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Determination of an Equilibrium Determination of an Equilibrium Constant, KeqConstant, Keq
Equilibrium
Equilibrium Constant
Data Collection and Calculation
Beer’s Law
Calibration Curve/ Working curve
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Learning ObjectivesLearning Objectives
• Practice colorimetric measurement
• Use Beer’s Law to determine concentration of FeSCN2+
• Calculate equilibrium constant, Keq, for the formation of FeSCN2+
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Equilibrium ConstantEquilibrium Constant
• General expression:
aA + bB ⇔ cC + dD K = [C]c[D]d/[A]a[B]b
• Specific expression for this lab:
Fe3+(aq) + HSCN(aq) ⇔ H+(aq) + FeSCN2+(aq)
Keq = Kf = [H+] [FeSCN2+]/[ Fe3+][ HSCN]
The numbers on the right side of the formula are concentrations at equilibrium.Therefore:
Keq = Kf = [H+]eq [FeSCN2+]eq /[ Fe3+]eq[ HSCN]eq
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Calculations (to determine equilibrium concentrations)
Calculations (to determine equilibrium concentrations)
Important formula to remember:
For dilutions:
C2V2 = C1V1
C2 = C1V1/V2
C1 = Concentration of a species at the beginning of dilutionV1 = Volume of a species at the beginning of dilutionC2 = Concentration of a species at the end of dilutionV2 = Volume of a species at the end of dilution
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Calculations (three tables)Calculations (three tables)
Important formula to remember: For dilutions: C2 = C1V1/V2Here: C1 = 6.0 x 10-4 M (for HSCN)
See table on page 40 of lab manual
3.0 x 10-5 M
Table 1: Standard curve (working curve) table
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The Second table: Equilibriummeasurement table
The Second table: Equilibriummeasurement table
Important formula to remember: For dilutions: C2 = C1V1/V2Here: C1 = 2.00 x 10-3 M (for Fe(NO3)3 or Fe3+ for short)
or C1 = 2.00 x 10-3 M (for HSCN)
See Table on page 43 of lab manual
0
0
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The Third tableThe Third table
Concentration Trial #1 Trial #2 Trial #3 Trial #4 Trial #5Initial [Fe3+]0 Initial [HSCN]0 Initial [H+]0 [H+]eq [FeSCN]eq [Fe3+]eq [HSCN]eq
[Fe3+]0 Determined from initial solution preparation (see the table on page 43). [HSCN]0 Determined from initial solution preparation (see the table on page 43). [H+]0 Determined from initial solution preparation, [H+]0 = 0.5 M. [H+]eq [H+]eq = [H+]0 = 0.5 M (because H+ balance and H+ is always in excess amount). [FeSCN]eq Determined from colorimetry (see below). [Fe3+]eq [Fe3+]eq = [Fe3+]0 - [FeSCN]eq (because of Fe balance: [Fe3+]0 = [Fe3+]eq + [FeSCN]eq) [HSCN]eq [HSCN]eq = [HSCN]0 - [FeSCN]eq (because HSCN balance: [HSCN]0 = [HSCN]eq + [FeSCN]eq)
Therefore, the key of the experiment is to determine [FeSCN]eq
1.00 x 10-3 M2.00 x 10-4 M
0.5 M0.5 M
[FeSCN]eq
1.00 x 10-3 - [FeSCN]eq M2.00 x 10-4 - [FeSCN]eq M
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How to determine [FeSCN]eq?How to determine [FeSCN]eq?
Use the Beer’s Law
August Beer (July 31, 1825 - November 18, 1863), German physicist.Pierre Bouguer (February 16, 1698 – August 15, 1758), French mathematician
• FeSCN2+ is a deep red colored complex.
• The more FeSCN2+ in solution, the darker the solution appears.
• There is a direct relationship between the color of [FeSCN2+] and its absorbance.
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Spectronic 20 SpectrophotometerSpectronic 20 Spectrophotometer
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Beer’s LawBeer’s Law
A: absorbanceIo: Intensity of incident light = photocell current of the blank solutionI : Intensity of transmitted light = photocell current of your sampleε: molar absorptivity constantb: path length (= 1 cm in our experiment)c: concentration of the sample
A = log (Io/I) = εbc
b
ε
, c
ASlope = ε = A/c
when b = 1 cm
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Calibration Curve(Working Curve)
Calibration Curve(Working Curve)
N u m b e r o f S tu d e n t
0 2 4 6 8 10 1 20
5
10
15
20
25
Num
ber o
f san
dwic
h
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Lab Technique TipsLab Technique Tips
• Cuvettes should be handled on the FROSTED side only (and with finger cots).
• All solutions should be made using volumetric techniques.• You must take a reading immediately after making the
solution (the solution degrades).• There are two different stock solutions of different
concentrations for each chemical. Make sure you use the right one!
• Set up your NetFiles account in order to save your data!
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Review and Preview
• Review today’s lecture:
• Read Lab 4 (pages 37-46)
• Preview next lecture (Determination of the Solubility of
Calcium Sulfate)
• Read Lab 5 (pages 47-55)
• Prepare and submit pre-Lab