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

Learning ObjectivesLearning Objectives

• Practice colorimetric measurement

• Use Beer’s Law to determine concentration of FeSCN2+

• Calculate equilibrium constant, Keq, for the formation of FeSCN2+

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

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

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

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

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

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.

Spectronic 20 SpectrophotometerSpectronic 20 Spectrophotometer

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

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

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!

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