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Chemical Energetics

Topic 3 of 4

Read an energy profile

The barrier and the overall change are different quantities.

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

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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.

Worked example

Construct 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.

  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.

Check your understandingAn exothermic reaction has a large activation energy. Is that contradictory?Think it through, then reveal the answer
No. Activation energy is the initial barrier; enthalpy change compares final and initial levels. A reaction can require ignition yet release energy overall.