What is an exothermic reaction?

What is an exothermic reaction?
Top Image: DenisKot - iStock

Concepts: exothermic reactions; heat

Skills: observing, collecting data

What you need

  • Quick-rising dry yeast - 30 mL or 16 g
  • Hydrogen peroxide solution - 60 mL (3% to 5%, found in most drug stores)
  • Thermometer
  • Small bowl
  • Spoon
  • Pencil and paper
  • Measuring cup
  • Measuring spoon
  • Clock or stopwatch that indicates seconds

Safety First!

Always wear safety glasses when working with chemicals that could splash or spill into your eyes.

What to do

  1. Record the room temperature and then place the thermometer in the bowl.
  2. Predict what will happen to the temperature when the hydrogen peroxide and yeast are combined.
  3. Record your prediction on paper.
  4. Pour 60 mL of hydrogen peroxide solution into the bowl and record the temperature.
  5. Add 30 mL of yeast to the bowl and stir the mixture.
  6. Record time and temperature readings every 30 seconds for a total of 4 minutes.


What’s happening?

Hydrogen peroxide will naturally decompose into water and oxygen gas and a small amount of heat energy. This decomposition reaction usually happens slowly. You can observe the speed of the decomposition happening as bubbles form when the hydrogen peroxide is poured into a container.

When yeast is mixed in with the hydrogen peroxide, the yeast causes this decomposition chemical reaction to occur much more quickly. The hydrogen peroxide changes into water and oxygen gas much more quickly. The observed bubbles and foam is the oxygen gas being formed very rapidly.

Because the reaction is occurring much more quickly than it normally would, the amount of heat energy produced is much more noticeable. The increase in temperature can be determined by examining the overall increase in temperature during the reaction. Any reaction that gives of heat is called an exothermic reaction.

The yeast is not reacting with the hydrogen peroxide. Rather it is acting as a catalyst. A catalyst is a substance that causes a chemical reaction to speed up but is not “used” up during the reaction.

Why does it matter?

Exothermic reactions occur regularly and are important for everyday life. Specifically, all combustion reactions (reactions with oxygen, such as burning fuel) are exothermic. Fire is one of the most common and useful combustion reactions. Packaged heating pads for muscle strains or for heating hands or feet in winter produce their heat from an exothermic reaction that occurs when the chemicals inside the packet are combined by crushing the packet.

Investigate further!

A much more spectacular demonstration of this decomposition reaction can be seen by adding a small squirt of liquid dishwashing detergent to the hydrogen peroxide before adding the yeast. The dishwashing liquid will help trap the oxygen and will cause the foaming action to be more dramatic. Warning: this can be messy! Add a couple of drops of food coloring before adding the yeast for a more colourful result.

For an even more spectacular result, pour the hydrogen peroxide, soap and food coloring mixture in a 2 litre pop bottle and mix. Dissolve the yeast in about 100 mL water and then pour it into the bottle quickly. The resulting foam will be forced out through the small opening of the bottle. Try repeating the experiment in different types of bottles to observe the differences.