Performance Benchmark P.12.A.6
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.
2 Na (s)  Cl2 (g)  2 NaCl (s)
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.
Ba(OH)28H2O (s)  2 NH4SCN (s)  Ba(SCN)2 (s)  2 NH3 (g)  10 H2O (l)
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:
Exothermic processes
Endothermic processes
making ice cubes
melting ice cubes
formation of snow in clouds
conversion of frost to water
vapor
condensation of rain from water vapor
evaporation of water
a candle flame
baking bread
rusting iron
cooking an egg
burning sugar
producing sugar by
photosynthesis
Combining atoms to make a molecule in the
gas phase
splitting a gas molecule apart
crystallizing liquid salts (as in sodium acetate
in chemical handwarmers)
mixing water and ammonium
nitrate (cold pack)
Figure 5. Examples of endothermic and exothermic processes. Excerpt taken from:
http://antoine.frostburg.edu/chem/senese/101/thermo/faq/exothermic-endothermic-examples.shtml
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#End
othermic%20and%20Exothermic
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.
Performance Benchmark P.12.A.6
Students know chemical reactions either release or absorb energy. E/S
Answers to Sample Test Questions
1.
2.
3.
4.
5.
6.
(b)
(c)
(a)
(b)
(c)
(d)
Performance Benchmark P.12.A.6
Students know chemical reactions either release or absorb energy. E/S
Intervention Strategies and Resources
The following list of intervention strategies and resources 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/2energychan
gesrev1.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+c
hemical+reaction