Energy and Biochemical
Reactions
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: September 23, 2014
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
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Chapter 1. Energy and Biochemical Reactions
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Energy and Biochemical
Reactions
Define energy.
Distinguish between an exothermic reaction and an endothermic reaction.
Describe the role of energy in chemical reactions.
Explain the significance of activation energy.
What is energy? Where does your energy come from? Can energy be recycled?
This team of ants is breaking down a dead tree. A classic example of teamwork. And all that work takes energy. In
fact, each chemical reaction - the chemical reactions that allow the cells in those ants to do the work - needs energy
to get started. And all that energy comes from the food the ants eat. Whatever eats the ants gets their energy from
the ants. Energy passes through an ecosystem in one direction only.
Chemical Reactions and Energy
Chemical reactions always involve energy. Energy is a property of matter that is defined as the ability to do work.
When methane burns, for example, it releases energy in the form of heat and light. Other chemical reactions absorb
energy rather than release it.
Exothermic Reactions
A chemical reaction that releases energy (as heat) is called an exothermic reaction. This type of reaction can be
represented by a general chemical equation:
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Reactants → Products + Heat
In addition to methane burning, another example of an exothermic reaction is chlorine combining with sodium to
form table salt. This reaction also releases energy.
Endothermic Reaction
A chemical reaction that absorbs energy is called an endothermic reaction. This type of reaction can also be
represented by a general chemical equation:
Reactants + Heat → Products
Did you ever use a chemical cold pack? The pack cools down because of an endothermic reaction. When a tube
inside the pack is broken, it releases a chemical that reacts with water inside the pack. This reaction absorbs heat
energy and quickly cools down the pack.
Activation Energy
All chemical reactions need energy to get started. Even reactions that release energy need a boost of energy in order
to begin. The energy needed to start a chemical reaction is called activation energy. Activation energy is like the
push a child needs to start going down a playground slide. The push gives the child enough energy to start moving,
but once she starts, she keeps moving without being pushed again. Activation energy is illustrated in Figure 1.1.
FIGURE 1.1
Activation Energy. Activation energy provides the “push” needed to start a chemical reaction. Is the chemical reaction in
this figure an exothermic or endothermic
reaction?
Why do all chemical reactions need energy to get started? In order for reactions to begin, reactant molecules must
bump into each other, so they must be moving, and movement requires energy. When reactant molecules bump
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Chapter 1. Energy and Biochemical Reactions
together, they may repel each other because of intermolecular forces pushing them apart. Overcoming these forces
so the molecules can come together and react also takes energy.
An overview of activation energy can be viewed at http://www.youtube.com/watch?v=VbIaK6PLrRM (1:16).
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/202
As you view Activation energy, focus on these concepts:
1. the role of activation energy,
2. what an energy diagram demonstrates.
Summary
• Chemical reactions always involve energy. A chemical reaction that releases energy is an exothermic reaction,
and a chemical reaction that absorbs energy is an endothermic reaction. The energy needed to start a chemical
reaction is the activation energy.
Review
1.
2.
3.
4.
5.
6.
What is Energy?
Why do living organisms need energy?
What is an exothermic reaction?
What is the general chemical equation for an endothermic reaction?
What is the activation energy?
Why do all chemical reactions require activation energy?
References
1. Hana Zavadska. Activation energy diagram. CC BY-NC 3.0
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