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Equilibrium Constant Today’s Experiment:

Equilibrium Constant Today’s Experiment: Fe 3+ (aq) + HSCN(aq) FeSCN 2+ (aq) + H + (aq) orange colorless dark red colorless Determine [FeSCN 2+ ] using Spec20 and Beer’s Law

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Equilibrium Constant Today’s Experiment:

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  1. Equilibrium Constant • Today’s Experiment: Fe3+(aq) + HSCN(aq) FeSCN2+(aq) + H+(aq) orange colorless dark red colorless • Determine [FeSCN2+] using Spec20 and Beer’s Law • Determine the other concentrations from an ICE Table • Calculate K at three different Temperatures • Use the Temperature data to determine DH, DS, and DG for the reaction • Take a look at the Pre-Lab Problems • Beer’s Law and Making a Calibration Curve • Colored compounds absorb light that is shined through them • A = elC Absorbance = (Extinction Coefficient)(length)(Concentration)

  2. 3. We will use test tubes #1-5 to make a calibration curve using Beer’s Law 4. Excess Fe3+ (0.200 M) pushes the reaction to the right: [HSCN]o = [FeSCN2+] 5. We must use Fe3+ solution as a blank to cancel out Fe3+ absorbance 6. Record %T and calculate A for the five different [HSCN] concentrations 7. Plot A vs. [FeSCN2+] to give a straight line. Calibration Curve 8. Once you find the Absorbance of any other [FeSCN2+] solution, you can find its concentration from the calibration curve. Slope = el A [FeSCN2+]

  3. Procedure for finding K (Tubes #6-9) • Make 4 different solutions of HSCN, Fe3+ • Find A with the Spec20 • Find [FeSCN2+] from the Calibration Curve • Use an ICE Table to find all the other concentrations • All solutions are made with 0.500 M HNO3, so [H+] = 0.500 • Other initial concentrations are found using the Dilution Equation

  4. Slope = -DH/R Intercept = DS/R lnK 1/T V. Procedure for finding DH, DS, and DG (Tubes #6-9 at different T’s) • You will use the same tubes (#6-9) at an ice bath (around 5 oC) • Hot tap water bath temperature (around 45 oC) [Not too hot! Boils off HSCN!) • You already have the room temperature data from these tubes (around 25 oC) • Plot lnK vs. 1/T for your three different temperatures • Use the following equations to calculate DH, DS, and DG for the reaction • Notes: • Use parafilm to cover the test tubes as you mix the solutions thoroughly • Spec20: 0% with nothing in it; 100% with Iron Solution only as Blank • Fill cuvet with most dilute first, rinse with next most dilute, and so on

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