AP CHEMISTRY
THERMODYNAMICS
GUIDED INQUIRY LAB
Name: _______________________
PARTNER___________________
LAB: SOLUBILITY AND THERMODYNAMICS
INTRODUCTION
In this experiment, we will set out to investigate the thermodynamic quantities associated with the
dissolution of potassium nitrate, KNO3, in water.
KNO3 (s)

K+ (aq)
+
NO3- (aq)
CONTEXT : Potassium nitrate dissolves in water like all nitrate salts. Energy changes arise in the
dissolution process due to breaking ionic bonds and the formation of interactions between the ions and
the solvent. Plausible arguments can be made for classifying the dissolution of the salt as a physical or
chemical process.
We know that energy changes are associated with all chemical and physical processes due the changes in
kinetic and potential energy as the ionic bonds are broken and the interactions of the ions and solvent
follow. In this lab, you will endeavor to determine the thermodynamic quantities associated with the
dissolution of KNO3: the enthalpy change, the entropy change and the Gibbs free energy change.
MATERIALS
Solid KNO3
Water bath
2 25 x 200 mm test tube or a 100 mL graduated cylinder with removable plastic bottom.
PRE-LAB QUESTIONS:
1. What variables must be placed on the x and y-axes when graphing the solubility of
KNO3?
2. Draw a graph based on your answer to question one and sketch your predicted slope?
3. What does each point on the solubility curve represent?
4. Describe what you will observe in the test tube when the saturation point has been reached.
5. Draw a particulate diagram of the species that exist at the saturation point.
6. Write the equilibrium expression.
7. Draw the experimental setup you use and list what measurements you must make in order to
construct a solubility curve.
AP CHEMISTRY
THERMODYNAMICS
Name: _______________________
GUIDED INQUIRY LAB
PARTNER___________________
8. How is the mathematical expression you wrote in Question 6 related to Gibb’s Free Energy?
9. Predict the sign of ∆S and ∆H for the dissolution of KNO3.
PROCEDURE
CAUTION: Potassium nitrate is a strong
oxidizer and skin irritant. Avoid contact
with skin, eyes and mucous membranes.
Avoid shock, heat, and contact with
combustible materials.
1. Weigh 20.0 g of potassium nitrate and
transfer it to a large 25x200 mm test tube.
2. Add 15 mL of water and heat the test tube in
a boiling water bath, stirring until all of the
KNO3 has dissolved.
3. Determine and record the volume of the
potassium nitrate solution. This can be done
by filling another large 25x200 mm test tube
with water until the volumes in both tubes
are the same, then measuring the volume in
the test tube filled with water in a graduated
cylinder. If you are using the graduated
cylinder, the total volume can just be read.
4. Remove the test tube with the potassium
nitrate from the water bath and allow it to
cool while slowly stirring the solution with a
thermometer.
5. Record the temperature when crystals first
appear. It is assumed at this temperature
that the system is at equilibrium, and it is
possible to calculate the concentration of the
ions.
6. Add 5 mL of water to the test tube and warm
the mixture in a water bath again until the
solid has all dissolved. Determine the
solution volume as before and record it.
7. Cool the solution slowly and record the
temperature at which crystals first appear.
8. Repeat the cycle (steps 6 and 7) adding 5 mL
of water, heating and cooling until crystals
appear near room temperature. This should
take 5 to 7 determinations.
DATA
Construct a data table
Calculations and Questions
1.
Construct a solubility curve for KNO3.
2.
Calculate the equilibrium constant at each temperature for the dissolution of KNO3.
3.
Based on your calculated value your equilibrium constant, what do you predict about the magnitude
of the value of ∆G, Gibbs free energy, at the different temperatures?
4.
Calculate the numerical value of ∆G for each of your temperatures.
5.
Construct a graph of the natural logarithm of K, ln K, versus
1
for each individual temperature.
T
6. What value can be determined from the slope of this graph? Explain and determine its value.
(Hint: ∆G = -RTlnK and ∆G = ∆H - T∆S, therefore, -RTlnK = ∆H - T∆S or lnK = -



H 1
S
( ) +
)
R T
R

7. Use the values of ∆G and ∆H to calculate the value of ∆S at each temperature.
CONCLUSION: Have each lab group report their findings to Google.docs. Follow this with an analysis
of the class data.
1.
Silberman, Robert G., SUNY at Cortland N.Y., filtrates and Residues, Journal of Chemical Education,
May, 1996, p. 426.
Acton-Boxborough Regional High School