Equilibrium ConstantsWarm Up Spec 20’s NOW!

First things first…

Turn on Spec 20’s to warm up

Quiz Lab

Finding equilibrium constant Kc

Overall Balanced Equation: [Fe(H2O))6](NO3)3 + Na(NCS) ↔ [Fe(H2O)5(NCS)](NO3)2 + H2O + NaNO3

NET IONIC: [Fe(H2O) 6]

3+ + (NCS)1- ↔ [Fe(H2O)5(NCS)]2+ + H2O clear clear red

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NCSFe

NCSFeKc

SIMPLIFIED: [Fe]3+ + [(NCS)]1- ↔ [Fe(NCS)]2+ + H2O

“ICE” Box – Finding the equilibrium concentration

[Fe]3+ [(NCS)1- ] [Fe(NCS)]2+

Initial Conc

M1V1 = M2V2

[A]M1V1 = M2V2

[B]No product formed

yet!

Change of Conc -x (x is the limiting rgnt’s conc)

-x +x

Equil Conc[A]-x [B]-x +x

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NCSFe

NCSFeKc

xBxA

xKc

Part A: Making Calibration Curve

Test Tube Fe(NO3)3 NaNCS HNO3

1x10-4

M 0.100 M 0.100 M1 1.00 mL 5.00 mL 4.00 mL2 2.00 mL 5.00 mL 3.00 mL3 3.00 mL 5.00 mL 2.00 mL4 4.00 mL 5.00 mL 1.00 mL5 5.00 mL 5.00 mL 0.00 mL

***Note Change: You are not making the test tube solutions, it has been pre-made in the proper ratios. *****

For each test tube:Set Spec to 450 nm wavelength1. Blank the spec, using water

solution (go to Crystal Violet Lab on how to blank spec)

2. Now insert test tube with solution (1/2 full), record the %T

Part B: Finding your sample conc

V2

Fe(NO3)3 NaNCS HNO3

0.0025 M 0.0025 M 0.100 M1 1.0 1.0 5.0 7.02 1.0 1.5 4.5 7.03 1.0 2.0 4.0 7.04 1.0 2.5 3.5 7.05 1.0 3.0 3.0 7.06 2.0 1.0 4.0 7.07 2.0 1.5 3.5 7.08 2.0 2.0 3.0 7.09 2.0 2.5 2.5 7.010 2.0 3.0 2.0 7.0

Volumes (mL)Test Tube Total

Volume

1. Lab TA will assign you three test tubes

2. Make the test tube solutions

3. Blank the spec before finding the %T for

each test tube solution

End of class

Lab Clean up (check out by lab prep) Turn in :

Lab notebook yellow carbon copies, pre-lab notebook copies, filled out data table

Formal Lab

Typed hard copy by next week in class Title Page, Data, Graph, Calculations,

Discussion/Analysis, Reference

Formal lab continue

Data table (via excel) Calibration curve raw/analyzed data with columns:

Columns : (Test Tube #, Reagent Volumes, %T,Abs, [Fe(NCS)]2+

Assigned Test Tube Raw Data Data With columns: (Test Tube #, Reagent Volumes, %T, Abs, [Fe(NCS)]2+

Assigned Test Tube Analyzed Data with columns Test Tube #, Equilibrium Conc Fe3+, Equil Conc NCS-, Kc,

Ave Kc for all tubes, and Stdev of Kc

Calc Part 1: Finding Initial Conc

Part 1 Solve for calibration and assigned test tubes

Calcs A and B: Determine [Fe]3+ and [(NCS)1-] at the start (Initial Conc) You made a solution so the concentration is diluted and

thus DIFFERENT from bottle conc.

MstockVstock = MsolnVsoln

M1V1 = M2V2

Where: V2 = Vtotalsoln = 7.00 mL

Calibration curve graph

For your calibration curve data, you will make a graph of [Fe(NCS)]2+ vs. Absorbamce

The initial conc of Fe(NO3)3 = conc [Fe(NCS)]2+

Part 2: Make Calibration Curve

y = 7355.7x + 0.0131R2 = 0.9986

0.0000

0.0500

0.1000

0.1500

0.2000

0.2500

0.3000

0.3500

0.4000

0.4500

0.000E+00 1.000E-05 2.000E-05 3.000E-05 4.000E-05 5.000E-05 6.000E-05

Ab

so

rba

nc

e

[Fe(NCS)]2+ (M)

Calibration Curve For Individual Data

Part 2: Find reacted conc

(Make calibration curve, get linear trendline)

Solve for assigned test tubes

Calc C: Determine [Fe(NCS)]2+ of unknowns Use linear trendline to find equil [Fe(NCS)]2+

Calc D and E: Determine the reacted conc (Change Concentration) [Fe]3+ and [(NCS)1-] reacted = equil [Fe(NCS)]2+

Calc F and G: unreacted [Fe]3+ and [(NCS)1-] (Equilibrium Conc) Calc A-D and B-E Aka Conc beginning – Conc reacted

Calc H: Find Kc!! Get ave Kc Get stdev kc

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NCSFe

NCSFeKc

Conclusion

1 page double spaced Discuss the kc’s you got –

precision/accuracy, trends Discuss the graph – what you got and what it

means Error analysis