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K324 / 2027
Chemical Energetics

Lesson 3 of 4

Read an energy profile

The barrier and the overall change are different quantities.

O-Level 6092 (2026) / SEC G3 K324 (2027)

Pure only
  • I can represent energy profiles.Syllabus

    Syllabus K324, 9(b). Represent energy profiles

Read an energy profile

The barrier and the overall change are different quantities.

An energy profile plots the energy of the reacting system against reaction progress, not time. Reactants start on one level, the path rises to an energy barrier, and products finish on another. Activation energy, Ea, is the minimum energy barrier that must be overcome for a successful reaction. Read it from the reactant level to the peak; read ΔH from reactant level to product level.

An exothermic profile

The reactants lie above the products; the activation barrier rises from reactants to the peak, while the enthalpy change points down towards products.

Products are lower: dH represents negative enthalpy change (ΔH). Ea runs upward from the reactant level to the peak. The vertical scale is schematic; the horizontal axis is reaction progress, not time.

For an endothermic profile, draw products above reactants, so ΔH is positive. There is still an activation barrier above the reactant level. A catalyst provides a lower-barrier route but leaves reactant and product levels unchanged, so it changes the rate rather than ΔH.

An endothermic profile

Products finish above reactants. Activation energy still runs from the reactant level to the peak; the enthalpy increase is smaller than the full barrier.

Products are higher: dH represents positive enthalpy change (ΔH). Ea runs upward from the reactant level to the peak. The vertical scale is schematic; the horizontal axis is reaction progress, not time.

Watch out for this

A student says: "On an energy profile, Ea is a double-headed arrow between the reactant level and the peak." What is wrong with this?

Show the answer

Draw Ea and delta H as arrows with one head. The Ea arrow starts at the reactant level and points up to the peak. The delta H arrow starts at the reactant level and points to the product level: down for an exothermic reaction, up for an endothermic one. A double-headed arrow does not show the direction of the change, and mark schemes give no mark for it. Label both axes: energy, and progress of reaction.

Worked exampleConstruct a profile from energy levels

On a relative energy scale in kJ mol-1, reactants are at 80, products at 30 and the uncatalysed peak at 140. Sketch and label the profile.

Try it first. Then show the worked stepsHide the worked steps
  1. Label the vertical axis energy / kJ mol-1 and the horizontal axis reaction progress. Draw the reactant level at 80 and the lower product level at 30.
  2. Join them by a curve that rises to 140 before falling to the product level. Draw an upward activation-energy arrow from the reactant level to the peak: Ea = 140 - 80 = 60 kJ mol-1.
  3. Draw the enthalpy-change arrow from reactants down to products: delta H = 30 - 80 = -50 kJ mol-1. For a catalysed route, lower only the peak.
Answer

An exothermic profile: products are lower, delta H is negative, and Ea is measured up from the reactants, not from zero. The energy zero is arbitrary; differences determine the labels.

Try it step by step

Work it out in steps: Enthalpy change, then Activation energy, then Exothermic or endothermic?

  1. Problem 1. On an energy profile in kJ mol-1, the reactants are at 90, the products at 40 and the peak at 150. Give each energy as a whole number of kJ mol-1, with its sign.
  2. Problem 2. On an energy profile in kJ mol-1, the reactants are at 20, the products at 85 and the peak at 130. Give each energy as a whole number of kJ mol-1, with its sign.
  3. Problem 3. On an energy profile in kJ mol-1, the reactants are at 110, the products at 35 and the peak at 160. Give each energy as a whole number of kJ mol-1, with its sign.
Show the answer

Problem 1.

Enthalpy change: delta H = products - reactants = 40 - 90 = -50 kJ mol-1.

Activation energy: Ea = peak - reactants = 150 - 90 = 60 kJ mol-1. Measure it up from the reactants, not from zero.

Exothermic or endothermic?: The products are lower than the reactants and delta H is negative, so the reaction is exothermic.

Problem 2.

Enthalpy change: delta H = 85 - 20 = +65 kJ mol-1.

Activation energy: Ea = 130 - 20 = 110 kJ mol-1.

Exothermic or endothermic?: The products are higher than the reactants, so the reaction is endothermic.

Problem 3.

Enthalpy change: delta H = 35 - 110 = -75 kJ mol-1.

Activation energy: Ea = 160 - 110 = 50 kJ mol-1.

Exothermic or endothermic?: The products are lower and delta H is negative, so the reaction is exothermic.

Check your understanding

An exothermic reaction has a large activation energy. Is that a contradiction?

  1. No. A large Ea makes delta H more negative, so a high barrier is normal for exothermic reactions.
  2. No. Ea is the barrier at the start, while delta H compares the products with the reactants.
  3. Yes. An exothermic reaction gives out energy, so it has no need to take any in at the start.
Show the answer

No. Ea is the barrier at the start, while delta H compares the products with the reactants.

A reaction can need energy to get started (a high Ea) and still release more energy overall. Burning fuel needs a spark but is strongly exothermic.

Check your understanding

In an energy profile for the endothermic reaction CaCO3(s) -> CaO(s) + CO2(g), how are the arrows for Ea and delta H drawn?

  1. Ea runs from the product level to the peak; delta H points down to the reactants.
  2. Both are double-headed arrows between the levels, with delta H labelled negative.
  3. Both start at the reactant level: Ea up to the peak, delta H up to the products.
Show the answer

Both start at the reactant level: Ea up to the peak, delta H up to the products.

Use single-headed arrows. Ea goes from CaCO3 up to the top of the curve. delta H goes from CaCO3 up to CaO + CO2, which sit higher, so delta H is positive.