3.1.5Inquiry Process
Activation Energy- Skills — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 105 of 160
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 5Reading an Energy Profile
An energy profile shows why a reaction needs a push to start and how enzymes help.
Axes
- Axes The vertical axis shows the energy of the reacting system and the horizontal axis shows progress of the reaction from reactants to products.
Reactant level
- Reactant level The starting energy of the reactants is drawn on the left, and the products are drawn on the right at their own energy level.
Activation energy
- Activation energy The height of the peak above the reactant level, which is the energy that must be supplied for the reaction to start.
Overall energy change
- Overall energy change The difference between the energy levels of reactants and products, which does not depend on the presence of an enzyme.
Transition state
- Transition state The unstable arrangement at the top of the peak, in which bonds are partly broken and partly formed.
Part 2 of 5Enzyme Effect on the Profile
Enzyme Effect on the Profile
Lower peak
- Lower peak With an enzyme the peak is lower, so the activation energy is smaller and a larger fraction of molecules can react.
Same start and end
- Same start and end Reactants and products stay at the same energy levels, so an enzyme does not change the overall energy released or taken in.
Faster rate only
- Faster rate only The enzyme makes the reaction reach equilibrium sooner but does not change how far the reaction goes.
How it lowers Ea
- How it lowers Ea Binding of the substrate to the active site strains bonds and holds molecules in the right position,
- which stabilises the transition state.
| Feature | Without enzyme | With enzyme |
|---|---|---|
| Activation energy | Higher | Lower |
| Reactant energy | Unchanged | Unchanged |
| Product energy | Unchanged | Unchanged |
| Reaction rate | Slow | Fast |
Part 3 of 5Types of Energy Change
Exergonic
- Exergonic The products have less energy than the reactants, so energy is released;
- catabolic reactions are usually of this type.
Endergonic
- Endergonic The products have more energy than the reactants, so energy must be supplied;
- anabolic reactions are usually of this type.
Activation still needed
- Activation still needed Even exergonic reactions need activation energy first, which is why glucose does not burn spontaneously in the body.
Coupling
- Coupling Energy from ATP breakdown is used to drive endergonic reactions in cells.
Part 4 of 5Worked Example
Question
- An uncatalysed reaction has reactants at 30 kJ mol⁻¹, a peak at 105 kJ mol⁻¹ and products at 10 kJ mol⁻¹.
- With an enzyme the peak is at 70 kJ mol⁻¹.
- Calculate the activation energy with and without the enzyme, the energy change of the reaction, and the percentage reduction in activation energy.
- Without enzyme105 - 30 = 75 kJ mol⁻¹.
- With enzyme70 - 30 = 40 kJ mol⁻¹.
- Energy change10 - 30 = -20 kJ mol⁻¹, so 20 kJ mol⁻¹ is released.
- Reduction(75 - 40) / 75 x 100 = 46.7%.
Answer
- The enzyme lowers Ea from 75 to 40 kJ mol⁻¹ (a 47% reduction);
- the reaction releases 20 kJ mol⁻¹ either way.
Part 5 of 5Exam Checklist & Tips
3.1.5 – Key Points for the Exam
- Activation energy The minimum energy needed to start a reaction.
- Transition state The unstable high-energy state between reactants and products.
- Exergonic A reaction that releases energy.
- Endergonic A reaction that needs a net input of energy.
Common Mistakes
- Drawing the product level at a different height for the catalysed pathway; only the peak changes.
- Thinking enzymes supply the activation energy rather than lower it.
- Measuring activation energy from zero instead of from the reactant level.
- Saying that enzymes make an endergonic reaction release energy.
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