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Equilibrium Constant Today’s Experiment:

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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:

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)
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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+]

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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
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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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