# 3.23: Endothermic Reactions

Difficulty Level: At Grade Created by: CK-12
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Did you ever use an instant ice pack like this one? You don’t have to pre-cool it in the freezer. All you need to do is squeeze the pack and it starts to get cold. How does this happen? The answer is an endothermic chemical reaction.

### What Is an Endothermic Reaction

All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. In some chemical reactions, called exothermic reactions, more energy is released when new bonds form in the products than is needed to break bonds in the reactants. The opposite is true of endothermic reactions. In an endothermic reaction, it takes more energy to break bonds in the reactants than is released when new bonds form in the products.

### Energy Change in Endothermic Reactions

The word endothermic literally means “taking in heat.” A constant input of energy, often in the form of heat, is needed to keep an endothermic reaction going. This is illustrated in the Figure below. Energy must be constantly added because not enough energy is released when the products form to break more bonds in the reactants. The general equation for an endothermic reaction is:

Reactants + Energy → Products

Note: ΔH represents the change in energy.

In endothermic reactions, the temperature of the products is typically lower than the temperature of the reactants. The drop in temperature may be great enough to cause liquids to freeze. That’s what happens in the endothermic reaction at this URL: http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/chemreac/energychangesrev1.shtml

Q: Now can you guess how an instant cold pack works?

A: Squeezing the cold pack breaks an inner bag of water, and the water mixes with a chemical inside the pack. The chemical and water combine in an endothermic reaction. The energy needed for the reaction to take place comes from the water, which gets colder as the reaction proceeds.

### Photosynthesis

One of the most important series of endothermic reactions is photosynthesis. In photosynthesis, plants make the simple sugar glucose (C6H12O6) from carbon dioxide (CO2) and water (H2O). They also release oxygen (O2) in the process. The reactions of photosynthesis are summed up by this chemical equation:

6 CO2 + 6 H2O → C6H12O6 + 6 O2

The energy for photosynthesis comes from light. Without light energy, photosynthesis cannot occur. As you can see in the Figure below, plants can get the energy they need for photosynthesis from either sunlight or artificial light.

### Summary

• An endothermic reaction is a chemical reaction in which more energy is needed to break bonds in the reactants than is released when new bonds form in the products.
• A constant input of energy, often in the form of heat, is needed to keep an endothermic reaction going.
• One of the most important series of endothermic reactions is photosynthesis. The energy needed for photosynthesis comes from light.

### Vocabulary

• endothermic reaction: Chemical reaction that needs a constant input of energy to continue because it takes more energy to break bonds in the reactants than is released when new bonds form in the products.

### Practice

Watch the series of slides and animations at the following URL. Then compare the energy changes in the reaction of baking soda with vinegar and the reaction of baking soda solution with calcium chloride.

### Review

1. What is an endothermic reaction?
2. Why is the temperature of products likely to be lower than the temperature of reactants in an endothermic reaction?
3. Describe an example of an endothermic reaction.

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### Vocabulary Language: English

TermDefinition
endothermic reaction Chemical reaction that needs a constant input of energy to continue because it takes more energy to break bonds in the reactants than is released when new bonds form in the products.

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