g of KNO3 temp craystized (C)

1.533

22.0

3.553

44.9

5.590

66.9

7.640

76.8

M (mol/L) Temp (K)

3.03E-04

295.0

7.03E-04

317.9

1.11E-03

339.9

1.51E-03

349.8

(-RT ln ksp

180

160

Solubility (g/100mL)

delta g

Solubility (g/100mL)

Temp (C)

30.66

22.0

71.06

44.9

111.8

66.9

152.8

76.8

140

120

100

80

60

40

20

0

1

check

delta G

M^2

M^2

LN M^2

RT

delta G

delta G (J/mol)

4.01E+04 9.21E-08

9.12 2.21047 -2476.23 -5.47E+03 3.98*10^4

3.87E+04 4.95E-07

4.94 1.597365 -2668.45 -4.26E+03 3.87*10^4

3.88E+04 1.22E-06

1.22 0.198851 -2853.12 -5.67E+02 3.85*10^4

3.81E+04 2.29E-06

2.29 0.828552 -2936.22 -2.43E+03 3.78*10^4

Temp (K) delta G

295 -5473.632

317.9 -4262.494

339.9 -567.3455

349.8 -2432.811

0

Solubility Table of KNO3

Delta G (J/mol)

-1000

290

300

310

320

330

-2000

-3000

-4000

-4262.493671

-5000

-5473.631643

-6000

1

2

3

Temp (C)

4

Temp (K)

330

340 -567.3454814

350

360

-2432.811412

4262.493671

Temp (K)

g of KNO3 temp craystized (C)

1.533

22.0

3.553

44.9

5.590

66.9

7.640

76.8

Solubility (g/100mL)

Temp (C)

30.66

22.0

71.06

44.9

111.8

66.9

152.8

76.8

M (mol/L)

3.03E-04

7.03E-04

1.11E-03

1.51E-03

Temp (K)

295.0

317.9

339.9

349.8

Temp (K) delta G

295

-5450

317.9

-10300

339.9

-13600

349.8

-15800

Solubility and Thermodynamics

In this lab you will construct a solubility curve for the salt KNO3. Then, from that solubility curve, you will

determine the thermodynamic values of ï„G, ï„H, and ï„S for the process of dissolving KNO3.

Part 1 â€“ Solubility Curve for KNO3 (to be completed during the first lab period)

1. You will prepare a solubility curve for KNO3 over the temperature range of about from room

temperature up around 75oC. To start this process, you will first determine the solubility of KNO3 at

room temperature. Do this in a test tube containing 5.00 mL of distilled water. Discuss as a group how

you will measure this. Write down a step-wise procedure. Before you perform your measurement,

check with your instructor. Once you have the instructorâ€™s okay, carry out the measurement and record

your result on your data sheet. Retain the mixture you made for future use.

2. Next, you will determine if the solubility of KNO3 increases or decreases with temperature using your

solution from step 1 above. Make a hot bath (beaker of water on a hot-plate) and an ice-water bath.

You will place the test tube in each of these solutions. Before you do this, answer the following

questions and show your instructor.

a. What type of solution do you have in your test tube right now: unsaturated, saturated, or

supersaturated?

b. If the solubility of KNO3 increases with increased temperature what will happen when you place

the test tube in the hot bath? The cold bath?

c. If the solubility of KNO3 decreases with increased temperature what will happen when you place

the test tube in the hot bath? The cold bath?

*Retain your test-tube of KNO3 for the next step. If your solution is warm, let it cool to room temp.

before proceeding with the next step (you can put it in the ice-bath to speed the cooling process).

3. Next, you will measure the solubility of KNO3 at a higher temperature. You will do this a little bit

differently than how you did it at room temperature. Add a little more KNO3 to the test tube and record

the exact amount added. Heat the test tube in the hot bath until all of the solid dissolves. Place a

temperature probe in the test tube. Remove the test tube from hot bath to let it cool. Observe the

solution carefully and watch for precipitate/crystals to just begin to form. Record the temperature at

which this occurs.

a. What type of solution do you have at the point crystals begin to form: unsaturated, saturated,

or supersaturated?

b. Assuming the mass of the crystals is negligible, can you calculate the concentration of the

solution is at this point? What is it?

c. If you know the answers to the above two questions, then you know the temperature at which

a solution of this concentration becomes saturated. In other words, you know the solubility

limit at that temperature!

At this point you should have two temperature/solubility data points, but you will need more to construct

a nice solubility curve for KNO3. Repeat step 3 with different amounts of KNO3 to find the solubility at

different temperatures. Keep track of your data in table form on your data sheet. When you think you

have enough data for a nice curve, make a graph on Excel and show your instructor. *Be sure to save a

copy of your Excel file or email it to yourself. You will need this data for part 2 of this experiment.

4. Use your graph to estimate the solubility of KNO3 at 10oC, 50oC, and 90oC. Show instructor.

**Completing 1-4 above will constitute your â€œgroup planning questionsâ€ for this experiment.

**Be sure to completely record all your data/observations on the provided data sheets (grid paper). This will be

collected as your â€œindividual notesâ€ for this experiment.

Part 2 â€“ Thermodynamics of the KNO3 dissolution process

Dissolving KNO3 in water can be viewed as an equilibrium process. As solid KNO3 dissolves, it dissociates

into its component ions. When a solution is saturated (is at the solubility limit), some KNO3(s) is not

completely dissolved and we have both the solid and the ions present in equilibrium with each other.

KNO3(s) â‡ŒK+(aq) + NO3-(aq)

1) Write the equilibrium expression for this reaction.

2) Calculate the value of the equilibrium constant for this reaction at each of the temperatures found in Part

1 of this experiment. (Hint: You should be able to calculate concentration of the ions at this point using

the data from your solubility graph. To simplify things, you may approximate the solution volume to be

the same as the solvent volume.)

3) Calculate ï„G Â° at each temperature using the equilibrium constants you found in #2 above.

4) Use Excel to make a plot of ï„G versus the temperature in Kelvin. Fit a linear trendline to this data and

display the trendline equation and R2 value on the plot.

5) We know that the free energy change for a reaction can be expressed as ï„G = ï„H âˆ’ Tï„S. This

expression can easily be rearranged to

ï„G = âˆ’Tï„S + ï„H

This is a linear equation (y = mx + b). You fit a linear equation to your data in step 4 above. What do

the slope and intercept of your graph correspond to?

6) Calculate the values of ï„H and ï„S for the reaction KNO3(s) â‡Œ K+(aq) + NO3-(aq).

**Show the instructor your answers for 1-6 above. This will constitute your group summary questions for

this experiment**

Lab Report

Following the lab report guidelines already provided, you will write a lab report that summarizes the results

from both part 1 and part 2 of this experiment. (E.g., provide a solubility curve and all of the important

thermodynamic values calculated in part 2). Your introduction should provide the background on how the

solubility was measured and how the thermodynamic values were calculated.

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