Content Benchmark P.12.A.6
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Students know chemical reactions either release or absorb energy. E/S

All chemical reactions involve a change in energy. Sometimes the change that occurs is so small that it is not detected, but often this change results in an increase or a decrease in the temperature of the system.

In chemical reactions, bonds are broken and new bonds are formed. Energy must be added to break the bonds that hold atoms of the reactant together. Conversely, when bonds are formed to make product, energy is released. If the amount of energy released when the product forms is greater than the amount of energy required to break the bonds of the reactant, energy will be released. However, if the amount required to break bonds is more than the amount released when new bonds form, energy must be taken in from the environment.

Reactions which release energy are called exothermic reactions. In an exothermic reaction, the energy of the reactants is greater than that of the products. The difference in energy is the amount of energy (usually transferred via heat) that flows from the reaction to the surroundings causing the temperature to increase.

Energy level diagrams can be used to represent the change in energy that occurs in chemical reactions. Figure 1 illustrates the energy change that occurs in an exothermic reaction.

Figure 1. Energy diagram of an exothermic reaction. (From http://woodchurchscience.edublogs.org/category/coursework/

An example of an exothermic reaction is that between sodium metal and chlorine gas. Once water is added the reaction occurs. Energy is transferred through heat and light.

Figure 2. Sodium and chlorine reaction. (From: http://www.angelo.edu/faculty/kboudrea/demos/ sodium_chlorine/sodium_chlorine.htm)

In endothermic reactions energy must be absorbed from the surroundings and a decrease in temperature is measured. In an endothermic reaction, the energy of the products is greater than that of the reactants. Figure 3 illustrates the changes in energy that occur in an endothermic reaction. The difference in energy when reactants are compared to products is equal to the amount of energy taken from the environment causing it to cool.

Figure 3. Energy diagram of an exothermic reaction. (From http://woodchurchscience.edublogs.org/category/coursework/)

An example of an endothermic reaction is that of barium hydroxide with ammonium thiocyanate.

The resulting mixture becomes cold enough to cause the container to freeze to a block of wood.

Figure 4. Reaction of barium hydroxide and ammonium thiocyanate. (From: http://jchemed.chem.wisc.edu/jcesoft/cca/CCA3/MAIN/ENDO2/PAGE1.HTM)

Frequently, energy must be added to initiate a reaction. This energy is called activation energy and an example of this is the striking of a match. Once the match starts to burn (and products form), energy is released which means that the reaction is exothermic. Figures 1 and 3 illustrate the relationship between activation energy which is the energy that must be added to start the reaction and the energy changes that result as reactant is converted to product.

Physical processes also involve changes in energy. Melting and vaporization are endothermic processes requiring the addition of energy. For example, when ice is added to a glass of water and then melts, the water temperature decreases indicating that the process of melting is endothermic. The evaporation of perspiration causes our skin to feel cool. Both of these processes are endothermic, absorbing heat from the surroundings and resulting in a decrease in temperature.

It follows that freezing and condensation (the opposite processes of those listed above) would then be exothermic processes releasing heat to the environment.

Common examples of endothermic and exothermic processes are listed below:

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Performance Benchmark P.12.A.6

Students know chemical reactions either release or absorb energy. E/S

Common misconceptions associated with this benchmark:

1. Students incorrectly believe that bonds store energy, breaking chemical bonds releases energy, and bond making requires energy.

Using the phrase “energy is stored in chemicals” leads students to believe that if the bonds are broken the energy will be released. However, energy is released to the surroundings only when a chemical reaction occurs in which the energy that is released in forming the products is greater than the energy needed to break the bonds in the reactants. For example, it is the chemical reaction of fossil fuels with oxygen that results in the release of energy.

To learn more about this misconception and other student misconceptions about basic chemistry, go to http://www.chemsoc.org/pdf/learnnet/rsc/miscon.pdf

2. Students incorrectly think that a candle burning is endothermic because heat transfer is needed to initiate the reaction.

While it is true that heat transfer is needed to initiate the reaction (activation energy), once the candle starts to burn, it continues to burn without the addition of energy. The reaction continues on its own releasing heat and light making the reaction exothermic.

Learn more on this and other chemistry misconceptions at http://educ.queensu.ca/~science/main/concept/chem/c07/C07CDTL1.htm

3. Students have difficulty identifying endothermic and exothermic processes.

If energy flows out of the reaction and raises the temperature of the surroundings, the reaction is exothermic. If energy flows into the reaction causing the surroundings’ temperature to decrease, the reaction is endothermic.

An interesting lesson that directly addresses this misconception is found at http://astlc.ua.edu/ENERGY/lessonplans/highschoollessons/
highschoollessonsx.htm#Endothermic%20and%20Exothermic

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Performance Benchmark P.12.A.6

Students know chemical reactions either release or absorb energy. E/S

Sample Test Questions

1. A reaction that releases heat is
a. endothermic.
b. exothermic.
c. neutral.
d. unchanged.

2. For an exothermic reaction, the products
a. have virtually no interaction energy.
b. are at a higher energy level than the reactants.
c. are at a lower energy level than the reactants.
d. are at the same energy level as the reactants.

3. The process of ice melting is
a. endothermic.
b. exothermic.
c. isothermic.
d. catalyzed.

4. If an endothermic reaction were occurring in a beaker,
a. the beaker would feel warmer than before the reaction occurred.
b. the beaker would feel cooler than before the reaction occurred.
c. the beaker would feel the same before and after the reaction.
d. the beaker would feel warmer then rapidly get cooler.

5. Which of the following is an endothermic process?
a. Water condensing
b. Water freezing
c. Water boiling
d. Water depositing

6. If a piece of metal is heated and then dropped into room temperature water,
a. the temperature of the metal will increase and the temperature of the water will increase.
b. the temperature of the metal will decrease and the temperature of the water will decrease.
c. the temperature of the metal will increase and the temperature of the water will decrease.
d. the temperature of the metal will decrease and the temperature of the water will increase.

Students know chemical reactions either release or absorb energy. E/S

Answers to Sample Test Questions
1. (b)
2. (c)
3. (a)
4. (b)
5. (c)
6. (d)

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PerformanceBenchmark P.12.A.6

Students know chemical reactions either release or absorb energy. E/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. General Discussion of Concepts Related to Energy and Chemical Reactions. The introductory topics of endothermic and exothermic reactions as well as the advanced topics of energy diagrams, catalysts, and bond energy calculations are included at this site. Animations of endothermic and exothermic reactions are included.

To access these, go to http://www.bbc.co.uk/schools/gcsebitesize/chemistry/
chemicalreactions/2energychangesrev1.shtml

An introductory discussion only of endothermic and exothermic reactions and activation energy can be found at http://www.visionlearning.com/library/module_viewer.php?mid=54

2. Endothermic and Exothermic Reaction Labs.
A lab based on hot and cold packs with an excellent student report sheet can be found at this site. Links are included that have videos on hot packs, explain how they work, and FAQ about them.

To access these labs, go to http://nobel.scas.bcit.ca/debeck_pt/science/hotColdPack/pack_p1.htm

Safe demos of an endothermic reaction (baking soda and citric acid) and an exothermic reaction (rusting of steel wool) that can be modified for student labs are found at http://chemistry.about.com/cs/generalchemistry/a/aa051903a.htm

How commercial cold and hot packs work are discussed at this site. Student- made simulations of cold and hot packs using ammonium nitrate and calcium chloride are monitored with probeware. These simulations are found at http://www.onsetcomp.com/information/labs/pdf_labs/6345_hotcoldpacks.pdf

The endothermic reaction between barium hydroxide and ammonium chloride which is cold enough to freeze water is described at this site. This reaction can be used as a demonstration, and the video of the reaction is also available.
http://dwb.unl.edu/Chemistry/DoChem/DoChem096.html

The exothermic reaction between vinegar and steel wool is monitored using temperature sensors. The law of conservation of mass is also confirmed in this lab found at:
http://www.pasco.com/experiments/chemistry/october_2002/home.html

3. Information Sport Creams
Students often confuse the action of sport creams with exothermic reactions. This site answers the question: How do sport creams work?
http://www.madsci.org/posts/archives/2000-09/969981432.Ch.r.html

4. Information About Self-Heating Cans
The self-heating can provides an unusual use for exothermic reactions. A can is dissected and the chemistry behind the process is revealed. For excellent photographs and a thorough discussion go to:
http://www.makezine.com/extras/48.html

5. Excellent Videos of Exothermic and Endothermic Reactions
The exothermic reaction between sodium metal and chlorine gas gives off intense heat and light. You can watch this video at http://www.youtube.com/watch?v=Mx5JJWI2aaw

The endothermic reaction between barium hydroxide and ammonium nitrate is cold enough to freeze water. A discussion of enthalpy and entropy is also included. You can view this video at http://www.youtube.com/watch?v=5RJLvQXce4A

A hot plate is used to supply the activation energy needed to ignite matches. To see this video, go to http://video.google.com/videoplay?docid=2211968460268385674
&q=energy+and+chemical+reaction

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