Measuring K for
The Reaction
• But we only want
• Make sure [Fe3+]o >> [SCN-]o
The Reaction
• Fe3+ can be very insoluble because it can act as a Lewis
Acid
• Drive equilibrium left by adding HNO3(aq)
Measuring Equilibrium Concentrations
Deep red color
• The complex absorbs in
the blue/green/yellow,
that is why it is red
• We can monitor the
concentration of the
product using Beer’s
Law
Beer’s Law
“The stronger the brew, the less light that goes through”
A = e ×l × éë FeSCN 2+ ùû
Path Length
Absorbance
Molar
Extinction
Coefficient
𝐴 = log
𝐼0
𝐼𝑡
Beer’s Law
A = e ×l × éë FeSCN 2+ ùû
• If we know  and l we can measure the concentration by
measuring the absorbance
• We can do this by calibrating the spectrometer, by measuring
the absorbance at known concentrations of [FeSCN2+]
• The slope of the calibration
curve is ε.l
Part A: Calibrating the Spectrometer
• But hang on how can we get known [FeSCN2+]eq to calibrate?
• Add a LOT of Fe3+, this will drive the reaction to completion
(Le Chatelier’s Principle)
• Then [SCN-]o ≈ [FeSCN2+]eq
Part A: Calibrating the Spectrometer
Flask #
0.200 M
Fe(NO3)3
(mL)
0.00200 M
KSCN
(mL)
0.1 M HNO3
(mL)
1
5.00
0.00
20.00
2
5.00
5.00
15.00
3
5.00
4.00
16.00
4
5.00
3.00
17.00
5
5.00
2.00
18.00
6
5.00
1.00
19.00
Pipette the reactants into a 25mL volumetric flask, and fill up to the calibration
mark on the flask with the 0.1M HNO3
Part A: Calibrating the Spectrometer
Flask #
0.200 M
Fe(NO3)3
(mL)
0.0020M
KSCN
(mL)
0.1 M
HNO3
(mL)
1
5.00
0.00
20.00
2
5.00
5.00
15.00
3
5.00
4.00
16.00
4
5.00
3.00
17.00
5
5.00
2.00
18.00
6
5.00
1.00
19.00
[Fe3+]o
[SCN-]o
[FeSCN2+]
≈ [SCN-]o
• Calculate the initial concentrations [Fe3+]o, [SCN-]o = [FeSCN2+]eq
• Measure A for each sample
[FeSCN 2+ ]eq » [SCN - ]o = 0.002M ×
[Fe3+ ]o = 0.200M ×
Abs
VSCN
25.00mL
5.00mL
= 0.04 M
25.00mL
Part A: Calibrating the Spectrometer
• Calibrate the SpectroVis spectrometer with distilled H2O filling
a cuvette
• Measure the absorbance spectrum of FeSCN2+
• Identify the wavelength of maximum absorption lmax
• Conduct a Absorbance vs. Concentration experiment on the
SpectroVis by selecting and choosing Abs vs Concentration
• Get slope of A vs. [FeSCN2+] curve = m
Part B: Measuring Kc
𝐹𝑒𝑆𝐶𝑁 2+
𝑒𝑞
𝐹𝑒𝑆𝐶𝑁 2+ 𝑒𝑞
𝐾𝐶 =
𝐹𝑒 3+ 𝑒𝑞 × 𝑆𝐶𝑁 −
𝐴
=
𝑚
𝐾𝐶 =
𝐹𝑒 3+
𝐴/𝑚
−
0 − 𝐴/𝑚 × 𝑆𝐶𝑁
We can get Kc values from 2-6 they should all be close to each other
0
𝑒𝑞
− 𝐴/𝑚