4.1.3Additional lessons
Energy Profiles — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 49 of 98
Part 1 of 6Reading a Profile
An energy profile is a graph that shows how enthalpy changes during one reaction.
Axes
- The vertical axis shows enthalpy or potential energy and the horizontal axis shows reaction progress from reactants to products.
Reactants and products
- Each is drawn as a horizontal level at its enthalpy;
- the gap between the two levels is the enthalpy change ΔH.
Activation energy
- The difference between the reactant level and the top of the peak, being the minimum energy colliding particles need to react.
Transition state
- The maximum on the curve represents an unstable arrangement where old bonds are partly broken and new ones partly formed.
Part 2 of 6Exothermic and Endothermic Shapes
Exothermic and Endothermic Shapes
Exothermic
- The products sit lower than the reactants, so the arrow for ΔH points downwards and ΔH is negative.
Endothermic
- The products sit higher than the reactants, so the arrow for ΔH points upwards and ΔH is positive.
Barrier always present
- Both types need a peak, since energy is needed to start breaking bonds even when the reaction ends up releasing energy.
Forward and reverse
- The reverse reaction has the opposite sign of ΔH and an activation energy measured from the product level to the peak.
| Quantity | How to read it |
|---|---|
| ΔH | Level of products minus level of reactants |
| Eₐ forward | Peak minus level of reactants |
| Eₐ reverse | Peak minus level of products |
Part 3 of 6Drawing a Profile
1
Draw labelled axes with enthalpy upwards and reaction progress along the bottom, then mark the reactants as a short horizontal line on the left.
2
Draw the product level lower than the reactants for an exothermic reaction or higher for an endothermic one.
3
Join the levels with a smooth curve that rises to a single peak between them.
4
Add a vertical arrow from reactants to the peak for the activation energy and another between the two levels for ΔH.
5
Label every arrow with its name, and write the sign of ΔH next to the arrow.
Part 4 of 6Link to Reaction Rate
Link to Reaction Rate
Higher barrier
- A reaction with a large activation energy has few collisions with enough energy, so it proceeds slowly at room temperature.
Not the same as ΔH
- A reaction can be strongly exothermic and still be slow if its activation energy is large.
Starting a reaction
- Heating or a spark supplies the activation energy, and an exothermic reaction can then keep itself going.
Catalysts
- A catalyst provides an alternative pathway with a lower peak, as seen in profiles with and without a catalyst.
Part 5 of 6Worked Example
Question
- A reaction has a forward activation energy of 120 kJ mol⁻¹ and ΔH = −45 kJ mol⁻¹.
- Find the activation energy of the reverse reaction.
- The reactants are 45 kJ mol⁻¹ above the products because ΔH is negative.
- The peak is 120 kJ mol⁻¹ above the reactants, so it is 120 + 45 = 165 kJ mol⁻¹ above the products.
- The reverse activation energy is measured from the products to the peak.
Answer Eₐ(reverse) = 165 kJ mol⁻¹
Part 6 of 6Exam Checklist & Tips
4.1.3 – Key Points for the Exam
- Energy profile A diagram of enthalpy against reaction progress showing the energy changes during a reaction.
- Activation energy The minimum energy that colliding particles must have for a reaction to occur.
- Transition state The highest-energy arrangement of atoms along the reaction pathway.
- Enthalpy change (ΔH) The difference between the enthalpy of the products and that of the reactants.
Common Mistakes
- Measuring ΔH from the top of the peak instead of between the reactant and product levels.
- Drawing an exothermic profile with no peak because energy is released overall.
- Placing activation energy arrows from the product level when describing the forward reaction.
- Writing ΔH without a sign or direction on the diagram.
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