activation energy

American  

noun

Chemistry.

1. the least amount of energy required to activate atoms or molecules to a state in which they can undergo a chemical reaction.

---

activation energy Scientific  

/ ăk′tə-vā′shən /

1. The least amount of energy needed for a chemical reaction to take place. Some elements and compounds react together naturally just by being close to each other, and their activation energy is zero. Others will react together only after a certain amount of energy is added to them. Striking a match on the side of a matchbox, for example, provides the activation energy (in the form of heat produced by friction) necessary for the chemicals in the match to ignite. Activation energy is usually expressed in terms of joules per mole of reactants.

---

Etymology

Origin of activation energy

First recorded in 1920–25

Example Sentences

For example, swapping chlorophyll and pheophytin in D2 could overcome the electron block, because chlorophyll needs lower activation energy than pheophytin.

From Science Daily • Oct. 13, 2025

Specifically, the transfer of electrons from pheophytin to plastoquinone in D2 requires twice as much activation energy as D1 -- a barrier that electrons seem unable to overcome, preventing energy from flowing forward.

From Science Daily • Oct. 13, 2025

The calculation of OTOCs showed that chemical reactions with a low activation energy at low temperatures where tunneling dominates can scramble information at nearly the quantum limit, like a black hole.

From Science Daily • Apr. 5, 2024

Account for the relationship between the rate of a reaction and its activation energy.

From Textbooks • Feb. 14, 2019

Even exergonic, energy-releasing reactions require a small amount of activation energy to proceed.

From Textbooks • Apr. 25, 2013