Equilibrium Constant
• Turn in CV Final Report and Pre-lab in folders
• Drop your Group Form
• Download example spreadsheet from Dr. Ng’s website
• Today – look at a dynamic equilibrium reaction between varying
concentrations of reactants for the following equation:
Fe3+(aq) +
clear
SCN-(aq)
clear
FeSCN2+(aq)
ORANGE
To determine the equilibrium constant for this reaction:
[ FeSCN 2 ]
K
[ Fe3 ][ SCN  ]
Equilibrium Constant
Step One:
Prepare the 8 solutions (7 reactions plus blank) on bottom of pg. 5
use one exclusive pipette per reactant
one for Fe+3 (Same as Fe(NO3)3) or [Fe(H2O)6]+3
one for SCN- (Same as KSCN)
one for DI water
Take enough of each solution to do mixing at your benchtop
(how much do you need?)
use medium or large test tubes *DO NOT USE SMALL**
LABEL YOUR SOLUTIONS!!!!
Equilibrium Constant
Step Two:
Calibrate the instrument (  = 447 nm ) (similar to last week)
0%T is door shut, no cuvet
100 %T is the prepared BLANK solution (bottom of
page 5)
Step Three:
Record % T for each solution (not absorbance, page 3, 4, 6, 7)
Use only one cuvet
No need to clean cuvet between solutions – simply dump cuvet
contents in a beaker from your drawer, and rinse with a little of
the next solution, then fill ¾ full with that solution
ALL WASTE IN WASTE BEAKER IN FUME HOOD!!!!!!!
Equilibrium Constant
Step Four, WHAT TO DO WITH THE DATA:
Make a Calibration Curve using given data on page 6 and an excel
spreadsheet
• y vs. x (Absorbance vs. concentration)
• From the calibration curve, determine linear equation
• Linear equation will be in the form
• y
= mx + b
• Abs = m*[Fe(SCN)]+2 + b (b is y intercept and should be 0 abs at 0 concentration)
• Abs = [·ℓ] * c
Beer’s Law
• Abs = [·ℓ] * [Fe(SCN)]+2
• [·ℓ] = m
• Get [·ℓ] => from equation on Calibration Curve
Equilibrium Constant
Calibration Curve for FeSCN2+
0.900
0.800
Absorbance
0.700
ε=5126.7 units!!!
0.600
0.500
0.400
Abs = 5126.7*C - 0.0028
R2 = 0.9998
0.300
0.200
0.100
0.000
0.0E+00
5.0E-05
1.0E-04
1.5E-04
2.0E-04
[Fe(SCN)]2+ (M)
Made from data on page 6, remember slope = ε * l, and l = 1 cm
Use information from linear equation of the calibration curve to convert absorbance
values into c=[FeSCN2+] for your 7 solutions!
Equilibrium Constant – Data Table
(make in EXCEL)
Using Dr. Ng’s example, make a similar table in EXCEL and input YOUR data
for your 7 solutions:
Equilibrium Constant – LAB REPORT
 Title Page
 Purpose
 Results
Calibration Curve with ε clearly marked on chart – full page and landscaped
excel data spreadsheet (formatted similar to example .pdf file with
gridlines and data)
sample calcs for each column of spreadsheet (choose one row)
significant figures and units
 Conclusion – purpose, errors, future suggestions, TWO post-lab questions
(see page 8 of laboratory procedure)
 Individual Checked-Pre-labs