Equilibrium Constant

I. Today’s Experiment:Fe3+(aq) + HSCN(aq) FeSCN2+(aq) + H+(aq)orange colorless dark red colorless

1. Determine [FeSCN2+] using Spec20 and Beer’s Law

2. Determine the other concentrations from an ICE Table

3. Calculate K at three different Temperatures

4. Use the Temperature data to determine H, S, and G for the reaction

II. Take a look at the Pre-Lab Problems

III. Beer’s Law and Making a Calibration Curve1. Colored compounds absorb light that is shined through them

2. A = lC Absorbance = (Extinction Coefficient)(length)(Concentration)

???][HSCN][Fe

]][H[FeSCNK

3

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.

%T

100logA

I

I%T

o

A

[FeSCN2+]

Slope = l

slope

A

ε

ACCεA

ll

IV. Procedure for finding K (Tubes #6-9)1. Make 4 different solutions of HSCN, Fe3+

2. Find A with the Spec20

3. Find [FeSCN2+] from the Calibration Curve

4. Use an ICE Table to find all the other concentrations

5. All solutions are made with 0.500 M HNO3, so [H+] = 0.500

6. Other initial concentrations are found using the Dilution Equation

Solution 6 Room Temperature

Species

Fe3+ HSCN FeSCN2+ H+

Initial Molar Concentration 0.00100 0.000200 0 0.500

Change in Molar Concentration

-x -x +x +x

Equilibrium Molar Concentration

0.00100 - x 0.000200 - x x 0.500 + x

Calculated Values 0.500

0.00020M0.010L

0.001L0.002M[HSCN]

V

VMM

VMVM

o2

112

2211

][HSCN][Fe

]][H[FeSCNK

3

2

V. Procedure for finding H, S, and G (Tubes #6-9 at different T’s)1. You will use the same tubes (#6-9) at an ice bath (around 5 oC)

2. Hot tap water bath temperature (around 45 oC) [Not too hot! Boils off HSCN!)

3. You already have the room temperature data from these tubes (around 25 oC)

4. Plot lnK vs. 1/T for your three different temperatures

5. Use the following equations to calculate H, S, and G for the reaction

6. Notes:

a. Use parafilm to cover the test tubes as you mix the solutions thoroughly

b. Spec20: 0% with nothing in it; 100% with Iron Solution only as Blank

c. Fill cuvet with most dilute first, rinse with next most dilute, and so on

R

ΔS

T

1

R

ΔH- lnK

STΔHRTlnKΔGoo

ooo

1/T

lnK

Slope = -H/RIntercept = S/R

KJ/mol 8.3145 R

Kelvins)in Temp. Roomat K average (use-RTlnK G

ST-HG

)(R)(interceptS

(slope)(R)ΔH