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3.26: Activation Energy

Difficulty Level: Basic / At Grade Created by: CK-12
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These cheerleaders have a lot of energy! Their role is to get the fans as excited as they are so everyone will cheer for the team. They use their energy to activate the crowd.  

Getting Started

Chemical reactions also need energy to be activated. They require a certain amount of energy just to get started. This energy is called activation energy. For example, activation energy is needed to start a car engine. Turning the key causes a spark that activates the burning of gasoline in the engine. The combustion of gas won’t occur without the spark of energy to begin the reaction.

Q: Why is activation energy needed? Why won’t a reaction occur without it?

A: A reaction won’t occur unless atoms or molecules of reactants come together. This happens only if the particles are moving, and movement takes energy. Often, reactants have to overcome forces that push them apart. This takes energy as well. Still more energy is needed to start breaking bonds in reactants.

Activating Endothermic and Exothermic Reactions

Some chemical reactions need a constant input of energy to take place. They are called endothermic reactions. Other chemical reactions release energy when they occur, so they can keep going without any added energy. They are called exothermic reactions.

Q: It makes sense that endothermic reactions need activation energy. But do exothermic reactions also need activation energy?

A: All chemical reactions need energy to get started, even exothermic reactions. Look at the Figure below. They compare energy changes that occur during endothermic and exothermic reactions. From the graphs, you can see that both types of reactions need the same amount of activation energy in order to get started. Only after it starts does the exothermic reaction produce more energy than it uses.

A Common Example

You have probably used activation energy to start a chemical reaction. For example, if you’ve ever struck a match to light it, then you provided the activation energy needed to start a combustion reaction. When you struck the match on the box, the friction started the match head burning. Combustion is exothermic. Once a match starts to burn, it releases enough energy to activate the next reaction, and the next, and so on. However, the match won’t burst into flames on its own.


  • All chemical reactions, including exothermic reactions, need activation energy to get started.
  • Activation energy is needed so reactants can move together, overcome forces of repulsion, and start breaking bonds.


  • activation energy: Energy needed to start a chemical reaction.


Watch the animated reactions at the following URL, and then answer the question below. http://www.youtube.com/watch?v=VbIaK6PLrRM

  1. What is the exothermic reaction in the video? What role does the flame play in this reaction?
  2. The exothermic reaction releases energy. What is this energy used for?
  3. What is the endothermic reaction in the video? What provides the activation energy for this reaction? What product results from this reaction?


  1. What is activation energy?
  2. Why do reactants need energy in order for a chemical reaction to begin?
  3. Make an original graph to represent the role of energy, including activation energy, in an endothermic reaction.

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Difficulty Level:



7 , 8

Date Created:

Oct 31, 2012

Last Modified:

Apr 14, 2014
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