Reading and Drawing Energy Profiles
Read and draw exothermic and endothermic profiles, distinguish activation energy from enthalpy change, and add a catalysed pathway with the same endpoints.
On this page
An energy profile connects the reaction’s overall energy change to the energy barrier along its pathway. Read the two endpoints to classify the reaction; read the rise from reactants to the peak for activation energy. Then practise drawing those relationships yourself.
Use Introduction to Energy Changes if you need to revisit the reacting system, surroundings or sign of Δ H.
Read the axes and the two vertical differences
| Feature | What it tells you | What it does not tell you |
|---|---|---|
| Vertical axis: energy | Relative energy levels along the pathway | The mixture’s temperature |
| Horizontal axis: progress of reaction | Position along the reaction pathway | Elapsed time or reaction rate |
| Reactant-to-product difference: Δ H | Overall enthalpy change of the system | The size of the activation barrier |
| Reactant-to-peak difference: Eₐ | Activation energy for the forward reaction | The energy required to break every reactant bond into separate atoms |
Activation energy is the minimum energy reacting particles need to react along that pathway. An exothermic reaction can still need a spark or warming because the particles must overcome a barrier before products form.
These are qualitative schematics, not measured energy curves. The shape and distances are not numerical bond-energy data.
Exothermic profile
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Products are below reactants, so the reacting system has transferred energy to its surroundings and Δ H < 0. The profile still rises above the reactants: activation energy is positive even though the overall enthalpy change is negative.
Endothermic profile
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Products are above reactants, so the system has gained energy overall and Δ H > 0. Measure activation energy from the reactant level, not from the bottom of the page or from the product level.
Worked drawing: an exothermic reaction
Modelled example 1
Draw the relationships rather than memorising a hump
Problem
View solution step by step
Set up the axes
Method
Label the vertical axis energy and the horizontal axis progress of reaction.Reason
The diagram represents energy along a pathway, not temperature against time.Working
No numerical scale is required for this qualitative sketch.Place the endpoints
Method
Draw the reactant level, then a lower product level. Label both.Reason
Heat release leaves products lower in energy than reactants.Working
Products lower; Δ H < 0.Draw the pathway
Method
Join the two levels with a curve that rises to a peak above both levels.Reason
The reaction has an activation barrier even though it is exothermic.Working
The hump is not the net heat released.Label the differences
Method
Label the vertical rise from reactants to the peak as activation energy. Draw the enthalpy-change arrow downward from the reactant level to the product level.Reason
The two arrows represent different quantities.Working
Eₐ: reactants to peak; Δ H: reactants to products.
Guided drawing: change the reaction type
Sketch the endothermic case on paper before revealing the steps. The choices below check the relationships; your drawing still needs the labelled axes, curve and arrows.
Guided practice 2
Draw an endothermic profile
Problem
A reaction absorbs heat overall. Draw an energy profile and label activation energy and enthalpy change. Which relationships must your drawing show?
Try this before viewing the solution
Hints
Hint 1: follow the system
The reacting system absorbs energy, so its final level must be higher.
Hint 2: separate the intervals
The product level determines ΔH; the peak determines activation energy.
View solution step by step
Place the levels
Method
Label the axes and put products above reactants.
Reason
The system gains energy overall.Working
Positive Δ H.Add the barrier
Method
Draw a curve rising to a peak above both levels and ending at the products.
Reason
The activation barrier must be distinguished from the final product level.
Working
Eₐ runs from the reactant level to the peak.
Complete the labels
Method
Label reactants, products, activation energy and the upward ΔH arrow.
Reason
A recognisable hump alone does not communicate the two energy differences.
Working
Compare with the endothermic profile above.
Add a catalyst without changing the reaction’s endpoints
A catalyst provides an alternative pathway with a lower activation energy. For the same reaction and conditions, reactants and products have the same energy levels as before: Δ H and the reaction type stay the same.
Try independently: Copy your endothermic profile. Add a dashed catalysed pathway and a key distinguishing the two paths. Check that its peak is lower than the uncatalysed peak but still above the product level. Explain why moving the product level downward would be an error.
Show a comparison and explanation
Swipe or scroll sideways to inspect the complete overview.
The dashed pathway has a lower activation barrier and shares the same endpoints. Moving the product level would change the overall reaction enthalpy; that is not the effect of a catalyst. Measure both activation energies from the same reactant level.
Try independently: Read an endothermic profile in reverse, from its products back to its reactants. Is the reverse reaction exothermic or endothermic? Which energy level is now the starting level for its activation energy?
Show answer and reasoning
The reverse reaction is exothermic: it starts at the higher level and ends at the lower level. Its Δ H has the opposite sign. Its activation energy is measured from the original product level to the same peak, because those products are now the reverse reaction’s reactants. Do not reuse the forward activation-energy interval.
Use the Chemical Energetics topic check to practise interpretation and energy reasoning. Keep practising drawings on paper: recognition questions alone do not demonstrate that you can construct a complete profile.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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