Endothermic and Exothermic Reactions

Talk It Over

When chemical changes happen, is there an energy change as well? Do some reactions give off heat while others absorb it? How can you find out?

Get

• 4 juice glasses
• Vinegar
• Scissors
• Steel wool pad
• Packet of dry yeast
• Hydrogen peroxide
• Baking soda
• Epsom salts
• NoSalt salt alternative (contains potassium chloride)
• Water
• Small spoons
• Digital instant-read thermometer*
• Sink or plastic tub

Go

1. You will make the mixtures shown in the chart above one at a time. Here's how:
   1. Fill a clean juice glass about half full of the liquid shown in the left column.
   2. Take and record the temperature of the liquid.
   3.  
   4. Add to the liquid a little of the material shown in the right column.
   5. Wait 2 minutes or longer and take the temperature again. Record.
2. Repeat the procedure for all five reactions. Use clean materials each time.
3. For each reaction, subtract to find the temperature change:

Ending Temperature - Starting Temperature = Temperature Change

If your answer is a + (positive) number, the liquid got hotter and the reaction released energy.

If your answer is a - (negative) number, the liquid got colder and the reaction absorbed energy.

Stay Safe

Don't touch the glasses after the reactants are mixed. Some of them may get very warm. Control spills and fizz-overs by doing these experiments in a sink or plastic tub.

Go Easy

Try only two reactions: vinegar/steel wool and vinegar/baking soda. Ask an adult to help you calculate and understand positive and negative temperature changes.

Go Far

Chemical reactions that release energy are exothermic. Reactions that absorb energy are endothermic. You can learn more about them using this procedure:

Note: Do not try this experiment with any electrical or battery-operated device. Do not open the packs or release the chemicals inside them. Dispose of them after use according to the directions on the package.

1. At your local drugstore, purchase instant cold packs and hot packs—the kind athletes use to relieve strains and sprains. Find a brand of cold pack that contains ammonium nitrate and water. Find a brand of hot pack that contains calcium chloride and water. You may want to try more than one size or brand.
2. For each kind of pack you want to test, put a measured amount of room temperature water in a pitcher or bucket. You want water deep enough to submerge the pack, but not much more.
3. Take the temperature of the water.
4. Activate the pack according to the directions on the package. Submerge the pack in the water. Hold it under the water's surface with a long-handled, wooden spoon, if you need to.
5. Start a stopwatch or timer and take the temperature of the water every minute.
6. How well do the packs work? Do some work better than others?

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