3.1.5Additional lessons
Activation Energy Skills — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 54 of 142
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 5Activation Energy
Definition
- Definition Activation energy is the energy that reactants must gain before a reaction can start.
Transition state
- Transition state Bonds are strained and partly broken at the peak of the energy profile before products form.
Reaction progress
- Reaction progress The x axis of a profile shows how far the reaction has progressed from reactants to products.
Energy axis
- Energy axis The y axis shows the energy of the system at each stage of the reaction.
Overall change
- Overall change The difference between the energy of reactants and products shows whether energy is released or taken in.
Part 2 of 5Effect of an Enzyme
1
An enzyme binds the substrate and holds it in a position that favours the reaction.
2
This lowers the energy needed to reach the transition state, so the peak is lower.
3
A larger proportion of collisions have enough energy to react, so the rate increases.
4
The energy of the reactants and products is unchanged, so the overall energy change is the same.
5
The enzyme affects how fast equilibrium is reached, not where it lies.
Part 3 of 5Drawing the Diagram
Drawing the Diagram
Axes
- Axes Label the vertical axis energy and the horizontal axis progress of reaction.
Curves
- Curves Draw one hump for the uncatalysed reaction and a lower hump for the enzyme-catalysed reaction.
Arrows
- Arrows Mark the activation energy as an arrow from the reactant level to the top of the hump.
Net change
- Net change Show the difference between the reactants and products levels as a separate arrow.
Part 4 of 5Worked Example
Question
- A reaction has activation energy 75 kJ mol⁻¹ without an enzyme and 45 kJ mol⁻¹ with an enzyme.
- The reactants have 20 kJ mol⁻¹ and the products 5 kJ mol⁻¹ on the same scale.
- State the effect of the enzyme and the net energy change.
- Reduction in activation energy = 75 − 45 = 30 kJ mol⁻¹.
- Percentage reduction = 30 ÷ 75 × 100 = 40 %.
- Net change = 5 − 20 = −15 kJ mol⁻¹, so 15 kJ mol⁻¹ is released, with or without the enzyme.
Answer
- The enzyme lowers activation energy by 30 kJ mol⁻¹ (40 %);
- the net change stays at 15 kJ mol⁻¹ released (example data).
Part 5 of 5Exam Checklist & Tips
3.1.5 – Key Points for the Exam
- Activation energy The minimum energy needed to start a chemical reaction.
- Catalyst A substance that speeds up a reaction without being used up.
- Exergonic A reaction that releases energy, in which products have less energy than reactants.
- Endergonic A reaction that takes in energy, in which products have more energy than reactants.
Common Mistakes
- Drawing the catalysed curve ending at a different level from the uncatalysed one.
- Saying that enzymes provide the activation energy; they reduce the amount needed.
- Measuring activation energy from zero instead of from the energy of the reactants.
- Claiming that the enzyme changes the overall energy released by the reaction.
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