Ammonium Nitrate – Heat of Solution Lab
Written for PULSE by Christopher Martin & Stephanie Nardei
Editor: Stephanie Nardei
Time:
1 class period
Preparation
30 minutes
Time:
Materials: Heat of Solution of ammonium nitrate worksheet
Ammonium nitrate
Water
Thermometers
Graduated cylinders
Styrofoam cups
Abstract
Students will measure the energy change caused by dissolving one mole of ammonium nitrate in
water.
Objectives
Students will be able to:
1. Quantify the relationship
between temperature,
energy and heat
2. Understand that an
endothermic reaction
causes a decrease in
temperature.
National Science
Education Standards
National Science Standard
The Physical Setting
E. Energy Transformations
Different energy levels are associated with different configurations of atoms and molecules. Some
changes of configuration require an input of energy whereas others release energy.
Arizona Science Standard
Strand 5: Physical Science
Concept 4: Chemical Reactions
Investigate relationships between reactants and products in chemical reactions.
PO 6.
PO.10
Explain the energy transfers within chemical reactions using the law of conservation of energy.
Concept 3: Conservation of energy and increase in disorder
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PO.6 Distinguish between heat and temperature.
Teacher Background
Calorimeters are designed to be well-insulated, so no heat is gained from or lost to the
surroundings. If no heating element is used to introduce heat in the system, the total heat
(q) for the entire calorimeter system must equal zero. The total heat can be split into heats
for each component in the system.
Imagine a reaction in which solid ammonium nitrate (a component in some fertilizers and an
explosive) is dissolved in water to produce an aqueous ammonium nitrate solution.
NH4NO3 (s)
NH4+ (aq) + NO3- (aq)
The heat (qrxn) for this reaction is called the heat of solution for ammonium nitrate. When the
reaction is finished, the system contains two substances, the calorimeter itself and the
aqueous solution, and there is a heat associated with each component. The heat balance
for this experiment is thus
0 = q = qrxn + qcal + qsoln
The basic strategy in calorimetry is to use a temperature change and a heat capacity to
determine a heat. In this experiment all substances start at the same initial and final
temperatures.
qcal = Ccal ΔT = Ccal ( Tf - Ti )
qsoln = Csoln ΔT = msoln ssoln ( Tf - Ti )
One typically determines the heat capacity of the aqueous solution (Csoln) from the mass of
the solution (msoln) and the specific heat capacity of the solution (ssoln). The mass of the
solution is the sum of the masses of the water and ammonium nitrate originally placed in the
calorimeter. The specific heat capacity of the aqueous solution is usually close to that of
pure water (4.184 J oC-1 g-1).
The objective of this experiment is to determine the heat of reaction (in this case a heat of
solution). The above equations can be combined and rearranged to yield a working
equation:
qrxn = - qcal - qsoln = - ( Ccal + msoln ssoln )( Tf - Ti )
Just as the heat capacity of a substance is an extensive property, so the heat of solution is
an extensive property. It is generally more convenient to report intensive properties, thus the
heat capacity of a substance is usually reported as a specific heat capacity, that is, the heat
capacity per gram of substance. Similarly one can report a specific heat of solution, which is
the heat a solution per gram of solute. More commonly, though, the molar heat of solution is
reported.
The molar enthalpy of solution (ΔHsoln) is the heat of solution (qrxn) per mole of solute (n). In
this experiment the reaction is performed under conditions of constant pressure and the
only work is "PV-work"; under these conditions the heat flow for the process equals the
enthalpy change for the process.
ΔHsoln =
qrxn
2
n
ABOVE INFORMATION TAKEN FROM:
http://www.chm.davidson.edu/ChemistryApplets/calorimetry/HeatOFSolutionOfAmmoniumNitrate.html
Related and Resource Websites
Chemistry: The Science in Context
http://www2.wwnorton.com/college/chemistry/gilbert/concepts/chapter13/ch13_1.htm
David N. Blauch Calorimetry
http://www.chm.davidson.edu/ChemistryApplets/calorimetry/HeatOFSolutionOfAmmoniumNitrate.
html
Brigham Young University Virtual ChemLab Community Heat of Solution Lesson
http://pulse.pharmacy.arizona.edu/resources/heatofsolution.pdf
Chemistry & Issues in the Environment (Ammonium Nitrate image taken from this website)
http://www.elmhurst.edu/~chm/onlcourse/chm110/chm110.html
Daniel High School Larry Jones Heat of Solution Reactions Lab
http://www.sciencebyjones.com/heat_of_solutions.htm
Ammonium nitrate on Wikipedia http://en.wikipedia.org/wiki/Ammonium_nitrate
Wikimedia Commons two dimensional image of Ammonium Nitrate
http://commons.wikimedia.org/wiki/Image:Ammonium-nitrate-2D.png
Material Safety Data Sheet on Ammonium Nitrate
http://www.jtbaker.com/msds/englishhtml/a6048.htm
Answers.com on Ammonium Nitrate http://www.answers.com/topic/ammonium-nitrate
The History of Ammonium Nitrate
http://web1.caryacademy.org/chemistry/rushin/StudentProjects/CompoundWebSites/2001/Ammo
niumNitrate/history.htm
Activity
1. Have students use a graduated cylinder 10 and measure 100 ml of water.
2. Next, have them transfer the water to the styrofoam cup.
3. Record the temperature of the water.
4. Make certain they weigh accurately, between two and three grams of NH4NO3.
5. Next, they add the NH4NO3 to the water, continually swishing the cup gently, and with
constant observation of the temperature until it remains constant for about 15-20
seconds.
6. Have them record this temperature.
7. After the students complete all data collection for this experiment, make sure they fill in
the information into the Data Table below.
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Data Table
Mass of 75.0 mL of water
Mass of NH4NO3
Initial temperature of water
Final temperature of solution
Temperature change
Closure
Have students answer the following questions so what they learned throughout the experiment
can be synthesized.
1. Is the dissolution of the NH4NO3 in water an exothermic or endothermic process?
2. Assuming 1 mL of water has a mass of 1 gram and the specific heat of the dilute solution
is the same as water, calculate the number of joules involved in the dissolution of the
NH4NO3. (use actual mass of H2O)
# of joules = 75 g x temperature change x 4.185 J/gºC
3. Record your answer: NH4NO3 ______________ J
4. How many moles of each substance were dissolved?
# of moles = grams dissolved
Molar mass
NH4NO3
_____________ mole
5. Calculate the number of joules that would be involved if one mole of the substance were
dissolved in water.
# of J =
mole
# of J (see question 2 above)
# of moles (see question 3 above)
6. If you have time, repeat the experiment with NaOH instead of NH 4NO3. You will need a
new Data Table.
7. Compare the results of the two experiments.
8. Ask students questions about the experiments which demonstrate their comprehension of
the material. Ask them for feedback.
Embedded Assessment
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Successful completion of the lab exercise and write up will demonstrate understanding.
Homework
If students did not finish the questions listed in the closure, you may have them do so for
homework.
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