Intermediate Topic 441 of 1102 Kinetics & Equilibrium
Activation Energy
The minimum energy colliding molecules need for reaction.
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In-depth explanation of Activation Energy
The activation energy Ea is the minimum energy that colliding molecules must possess for a reaction to occur, and it corresponds to the height of the energy barrier between reactants and products. Most activation energies for chemical reactions lie in the range of roughly 40 to 200 kJ per mol, and reactions with high barriers proceed slowly at ordinary temperatures. A higher activation energy makes the rate more sensitive to temperature, because the fraction of molecules with enough energy changes rapidly as temperature rises. Activation energy is not the same as the overall energy change of the reaction, which can be positive or negative.
Going deeper: Activation Energy
Key ideas: Estimate Ea from two rate constants: Ea = R ln(k2/k1) / (1/T1 - 1/T2), with R = 8.314 J per mol per K and a final answer in kJ per mol. A rough guide is that a rate doubles for each 10 K rise when Ea is near 50 kJ per mol around room temperature. Catalysts lower Ea of both directions equally.
Common mistakes and connections: A usual mistake is to forget that temperatures must be in kelvin or that R in J requires an answer in J, not kJ. Students also think Ea equals the enthalpy change; on a diagram Ea forward minus Ea reverse equals delta H. Another error is treating Ea as zero for fast radical recombination, which is nearly right but not exact. Links to the Arrhenius plot.
Visualization of Activation Energy
Ea is the height of the barrier from reactants to the top; it differs from the overall energy change of the reaction, here exothermic by 25 kJ/mol. The interactive lesson shows this as a live animation or chart.
Worked example and use case
With Ea = 50 kJ per mol, the rate near room temperature nearly doubles for a rise of about 10 degrees C.
Frequently asked questions about Activation Energy
What is Activation Energy?
The minimum energy colliding molecules need for reaction.
Is Activation Energy a beginner, intermediate or advanced topic?
Activation Energy is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Activation Energy?
Study these earlier topics in Kinetics & Equilibrium first: Steady State Approximation, Pre-equilibrium Approximation, Collision Theory.
What comes after Activation Energy?
The next topics in Kinetics & Equilibrium are Transition State Theory and Activated Complex.
Can I practise Activation Energy with quizzes?
Yes. The interactive lesson includes a self-check question on Activation Energy, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Activation Energy fits in Kinetics & Equilibrium
Activation Energy is topic 22 of 91 in Kinetics & Equilibrium.