3.2.4Physical Chemistry
Activation Energy — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 13 of 19
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 5Reading a Reaction Profile
Reactant level
- Reactant level The starting energy level shows the total chemical energy of the reactants before the reaction begins.
Product level
- Product level The final energy level shows the total chemical energy of the products once the reaction is complete.
Enthalpy change
- Enthalpy change ΔH is the difference between product and reactant energy levels, and it is negative for exothermic reactions.
Activation energy
- Activation energy Eₐ is the height from the reactant level up to the peak of the curve, the energy needed to start the reaction.
Peak of the curve
- Peak of the curve The peak represents the transition state, where old bonds are partly broken and new bonds are partly formed.
Section 2 of 5Exothermic and Endothermic Profiles
Exothermic and Endothermic Profiles
Energy flow
- Energy flow Exothermic reactions release energy to the surroundings, so the surroundings warm up.
Energy flow
- Energy flow Endothermic reactions take in energy from the surroundings, so the surroundings cool down.
Bond energy
- Bond energy The energy released when new bonds form is compared with the energy needed to break the old bonds.
Starting a reaction
- Starting a reaction Even exothermic reactions need activation energy, which is why a lit match is needed to start a fire.
| Feature | Exothermic | Endothermic |
|---|---|---|
| Sign of ΔH | Negative | Positive |
| Products compared with reactants | Lower in energy | Higher in energy |
| Arrow on the profile | Points downwards | Points upwards |
| Surroundings | Get warmer | Get colder |
Section 3 of 5Activation Energy and Collisions
1
Reacting particles must collide with enough kinetic energy to break the existing bonds before new bonds can form.
2
Only collisions with energy equal to or greater than the activation energy lead to a reaction.
3
A higher activation energy means a smaller fraction of collisions succeed, so the reaction is slower.
4
Heating gives particles more kinetic energy, so a larger proportion of them can reach the activation energy.
5
A catalyst lowers the activation energy, so a larger proportion of collisions succeed at the same temperature.
Section 4 of 5Worked Example
Question
- On a reaction profile the reactants are at 100 kJ/mol, the products at 60 kJ/mol and the peak at 175 kJ/mol.
- Calculate ΔH and the activation energy for the forward reaction, and state whether the reaction is exothermic.
- ΔH = energy of products − energy of reactants = 60 − 100 = −40 kJ/mol
- Activation energy = energy of peak − energy of reactants = 175 − 100 = 75 kJ/mol
- ΔH is negative, so the reaction is exothermic
Answer ΔH = −40 kJ/mol, Eₐ = 75 kJ/mol, and the reaction is exothermic.
Section 5 of 5Exam Checklist & Tips
3.2.4 – Key Points for the Exam
- Activation energy (Eₐ) The minimum energy colliding particles need for a reaction to take place.
- Enthalpy change (ΔH) The difference in energy between products and reactants, measured at constant pressure.
- Exothermic A reaction that releases energy to the surroundings, so ΔH is negative.
- Endothermic A reaction that takes in energy from the surroundings, so ΔH is positive.
- Reaction profile A graph of energy against the progress of a reaction.
Common Mistakes
- Measuring activation energy from the products; it is measured from the reactant level up to the peak.
- Giving ΔH a positive sign for an exothermic reaction.
- Describing activation energy as the energy released by the reaction.
- Thinking a catalyst changes the enthalpy change of the reaction.
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