#### Energy in Chemical Reactions

Energy in Chemical Reactions

Every chemical reaction is accompanied by a change in temperature. Sometimes the change is barely noticeable and other times it’s extreme. Sometimes the temperature goes up and other times it goes down. Try the two chemical reactions described in this activity to experience two different kinds of temperature changes.

PART A

Materials:
3% hydrogen peroxide (do not use a higher percentage)
Measuring spoons
Yeast (3 teaspoons)
Cup (paper or plastic)
Thermometer (use thermometers with red liquid only)
Watch (with a second hand)
Three people (at least)

Procedure:

1. Make a chart like the one below.

2. Pour 2 tablespoons of hydrogen peroxide into a cup. Place the thermometer into the cup. Hold the thermometer and cup so they do not fall over. Read the temperature and record it in the chart under “Time 0”.

3. Measure 1 teaspoon of yeast. Have one partner watch the thermometer and another look at the second hand

on a watch.

4. Dump all the yeast into the cup. Gently swirl the cup while one partner calls out the time every 10 seconds. When each 10 seconds is called, another partner should call out the temperature. The third partner should record the temperature in the chart. What did you observe?

 Time (Sec) 0 10 20 30 40 50 60 70 80 Temp. (0 C)

 Time (Sec) 90 100 110 120 130 140 150 160 170 Temp. (0 C)

PART B

Materials:
Vinegar
Baking soda
Measuring spoons
Water
Cup (paper or plastic)
Thermometer (use thermometers with red liquid only)

Procedure:

1. Make a chart like the one below.

2. Place 2 tablespoons of vinegar in the cup. Put the thermometer in the cup. Hold the thermometer and cup so they do not fall over. Read the temperature and record it in the chart under “Time 0”.

3. Measure 1 teaspoon of baking soda. Dump all the baking soda in the cup. Gently swirl the cup while one partner calls out the time every 3 seconds. When each 3 seconds is called, another partner should record the temperature in the chart. What did you observe?

4. Make a graph like the one below. Use the information on your chart to graph your results. During what period of time did the temperature change the most?

 Time (Sec) 0 3 6 9 12 15 18 21 24 Temp. (0 C)

 Time (Sec) 27 30 33 36 39 42 45 48 51 Temp. (0 C)

Make a graph like the one here. Use the information in your charts to graph your results. Use “X” marks for part A and “O” marks for part B. During what period of time did the temperature change the most? How about the least?

 Temp (deg C) 32 30 28 26 24 22 20 18 16 14 12 10 0 10 20 30 40 50 60 70 80 90 100 110 120 Time (sec)

Discussion;

In Part A of this activity, yeast was added to hydrogen peroxide. A chemical in the yeast causes a reaction in which the hydrogen peroxide breaks apart to form oxygen gas and water followed by a reaction of the oxygen with the carbohydrates of the yeast producing carbon dioxide and water. The energy released when the hydrogen peroxide broke apart was more than the energy required to form the reaction products; oxygen, carbon dioxide and water. Since the energy released was greater than the energy needed to form the products the excess energy was released as heat energy and the temperature went up. This reaction is called an exothermic reaction.

In Part B of this activity, baking soda was added to vinegar. Baking soda reacts with the vinegar to produce carbon dioxide gas, sodium acetate, and water. The energy released when the baking soda and vinegar reacted was less than the energy required to form the products; sodium acetate, carbon dioxide and water. Since the energy released was less than the energy required to form the products heat energy was drawn from the surrounding environment and the temperature went down. This reaction is called an endothermic reaction.

Learning experiences;
Skills
Measurement
Observation
Recording data
Charting
Graphing
Interpreting Graphs

Understanding
There is a transformation of energy from heat energy to chemical energy and from chemical energy to heat energy during some chemical reactions.*

The rate of a reaction decreases as the limiting reactant is used up.

Different reactions proceed at different rates.

*This is a good time to point out that the form of the energy needed to complete the reaction or released during the reaction is not always heat. For example, a battery, which utilizes a reversible chemical reaction, releases excess energy as electric energy when discharging and uses an input of electric energy when charging.