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Published byPedro Torell Modified over 2 years ago

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Equilibrium Constants Warm Up Spec 20’s NOW!

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First things first… Turn on Spec 20’s to warm up Quiz Lab

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Finding equilibrium constant Kc

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

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“ICE” Box – Finding the equilibrium concentration [Fe] 3+ [(NCS) 1- ] [Fe(NCS)] 2+ I nitial Conc M1V1 = M2V2 [A] M1V1 = M2V2 [B] No product formed yet! C hange of Conc -x (x is the limiting rgnt’s conc) -x+x E quil Conc [A]-x[B]-x+x

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Part A: Making Calibration Curve ***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 wavelength 1.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

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Part B: Finding your sample conc 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

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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

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Formal Lab Typed hard copy by next week in class Title Page, Data, Graph, Calculations, Discussion/Analysis, Reference

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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

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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. M stock V stock = M soln V soln M 1 V 1 = M 2 V 2 Where: V 2 = V totalsoln = 7.00 mL

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Calibration curve graph For your calibration curve data, you will make a graph of [Fe(NCS)] 2+ vs. Absorbamce The initial conc of Fe(NO 3 ) 3 = conc [Fe(NCS)] 2+

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Part 2: Make Calibration Curve

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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+

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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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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

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