Chemical Energy

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Author: M. Rae Nelson
Editor: Kristine Krapp
Date: 2010
Document Type: Topic overview; Experiment activity
Length: 2,076 words
Content Level: (Level 4)
Lexile Measure: 1150L

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Chemical energy is the energy stored within the bonds of atoms. A bond is the force that holds two atoms together. Different substances have bonds held together by different amounts of energy. When those bonds are released, a chemical reaction takes place, and a new substance is created. Chemical reactions that break these bonds and form new ones sometimes release the excess energy as heat and sometimes absorb heat energy from whatever is around them.

Thus, heat energy can be produced or absorbed during a chemical reaction. Reactions that release heat energy are called exothermic. Reactions that take in heat energy from the surrounding environment are called endothermic. Whether heat energy is given off or absorbed during a chemical reaction depends on the bonds that hold the atoms together.

In a chemical reaction, the original substances are called reactants. The new substances that are formed are called products. When the bonding structure of the products requires less energy than the bonding structure of the reactants, the excess energy may be released as heat. When the bonding structure of the products requires more energy than the structure of the reactants, it gets that energy by removing heat from its surroundings.

For example, when iron rusts, the iron atoms are combining with oxygen molecules in the air to form iron oxide. The chemical reaction of rusting breaks the bonds in the oxygen molecules, releasing heat energy. The bonds between the oxygen atoms and the iron atoms do not require as much heat energy as the bonds within the oxygen molecules, so a small amount of energy is released, making the reaction exothermic. The amount of heat released is quite small, and the reaction is normally quite slow, so rusting iron does not feel hot to us. Yet, the energy released can be measured with a thermometer. In the first experiment, you will observe the change in temperature resulting from rusting.

Some exothermic reactions are quite common. One is combustion, the burning of organic substances during which oxygen is used to form carbon dioxide and water vapor. The substances formed (ashes, for example) hold less heat energy than the substances burned held. The excess energy is released as heat. The reactions between some chemicals, such as aluminum oxide and iron oxide, can produce great amounts of heat. This reaction is used to produce very high temperatures for industrial purposes.

Endothermic reactions are more rare in nature, but scientists have found ways to create them. For example, an endothermic reaction occurs when you use a chemical cold pack. These packs contain a chemical in powder form that reacts with water. Squeezing the pack breaks down the wall separating the powder from the water. The reaction that occurs absorbs more energy than it releases, making the pack feel cold to you. In the second experiment, you will compare four chemical reactions and determine whether each one is exothermic or endothermic.

Words to Know

Atom
The smallest unit of an element, made up of protons and neutrons in a central nucleus surrounded by moving electrons.
Bond
The force that holds two atoms together.
Chemical energy
Energy stored in chemical bonds.
Chemical reaction
Any chemical change in which at least one new substance is formed.
Combustion
Any chemical reaction in which heat, and usually light, is produced. It is commonly the burning of organic substances during which oxygen from the air is used to form carbon dioxide and water vapor.
Control experiment
A set-up that is identical to the experiment but is not affected by the variable that will be changed during the experiment.
Endothermic
A chemical reaction that takes in heat energy.
Exothermic
A chemical reaction that gives off heat energy.
Heat
A form of energy produced by the motion of molecules that make up a substance.
Hypothesis
An idea in the form of a statement that can be tested by observation and/or experiment.
Molecule
The smallest particle of a substance that retains all the properties of the substance and is composed of one or more atoms.
Product
A compound that is formed as a result of a chemical reaction.
Reactant
A compound present at the beginning of a chemical reaction.
Variable
Something that can affect the results of an experiment.

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EXPERIMENT 1

Rusting: Is the chemical reaction exothermic, endothermic, or neither?

Purpose/Hypothesis

In this experiment, you will measure the heat energy released or absorbed by the chemical reaction of rusting, the transformation of iron and atmospheric oxygen into iron oxide. Before you begin, make an educated guess about the outcome of this experiment based on your knowledge of rusting. This educated guess, or prediction, is your hypothesis. A hypothesis should explain these things:

  • the topic of the experiment
  • the variable you will change
  • the variable you will measure
  • what you expect to happen

A hypothesis should be brief, specific, and measurable. It must be something you can test through observation. Your experiment will prove or disprove whether your hypothesis is correct. Here is one possible hypothesis for this experiment: "A rise in air temperature will show that rusting is an exothermic reaction."

In this case, the variable you will change is the number of rusting pads in each cup, and the variable you will measure is any change in air temperature. You expect the temperature to go up in the cups with rusting pads and the temperature to go up the most in the cup with the most pads.

As a control experiment, you will leave one cup empty and monitor any change in temperature there. If the temperature is higher in the cup with more pads and does not change in the empty cup, your hypothesis will be supported.

What Are the Variables?

Variables are anything that might affect the results of an experiment. Here are the main variables in this experiment:

  • the type of reactants used (iron in the pads and atmospheric oxygen)
  • the temperature of the environment in which the samples are tested
  • the number of rusting pads in each cup

In other words, the variables in this experiment are everything that might affect the temperature in the cup. If you change more than one variable, you will not be able to tell which variable had the most effect on the temperature.

Level of Difficulty

Moderate.

Materials Needed

  • 4 large Styrofoam cups
  • aluminum foil
  • 7 steel wool pads (not pads treated with detergent or soap)
  • vinegar
  • 4 digital laboratory thermometers
  • rubber or surgical gloves
  • paper towels
  • large bowl

Approximate Budget

$10. (If four thermometers are unavailable, the four parts of this experiment can be performed separately with one thermometer.)

Timetable

About 45 minutes.

How to Experiment Safely

Wear protective gloves when handling the steel wool and vinegar.

Step-by-Step Instructions

  1. Line the inside of each of the four cups with aluminum foil.
  2. Place the seven steel wool pads in the large bowl and soak them thoroughly in vinegar (to remove any coating and encourage rusting). Blot them dry with paper towels.
  3. Place one pad in the first cup, two pads in the second cup, and four pads in the third cup. The fourth cup will be empty--your control.
  4. Push the bulb of one thermometer gently into the steel wool in the first cup. Do not push the bulb down to or near the bottom of the cup. Cover the opening of the cup with aluminum foil. The stem on the thermometer must be visible.
  5. Repeat Step 4 for the second, third, and fourth (control) cups.
  6. Place all four cups where no other heat sources will affect their temperature.
  7. Prepare a chart similar to the one illustrated so you can record your observations.
  8. Observe and record any change in temperature in any of the four cups every 10 minutes. The rusting process will begin immediately, but the resulting change in temperature will be gradual and small. Make sure that external factors are not affecting the temperature, such as sunlight or heat from a lamp.

Summary of Results

Examine your results and determine whether your hypothesis is correct. Did the temperature rise higher when more wool pads were in the cup? Did it rise in the empty cup? If the reactions resulted in an increase in temperature, then rusting is indeed exothermic. Make sure that your chart shows clearly the result of the tests on each sample.

Troubleshooter's Guide

Few problems should arise if the steps in this experiment are followed closely. However, when doing experiments involving the mixing of substances, be aware that a number of variables--such as temperature and impurity of substances--can affect your results. Here is a problem that may arise, a possible cause, and a way to remedy the problem.

Problem: You observed little or no temperature change in the cups.

Possible cause: The steel wool is not rusting. Try soaking it in vinegar again for several minutes to remove any protective layers and then repeat the experiment.

Change the Variables

You can vary this experiment. Here are some possibilities:

  • Other metals will oxidize, though at much slower rates. See if you can measure the temperature change resulting from the oxidation of copper (loops of copper wire may be best).
  • Compare the heat energy released by different kinds of oxidation. What about the oxidation you can see occurring on the cut surface of an apple? Find a way to determine if that reaction is exothermic. Always check first with your teacher before altering the materials used in your experiments.

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EXPERIMENT 2

Exothermic or Endothermic: Determining whether various chemical reactions are exothermic or endothermic

Purpose/Hypothesis

In this experiment, you will measure the heat energy released or absorbed as four different chemicals (see the materials list) are mixed with water. You expect that the temperature of the solution will go up if the reaction is exothermic and go down if the reaction is endothermic. Before you begin, make an educated guess about the outcome of each reaction based on your knowledge of the chemicals and reactions involved. This educated guess, or prediction, is your hypothesis. A hypothesis should explain these things:

  • the topic of the experiment
  • the variable you will change
  • the variable you will measure
  • what you expect to happen

A hypothesis should be brief, specific, and measurable. It must be something you can test through observation. Your experiment will prove or disprove whether your hypothesis is correct. Here is one possible hypothesis for one of the reactions in this experiment: "Mixing water with calcium chloride will produce an exothermic reaction."

In this case, the variable you will change is the chemical being reacted with water, and the variable you will measure is the resulting temperature of the solution. In the case of calcium chloride, you expect the temperature to go up.

As a control experiment, you will measure the temperature in a beaker of distilled water with no chemical in it. If the temperature changes in the beakers with chemicals as predicted and remains steady in the control beaker, you will know your hypothesis is supported.

What Are the Variables?

Variables are anything that might affect the results of an experiment. Here are the main variables in this experiment:

  • the type of reactants used
  • the purity of the reactants
  • the temperature of the environment in which the samples are tested

In other words, the variables in this experiment are everything that might affect the temperature of the solutions. If you change more than one variable, you will not be able to tell which variable had the most effect on the temperature.

Level of Difficulty

Moderate; an adult's supervision is required.

Materials Needed

  • 5 glass beakers
  • 1 graduated cylinder
  • 1 glass stirring rod
  • 1 small spoon or spatula
  • 1 digital laboratory thermometer
  • 1 pint (500 milliliters) distilled water
  • 1 tablespoon (14 grams) calcium chloride
  • 1 tablespoon (14 grams) sodium hydrocarbonate
  • 1 tablespoon (14 grams) ammonium nitrate
  • 2 teaspoons (10 milliliters) concentrated sulfuric acid
  • safety glasses or goggles
  • rubber or surgical gloves

Approximate Budget

$25. (This experiment should be performed only with the appropriate lab equipment and materials. Ask your teacher about ordering the chemicals.)

Timetable

One hour.

How to Experiment Safely

This experiment involves dangerous and toxic substances. No part of this experiment should be performed without adult supervision. You must be especially careful handling the sulfuric acid, which is highly corrosive. Wear gloves and safety glasses or goggles at all times! When you are finished with the experiment, the chemicals used must be disposed of properly and with supervision. Ask your teacher for help in handling, neutralizing, and disposing of the sulfuric acid.

Step-by-Step Instructions

  1. Place the five beakers on a clean, stable surface and use the graduated cylinder to measure and pour 3½ tablespoons (about 50 milliliters) of distilled water into each one.
  2. Prepare a chart on which you will record your observations. Your chart should look something like the illustration.
  3. Place the thermometer in the first beaker and record the temperature on your chart. This sample, which contains only the distilled water, will be your control.
  4. Using the spoon or small spatula, add about half the sample of calcium chloride to the second beaker. Stir it gently until it is mixed with the distilled water.
  5. Place the thermometer in the beaker and note the temperature once each 30 seconds for five minutes. Record the temperatures on the chart. When you are done, be sure to rinse the thermometer with room-temperature distilled water.
  6. Repeat Steps 4 and 5 for the sodium hydrocarbonate and the ammonium nitrate. Remember to rinse the thermometer, stirring rod, spatula, or spoon in distilled water after each test.
  7. In the last beaker, slowly and gently add all of the sulfuric acid to the water. Be careful not to spill or splash the acid. Place the thermometer in the beaker and note the temperature once each 30 seconds for five minutes. Record the temperature changes on your chart. When you are done, be sure to rinse the thermometer.

Summary of Results

Examine your results and determine whether each of your hypotheses is correct. If any reactions resulted in an increase in temperature, those reactions are exothermic. If any reactions resulted in a decrease in temperature, they are endothermic. Make sure that your chart shows clearly the result of the tests on each set of reactants. It may be helpful to those viewing your results to see a diagram outlining the procedure you followed.

Troubleshooter's Guide

When doing experiments involving the mixing of substances, be aware that a number of variables--such as temperature and impurity of substances--can affect your results. When mixing substances, you must keep the mixing containers and utensils clean. Even tiny impurities in a mixture can drastically alter your results.

Here is a problem that may arise, a possible cause, and a way to remedy the problem.

Problem: You observed little or no temperature change in the beakers.

Possible cause: You are not placing enough of the solid reactants in the water. Try increasing the amount of solid reactant.

Change the Variables

You can vary this experiment by trying reactions involving different household materials or chemical compounds. Do not mix them with anything other than water. Always check first with your teacher before altering the materials used in your experiments.

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Design Your Own Experiment

How to Select a Topic Relating to this Concept

Other kinds of experiments can reveal interesting facts about endothermic and exothermic reactions. Our bodies produce exothermic reactions when we turn food into energy. Can you measure the amount of food energy available in a sample by burning it and measuring the resulting temperature change in a sample of water? Review the description of how cold packs work. Can you think of a way to design a homemade cold pack?

Check the Further Readings section and talk with your science teacher or school or community media specialist to start gathering information on chemical reaction questions that interest you.

Steps in the Scientific Method

To do an original experiment, you need to plan carefully and think things through. Otherwise, you might not be sure what question you are answering, what you are or should be measuring, or what your findings prove or disprove.

Here are the steps in designing an experiment:

  • State the purpose of--and the underlying question behind--the experiment you propose to do.
  • Recognize the variables involved, and select one that will help you answer the question at hand.
  • State a testable hypothesis, an educated guess about the answer to your question.
  • Decide how to change the variable you selected.
  • Decide how to measure your results.

Recording Data and Summarizing the Results

In the experiments included here and in any experiments you develop, strive to display your data in accurate and interesting ways. Remember that those who view your results may not have seen the experiment performed, so you must present the information you have gathered as clearly as possible. Including photographs or illustrations of the steps in the experiment is a good way to show a viewer how you got from your hypothesis to your conclusion.

Related Projects

Chemical energy is a basic and crucial part of life processes as well as technological processes. Projects that determine the energy produced by different fuels and compare the by-products of those fuels can help to demonstrate the necessity for developing alternative energy sources. Examining different reactions and determining their endothermic or exothermic rate can help us understand where so much of the energy we use goes.

Source Citation

Source Citation   

Gale Document Number: GALE|CV2644200005