Measuring K for

The Reaction But we only want Make sure [Fe3+]o >> [SCN-]o

The Reaction Fe3+ can be very insoluble because it can act as a Lewis Acid Drive equilibrium left by adding HNO3(aq)

Measuring Equilibrium Concentrations Deep red color The complex absorbs in the blue/green/yellow, that is why it is red We can monitor the concentration of the product using Beer’s Law

Beer’s Law Absorbance Path Length Molar Extinction Coefficient “The stronger the brew, the less light that goes through” Absorbance Molar Extinction Coefficient Path Length

Beer’s Law If we know ε and l we can measure the concentration by measuring the absorbance We can do this by calibrating the spectrometer, by measuring the absorbance at known concentrations of [FeSCN2+] The slope of the calibration curve is ε.l

Calibrating the Spectrometer But hang on how can we get known [FeSCN2+]eq to calibrate? Add a LOT of Fe3+, this will drive the reaction to completion (Le Chatelier’s Principle) Then [SCN-]o ≈ [FeSCN2+]eq

Calibrating the Spectrometer Flask # 0.200 M Fe(NO3)3 (mL) 0.00200 M KSCN 0.1 M HNO3 (mL) 1 5.00 0.00 20.00 2 15.00 3 4.00 16.00 4 3.00 17.00 5 2.00 18.00 6 1.00 19.00 Pipette the reactants into a 25mL volumetric flask, and fill up to the calibration mark on the flask with the 0.1M HNO3

Calibrating the Spectrometer Flask # 0.200 M Fe(NO3)3 (mL) 0.0020M KSCN 0.1 M HNO3 [Fe3+]o [SCN-]o [FeSCN2+] Abs 1 5.00 0.00 20.00 2 15.00 3 4.00 16.00 4 3.00 17.00 5 2.00 18.00 6 1.00 19.00 Calculate the initial concentrations [Fe3+]o, [SCN-]o = [FeSCN2+]eq Measure A for each sample