Year 12 / Reversibility and activation energy
Over the energy hill.
An energy profile compares reactants, products and a transition-state maximum. Activation energy is the gap from the starting level to this maximum. Enthalpy change is the product energy minus the reactant energy.
In an exothermic reaction, the products sit below the reactants. The reverse activation energy is therefore larger. A catalyst offers a different pathway with a lower barrier in both directions; reactant and product energies are unchanged.
INTERACTIVE MODEL
Find the activation barrier
Forward Ea: 60 · Reverse Ea: 90 kJ mol⁻¹
Illustrative exothermic profile. The catalyst lowers both activation barriers while ΔH stays −30 kJ mol⁻¹.
ΔH = Eproducts − Ereactants = Ea,forward − Ea,reverse
Assumed knowledge
Subtract energy levels. Units here are kJ mol⁻¹.
Learning checkpoints & source
YOUR SYLLABUS CHECKPOINT- Relate forward and reverse activation energies to enthalpy.
- Explain the effect of a catalyst on both directions.
READY TO TRY IT?