4.1.3Additional lessons
Energy Profiles — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 71 of 172
Part 1 of 5Parts of a Profile
Axes
- The vertical axis shows the enthalpy (potential energy) of the system and the horizontal axis shows progress of reaction.
Activation energy
- Eₐ is the minimum energy colliding particles must have for the reaction to occur, measured from reactants to the top of the peak.
Transition state
- The highest point on the curve is the transition state, an unstable arrangement of atoms that cannot be isolated.
Enthalpy change
- ΔH is the vertical difference between products and reactants, drawn downward for exothermic and upward for endothermic reactions.
Intermediates
- Multi-step reactions show a valley between two peaks, and the species at the valley is a real, though short-lived, intermediate.
| Feature | Exothermic | Endothermic |
|---|---|---|
| Products relative to reactants | Lower | Higher |
| ΔH sign | Negative | Positive |
| Reverse reaction | Larger Eₐ | Smaller Eₐ |
Part 2 of 5Sketching a Profile
1
Draw the axes and mark the reactants at the left at their starting enthalpy.
2
Draw a curve rising to a peak, labelling the height above the reactants as the activation energy.
3
Continue the curve down to the products, placing them below the reactants for exothermic reactions or above for endothermic ones.
4
Add a double-headed arrow between the reactant and product levels and label it ΔH.
5
Show a catalysed route as a second, lower peak that starts and ends at the same levels.
Part 3 of 5Effect of a Catalyst
Effect of a Catalyst
Lower Eₐ
- A catalyst provides an alternative pathway with a lower activation energy, so more collisions are successful.
Same ΔH
- Reactant and product enthalpies do not change, so the enthalpy change is the same.
Both directions
- The activation energy of the forward and reverse reactions are lowered by the same amount.
Not consumed
- The catalyst is regenerated at the end of the reaction and does not appear in the overall equation.
Part 4 of 5Worked Example
Question
- For a reaction Eₐ(forward) = 85 kJ mol⁻¹ and ΔH = −40 kJ mol⁻¹.
- Calculate the activation energy of the reverse reaction.
- The reaction is exothermic, so the products lie 40 kJ mol⁻¹ below the reactants.
- The reverse reaction must climb from the products to the same peak.
- Eₐ(reverse) = Eₐ(forward) − ΔH = 85 − (−40).
- Eₐ(reverse) = 125 kJ mol⁻¹.
Answer Eₐ(reverse) = 125 kJ mol⁻¹
Part 5 of 5Exam Checklist & Tips
4.1.3 – Key Points for the Exam
- Activation energy The minimum energy that colliding particles need for a reaction to occur.
- Transition state The highest-energy arrangement of atoms along the reaction pathway.
- Reaction intermediate A species formed in one step and used up in a later step.
- Catalyst A substance that speeds up a reaction by providing a lower-energy pathway and is not used up.
Common Mistakes
- Drawing a catalyst as changing the enthalpy of reactants or products.
- Measuring Eₐ from the baseline of the graph instead of from the reactant level.
- Placing products above reactants for an exothermic reaction.
- Describing the transition state as an intermediate that can be isolated.
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