Activation Energy
Reactions need a minimum energy to start.
About this topic
Activation energy (Ea) is the minimum energy colliding particles need for a reaction to occur. It is the energy barrier between reactants and the transition state, shown as the peak on an energy profile diagram. Higher activation energy means a slower reaction at a given temperature.
Even strongly exothermic reactions often need a spark to begin, because bonds must be partially broken before new ones form. At the top of the barrier is the activated complex, a short-lived arrangement of atoms that is neither reactants nor products. Increasing temperature gives more particles energy above Ea, and catalysts lower Ea. The Arrhenius equation, k = A e^(-Ea/RT), shows that rate constants rise sharply with temperature; a rule of thumb is that rates double for every 10 degree rise.
Example: a mixture of hydrogen and oxygen can sit unreacted at room temperature for years, but a spark gives enough energy to start the reaction, which then releases enough heat to sustain itself. Food spoils faster in warm kitchens and slower in fridges, since fewer molecules have enough energy. Platinum in catalytic converters allows reactions at lower temperature.
A misconception is that activation energy is the energy released. In fact, the energy released is ΔH. Also, a catalyst lowers Ea for both directions equally and does not change ΔH.
Key ideas
- Energy barrier
- Collisions must be energetic enough
- Catalysts lower it
Quick check
Use cases: where Activation Energy helps
- Homework: review the key idea behind Activation Energy before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Activation Energy without notes.
- Visual learning: play with the interactive lab to see how Activation Energy changes when you adjust its inputs.
- Teaching a friend: explain Activation Energy in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Activation Energy?
Reactions need a minimum energy to start.
What will I learn in Activation Energy?
Energy barrier. Collisions must be energetic enough. Catalysts lower it.
Is Activation Energy part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Activation Energy lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Activation Energy?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.