5.2.6Additional lessons
Energy Profiles With and Without Catalysts — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 101 of 172
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 4Reading an Energy Profile
An energy profile is a graph that follows the potential energy of the system as reactants become products.
Axes
- Axes The vertical axis shows potential energy and the horizontal axis is the reaction progress from reactants to products.
Activation energy
- Activation energy Eₐ is the vertical distance from the reactants up to the peak of the curve, which is the transition state.
Enthalpy change
- Enthalpy change ΔH is the vertical difference between the energy of the products and the energy of the reactants.
Sign of ΔH
- Sign of ΔH In an exothermic reaction the products lie lower than the reactants, so ΔH is negative.
Part 2 of 4Exothermic and Endothermic
Exothermic and Endothermic
Exothermic
- Exothermic Products are lower in energy than reactants;
- energy is released to the surroundings and the surroundings warm up.
Endothermic
- Endothermic Products are higher in energy than reactants; energy is taken in and the surroundings cool down.
Reverse reaction
- Reverse reaction The reverse reaction has Eₐ(reverse) = Eₐ(forward) − ΔH, so it faces a different barrier.
Transition state
- Transition state The highest-energy arrangement along the path is the transition state and it cannot be isolated.
| Feature | Exothermic | Endothermic |
|---|---|---|
| Products compared with reactants | Lower energy | Higher energy |
| ΔH | Negative | Positive |
| Surroundings | Temperature rises | Temperature falls |
Part 3 of 4Effect of a Catalyst
1
A catalyst provides an alternative pathway that has a lower activation energy than the uncatalysed route.
2
On the diagram the catalysed curve has a lower peak, but it starts at the same reactant level and ends at the same product level.
3
Because the start and end points are unchanged, ΔH is exactly the same with or without the catalyst.
4
The catalyst lowers Eₐ for both the forward and the reverse reaction by the same amount, so it speeds up both directions.
5
Many catalysed paths have several steps, so the catalysed curve may show two smaller peaks with an intermediate between them.
Part 4 of 4Exam Checklist & Tips
5.2.6 – Key Points for the Exam
- Energy profile A graph of potential energy against reaction progress showing activation energy and ΔH.
- Transition state The unstable, highest-energy arrangement of atoms at the peak of the profile.
- Intermediate A species formed in one step and used in the next, located in a dip between two peaks.
- Exothermic A reaction that releases energy to the surroundings, with a negative ΔH.
Common Mistakes
- Drawing the catalysed curve with products at a different energy level from the uncatalysed one.
- Measuring Eₐ from the baseline of the graph instead of from the reactant level.
- Confusing ΔH with Eₐ when labelling the arrows.
- Writing that a catalyst makes an endothermic reaction exothermic.
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