CourseCourseQuestionQuestionDocumentDocument

1.1.3—Stability and energy

Syllabus
First assessment 2025
Objective
1.1.3
Level
SL

Stability and Energy Profiles

Lower relative energy corresponds to greater relative stability. In an exothermic reaction the products are lower in energy than the reactants; in an endothermic reaction the products are higher.

products are lower in enthalpy than reactants; Ea runs from the reactant level to the transition-state peak; ΔH points downward and is labelled negative; heat release is assigned to the surroundings.
products are higher in enthalpy than reactants; Ea runs from the reactant level to the transition-state peak; ΔH points upward and is labelled positive; heat absorption is assigned from the surroundings.

Label the horizontal axis reaction coordinate and the vertical axis potential energy. Then read reactant and product levels, identify the sign and direction of ΔH, and distinguish ΔH from activation energy.

On a profile, ΔH is the vertical difference between product and reactant levels, while activation energy rises from reactants to the peak. A catalyst lowers the peak by changing the pathway but leaves the two energy levels, ΔH and the relative stability of reactants and products unchanged.

An energy profile identifies an activation barrier but does not by itself determine an observed rate: temperature, particle concentrations/collision frequency and the available pathway also matter. Use the diagram to compare energetic barriers only when the profiles and conditions make that comparison valid.

Interpreting Energy Profiles

[ 3 ]

The forward reaction is endothermic, uses iron(III) oxide as a catalyst, and takes place at 900 K .

Sketch the energy profile for the reaction, both with and without the catalyst, labelling ΔH\Delta H and the activation energies.

Measuring Enthalpy Summary

Retrieve the route: define system/surroundings transfer, classify endothermic or exothermic, read stability from energy profiles, then calculate Q and ΔH with the correct sign.

Check the energy direction, surroundings temperature, reactant/product levels, moles, units, and ΔH sign.