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# NCSU CH 202 - Determination of an Equilibrium Constant

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CH202 Lab 5!Utilize a pipet and stock solution to prepare standards !Generate a calibration curve using a spectrometer !Determine the equilibrium constant for the reaction:Goals :Fe3++SCN1-FeSCN2+colorlessredSafety :!Nitric acid (HNO3) is corrosive and an oxidant !Iron nitrate (Fe(NO3)3) is an oxidant!Sodium thiocyanate (NaSCN) is toxic and an irritant!Flush with lots of water if any solutions come into contact with your skinWaste:Please discard waste in the container on the side shelf.Determination of an Equilibrium ConstantPrelab AssistanceRemember when diluting stock solutions, to use the formula: CiVi= CfVfWhat is the concentration of Fe3+if 1.50 mL of 0.100 M Fe3+is added to 4.50 mL of water?Example :C1V1= C2V2 (0.100 M)(1.50 mL) = C2· (1.50 + 4.50 mL)0.025 M = C21.50 mL0.15 mmol Fe3+6.00 mL0.15 mmol Fe3+Experimental OverviewPart A involves preparing five solutions of FeSCN2+, measuring the absorbance, and generating a calibration plotBy loading solutions with Fe3+, the equilibrium is driven to the red FeSCN2+product. This is a practical use of Le Châtelier’s principle.Fe3++SCN1-FeSCN2+excessshift equilibriumFe3++SCN1-FeSCN2+colorless red – absorbs at 447Almost all SCN1-is converted to FeSCN2+intial:equilibrium:somenone∆:-some -some +somepresent ~0 someExperimental OverviewRepresentative absorbance spectrum of solution containing FeSCN2+Assuming all SCN1-is converted to FeSCN2+, a student can prepare a calibration curve relating absorbance to [FeSCN2+].150.00 250.00 500.00 750.00900.00Wavelengths [nm]Counts0.0000.1000.2000.3000.4000.5000.6000.7000.8000.900-0.100-0.200-0.300-0.400Experimental OverviewCalibration PlotFeSCN2+M0.50 10-4.1.00 10-4.1.50 10-4.2.00 10-4.2.50 10-4.Absorbance0.090 0.181.0.2710.3620.452The relationship between FeSCN2+concentration and absorption should be linearAbsorbance versus [FeSCN2+]y = 1809.1x - 0.0001R2 = 1-0.100.10.20.30.40.50 0.00005 0.0001 0.00015 0.0002 0.00025 0.0003[FeSCN2+]absorbanceExperimental OverviewIn Part B, students will prepare mixtures of Fe3+, SCN1-, and FeSCN2+. The measured absorbance and calibration plot will be used to determine the concentration of FeSCN2+.3mL0.00200 MFe3+3mL0.00200 MSCN1-0.00100M Fe3+0.00100M SCN1-measuredabsorbanceis 0.343From previous slide: y = 1809.1 x + 0.0001Absorbance = 1809.1·concentration + offset0.343 = 1809.1 · concentration + 0.00011.90 · 10-4= concentrationExperimental CalculationFe3+ + SCN1- FeSCN2+The initial concentrations of Fe3+and SCN1-and the measured equilibrium FeSCN2+concentration can be used to determine Kinitial 0.00100 0.00100 0.000∆-0.000190-0.00081 -0.000810.000190equil= values derived from logic-0.000190 +0.000190K =[FeSCN2+][Fe3+] [SCN1-]=(0.000190)(0.00081) (0.00081)=

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