- Confusing activation energy with the enthalpy change of the reaction.
- Believing that exothermic reactions have no activation energy.
- Saying that heating lowers the activation energy.
- Measuring Eₐ from the product level for the forward reaction.
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