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.