Pre-lab Questions

1. Define an endothermic reaction:
2. Define an exothermic reaction:

Problem

How can you determine if a reaction gives off energy or absorbs energy?

Purpose

Some reactions need energy to occur. Other reactions release energy when they occur. In this lab, you will use evidence to determine if a chemical reaction releases energy.

Materials (per group)

• 2 foam cups
• Black permanent marker
• 2 paper towels
• Citric acid
• Thermometer
• Hydrogen peroxide
• Stopwatch
• Beef liver
• Graduated cylinder
• Baking soda
• Teaspoon
• Spoon

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Wash your hands thoroughly before leaving the lab.
• Follow your teacher’s instructions for clean-up of materials and disposal of any chemicals.

• Do not consume lab solutions, even if they’re otherwise edible products.
• Food in the lab should be considered a chemical not for consumption.

Procedure

1. Take one foam cup and write the #1 on the outside. Take the other cup and write the #2 on the outside.
2. Measure 25 mL of citric acid into a foam cup.
3. Place the thermometer in the citric acid and record the initial temperature (⁰C) in the data table below.
4. Add a teaspoon of baking soda (sodium bicarbonate) to the cup and stir. The thermometer should still be in the cup.
5. Use a stopwatch to time 15-second intervals. After each 15 second interval, you should record the temperature of the reaction in the cup in the data table below.
6. Record the temperature 12 times, every 15 seconds, during a total of 3 minutes.
7. Take the thermometer out of the cup, rinse it and wipe it dry with a paper towel.
8. Measure 25 mL of hydrogen peroxide, and add it to the 2^nd foam cup.
9. Place the thermometer in the cup and record the temperature in the data table below.
10. Place a small piece of liver in the hydrogen peroxide.
11. Again, use a stopwatch to time 15-second intervals. After each 15 second interval, you should record the temperature of the reaction in the cup in the data table below.
12. Record the temperature 12 times, every 15 seconds, during a total of 3 minutes.
13. Take the thermometer out of the cup, rinse it and wipe it dry with a paper towel.

Data

Time	Temperature (°C)
	Citric Acid Solution	Hydrogen peroxide
Initial Temperature, 0 seconds
15 seconds
30 seconds
45 seconds
1 minute
1 minute, 15 seconds
1 minute, 30 seconds
1 minute, 45 seconds
2 minutes
2 minutes, 15 seconds
2 minutes, 30 seconds
2 minutes, 45 seconds
3 minutes, seconds

Analysis

1. What happened to the temperature in cup #1 during the 3 minutes of the reaction?
2. What type of reaction, endothermic or exothermic, occurred in this cup? How do you know?
3. What happened to the temperature in cup #2 during the 3 minutes of the reaction?
4. What type of reaction, endothermic or exothermic, occurred in this cup? How do you know?
5. How could you explain the changes in the temperatures of cup #1 and cup #2?
6. If you were to repeat the experiment, do you think you would get the same results?
7. Why would it be important to repeat the experiment?
8. What would happen if you used a slice of carrot instead of the liver?
9. If you added more baking soda to the citric acid, would it cause a greater temperature change?

Conclusion

Now that you have completed the lab, it is good to know how endothermic reactions and exothermic reactions are used in everyday living. The following are some endothermic and exothermic reactions:

Exothermic Reactions:	Endothermic reactions:
Snow clouds forming	Frost melting to water vapor
Rusting Iron	Melting Ice cubes
Freezing ice cubes	Baking bread
Burning sugar	Cooking eggs

Imagine that you are the CEO of a company that sells camping equipment. Why is it important to know about endothermic reactions and exothermic reactions when you are trying to create and sell products to the public?