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Equilibrium Constant
I.^
Today’s Experiment:
3+Fe
(aq)
+^
HSCN(aq)
FeSCN
2+(aq)
+^
+H(aq)
orange
colorless
dark red
colorless
1.^
Determine [FeSCN
2+^ ] 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 Curve
1.^
Colored compounds absorb light that is shined through them
2.^
A =
l C
Absorbance = (Extinction Coefficient)(length)(Concentration)
???
][HSCN] [Fe
] ][H
[FeSCN K^
3
2
^
- We will use test tubes #1-5 to make a calibration curve using Beer’s Law4. Excess Fe
3+^ (0.200 M) pushes the reaction to the right: [HSCN]
= [FeSCNo
2+^ ]
- We must use Fe
3+^ solution as a blank to cancel out Fe
3+^ absorbance
- Record %T and calculate A for the five different [HSCN] concentrations7. Plot A vs. [FeSCN
2+^ ] to give a straight line. Calibration Curve
- Once you find the Absorbance of any other [FeSCN
2+^ ] solution, you can find its
concentration from the calibration curve.
^
100 %T log A
I I %T
o
A
[FeSCN
2+^ ]
Slope =
l
A slope A ε C Cε A^
^
l
l
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
o C)
2.^
Hot tap water bath temperature (around 45
o C) [Not too hot! Boils off HSCN!)
3.^
You already have the room temperature data from these tubes (around 25
o C)
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
ΔS R
1 T
ΔH- R
lnK
ST
ΔH
RTlnK
ΔG
o
o
o o
o
^
1/T
lnK
Slope = -
H/R
Intercept =
S/R
K
J/mol
R
Kelvins)in
Temp.
Roomat
K
average
(use
-RTlnK
G
ST
G
)(R)
(intercept
S
(slope)(R)
ΔH
