http://www.ck12.org Chapter 3. Chemical Interactions
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 theFigure3.39. 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.
FIGURE 3.39
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.
Summary
- 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.
Vocabulary
- activation energy: Energy needed to start a chemical reaction.
Explore More
Watch the animated reactions at the following URL, and then answer the question below.
http://www.youtube.com/watch?v=VbIaK6PLrRM
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/202