FESCN Equilibrium

Determination of Formation Constant,
Kf of Thiocyanoiron(III), FeSCN+2
Dr. Fred Omega Garces
Chemistry 201
Miramar College

Chemical Equilibrium:
Finding the Formation Constant of
FeSCN2+ (aq)
Fe3 +(aq)

iron(III)

+

SCN–(aq)

FeSCN2+(aq)

D

thiocyanate

thiocyanoiron(III)

kf =

€

FeSCN2 +

[

]

Fe +3 [SCN− ]

[

]

Objective
The purpose of this experiment is to determine the constant formation, Kf, (equilibrium constant) for the formation of thiocyanoiron(III).

Fe3+ (aq) + SCN-(aq)

Kf
D

FeSCN2+ (aq)

Background Information
Consider the following reaction:
Fe3+ (aq) + SCN-(aq)

D

FeSCN2+ (aq)

The mass action expression iskf =

[FeSCN2+ ] eq
[Fe 3+ ] eq [SCN- ] eq

To determine the equilibrium constant, the equilibrium concentration of [Fe3+]eq , [SCN-] eq
€
and [FeSCN2+] eq will need to be known.

Chemcials
Fe3+ from Fe(NO3)3, yellowish
SCN- from kSCN, colorless
FeSCN2+ from kSCN, dark red absorbs blue λmax =447nm
According to Beer's Law, A = ε l [c] so the AFeSCN can be used to determined [FeSCN2+] eq .
2+

How can the concentrations of [Fe3+]eq and [SCN-] eq be determined by simply monitoring the [FeSCN2+] eq absorbance? Chemical Reaction
Initial Concentration of Fe3+ is known as well as the initial concentration of from SCN- .
Furthermore, the reaction is carried out so that the [Fe3+]i >> [SCNi , this ensures that the product [FeSCN2+] depends on the [SCN-] i .

Fe3+
Excess

+

SCN Limiting

!

FeSCN2+
Amount produced depends on limiting [SCN-]

How can the concentrations of [Fe3+]eq and [SCN-] eq be determined
2+
by
€ simply monitoring the [FeSCN ] eq absorbance?

Background Information
-The equilibrium process.
To determine Kf for FeSCN2+, all the concentrations of the specie in the reaction:
Fe3 +(aq)

+

SCN–(aq)

FeSCN2+(aq)

D

This can be done by investigating the reaction as the equilibrium is
established.