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Equilibrium Constants Warm Up Spec 20’s NOW!. First things first…. Turn on Spec 20’s to warm up Quiz Lab. Finding equilibrium constant Kc. SIMPLIFIED: [Fe] 3+ + [(NCS)] 1- ↔ [Fe(NCS)] 2+ + H 2 O. “ICE” Box – Finding the equilibrium concentration. Part A: Making Calibration Curve. - PowerPoint PPT Presentation
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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
“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
13
2
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
13
2
NCSFe
NCSFeKc