Sangeetha Pulapaka

Do you want to know the method to determine if a reaction is endothermic or not?

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Sangeetha Pulapaka
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Sangeetha Pulapaka

When a chemical reaction takes place, chemical bonds in the reactants are broken and new bonds are formed. Depending on the nature of the bonds involved, a reaction will either release energy to, or absorb energy from, the surroundings, as heat. When a process releases energy to the surroundings it’s called exothermic (from the Greek thermo meaning heat, and exo meaning outside). The most obvious effect of an exothermic chemical reaction is generally a rise in the temperature of the reaction mixture. However, some processes absorb heat energy from the surroundings in which case they’re called endothermic reactions (endo in Greek, means within). The effect of an endothermic chemical reaction is generally a fall in the temperature of the reaction mixture.

All combustion reactions (e.g burning of coal) are exothermic. Incredibly, the reaction between iron and moist air to produce rust is a very exothermic process that generates lots of heat. Unfortunately, this particular reaction takes place so slowly that the liberation of heat is undetectable. Pyrotechnics, explosives and fuels, on the other hand, all involve very fast and hugely exothermic chemical reactions.

Concepts: exothermic reactions; heat

Skills: observing, collecting data


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

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

  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 will happen:

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.

Here is the link to an experiment to observe a endothermic reaction