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As you just learned, chemical reactions usually involve the exchange of energy in some form. In the last activity, you learned about endothermic reactions that draws heat from the environment. In this activity, you will learn about the opposite reaction called an exothermic reaction.

An exothermic reaction is the opposite of an endothermic reaction. Where an endothermic reaction draws in heat, an exothermic reaction is a chemical reaction that releases energy by light or heat.

Exothermic reactions transfer energy to the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. The temperature increase can be detected using a thermometer. Below are some examples of exothermic reactions.

Another good example of an exothermic chemical reaction is the burning of natural gas. If you put your hands next to a natural gas flame, you feel heat; your hands become part of the surroundings, absorbing heat from the system.

In the last activity, you looked at how a cold pack works in an endothermic reaction. Now, let's take a look at how a hot pack works in an exothermic reaction. The process the heat pack goes through is known as an exothermic reaction. All liquids release heat when they freeze. For example, the water in an ice cube tray releases heat into the freezer compartment as the ice cubes solidify. When supercooled liquids freeze, their temperature rises to the melting point as they freeze.

That's the basic idea behind hot packs. A flexible plastic pillow pad is filled with a liquid (sodium acetate trihydrate) that is supercooled below its freezing point of 54° C. When you flex a tiny metal disk embedded in the pad, rough surfaces on the disk are exposed, which allows crystals to grow. The sodium acetate instantly begins to crystallize as the pad warms to a toasty 54° C.

Now let's take a look at how endothermic and exothermic reactions are different.

Click on the Play button below to watch a video that explains the difference between endothermic and exothermic reactions.

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