Activation Energy: A level Chemistry Definition

Alexandra Brennan

Written by: Alexandra Brennan

Reviewed by: Philippa Platt

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What is activation energy?

Activation energy (Eₐ) is the minimum energy required for reactant particles to successfully collide and form products in a chemical reaction. It represents the energy barrier that must be overcome for a reaction to proceed.

In reaction profile diagrams, activation energy is shown as the difference between the energy of the reactants and the peak of the energy curve. Catalysts lower activation energy by providing an alternative reaction pathway with a lower energy requirement, increasing the reaction rate.

Graph illustrating an exothermic reaction, showing energy decrease from reactants to products with a peak at the transition state; activation energy marked.
Reaction profile for an exothermic reaction
Graph illustrating an endothermic reaction; energy increases from reactants to products. ΔH is positive, and products are higher in energy.
Reaction profile for an endothermic reaction

Understanding activation energy is crucial in A Level Chemistry, particularly in reaction kinetics, collision theory, and the effect of temperature and catalysts on reaction rates.

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.

Philippa Platt

Author: Philippa Platt

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener

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