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# 1.15: Energy and Biochemical Reactions

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Practice Energy and Biochemical Reactions
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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:

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 below .

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 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&feature=related (1:16).

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.

### Practice

Use this resource to answer the questions that follow.

1. What is energy?
2. Why do living organisms need energy?
3. What is the main difference between potential and kinetic energy?
4. What is the original source of most energy used by living organisms on Earth?

### Review

1. What is the general chemical equation for an endothermic reaction?

2. Why do all chemical reactions require activation energy?

Feb 24, 2012

Sep 15, 2014