1.6.2Physical Chemistry
Energy Level Diagrams — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 32 of 102
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 a Diagram
Vertical axis
- Vertical axis The vertical axis shows the energy (enthalpy) of the chemicals, and the horizontal axis shows the progress of the reaction.
Reactant and product levels
- Reactant and product levels Horizontal lines mark the energy of the reactants on the left and of the products on the right.
Activation energy
- Activation energy Ea is the energy difference between the reactants and the top of the hump, which is the highest-energy point.
Enthalpy change
- Enthalpy change ΔH is the difference between the products and reactants, shown as an arrow from reactants to products.
Direction
- Direction In an exothermic reaction the products are lower than the reactants, and in an endothermic reaction they are higher.
Part 2 of 5Comparing Profiles
Comparing Profiles
Exothermic profile
- Exothermic profile The curve rises to a peak and falls to a level below the start, so ΔH is negative.
Endothermic profile
- Endothermic profile The curve rises to a peak and ends at a level above the start, so ΔH is positive.
Catalysed route
- Catalysed route A catalyst provides a path with a lower peak, shown as a second curve below the first, but ΔH is the same.
Reverse reaction
- Reverse reaction The reverse reaction has the opposite sign of ΔH and an activation energy equal to Ea(forward) − ΔH.
| Feature | Exothermic | Endothermic |
|---|---|---|
| Products compared with reactants | Lower | Higher |
| ΔH | Negative | Positive |
| Arrow for ΔH | Points down | Points up |
Part 3 of 5Drawing One
1
Label the vertical axis with energy and the horizontal axis with the reaction pathway or progress of reaction.
2
Draw a line for the reactants and a line for the products at the correct relative heights.
3
Join them with a smooth curve that rises to a peak above the reactants.
4
Add a vertical arrow from the reactant level to the peak and label it Ea.
5
Add a second arrow between the reactant and product levels, labelled ΔH, pointing in the correct direction.
Part 4 of 5Worked Example
Question
- A reaction has a forward activation energy of 120 kJ mol⁻¹ and ΔH = −50 kJ mol⁻¹.
- Calculate the activation energy of the reverse reaction.
- The reactants are 50 kJ mol⁻¹ higher in energy than the products, since the forward reaction is exothermic.
- For the reverse direction, the products of the forward reaction become the starting point.
- Ea(reverse) = Ea(forward) − ΔH = 120 − (−50).
- Ea(reverse) = 120 + 50 = 170 kJ mol⁻¹.
Answer The reverse reaction has Ea = 170 kJ mol⁻¹ and ΔH = +50 kJ mol⁻¹.
Part 5 of 5Exam Checklist & Tips
1.6.2 – Key Points for the Exam
- Energy level diagram A graph showing how the energy of reactants and products changes during a reaction.
- Activation energy The minimum energy needed for particles to react.
- Transition state The highest-energy arrangement of atoms at the peak of the profile.
- Catalyst A substance that speeds up a reaction by providing a route with lower activation energy.
Common Mistakes
- Drawing the activation energy arrow from the reactants to the products rather than to the peak.
- Showing the catalysed reaction with a different ΔH, which is wrong because catalysts do not alter ΔH.
- Pointing the ΔH arrow upwards for an exothermic reaction.
- Starting the curve at the axis instead of at the reactant energy level.
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