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
€ [FeSCN2+] will need to be known.
and
eq
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.
colorless
colorless
Fe3 +(aq)
+
SCN–(aq)
i
[Fe+3]i
[SCN-] i
Δ
-x
-x
[e]
[Fe+3]
i
-x
Blood red
[SCN-]
D
FeSCN2+(aq)
0
+x
i
- x
[FeSCN2+]
eq
Background Information
- Determining Concentration at Equilibrium
[FeSCN2+]
eq
= x
Determined by absorbance
Therefore
[SCN-] eq = [SCN-] i – x = [SCN-] i – [FeSCN2+]
[Fe+3] eq = [Fe+3] i – x = [Fe+3] i - [FeSCN2+]
eq
eq
Background Information
- Determining Concentration of [FeSCN2+] trials
[FeSCN2+]
eq
is red and absorbs blue light 470nm.
Standard solution prepared with large Fe+3 concentration added to small
amount of SCN-. This results in practically all the Fe+3 converting to SCN-.
[Fe+3](large amount)
100 x
+ [SCN-] (small amount) D
[FeSCN2+]
1x
(same amount [SCN-] )
~1
Astd g [FeSCN2+]std & for other rxns: Atrial# ∝ [FeSCN2+]trial#
From Beer's Law A = ε b [FeSCN ]
2+
Atrial#
1 [FeSCN2+ ]trial#
=
Astd
1 [FeSCN2+ ]std
€
Therefore
⇒ [FeSCN2+ ]trial# =
Atrial#
ε b [FeSCN2+ ]trial#
=
,
Astd
ε b [FeSCN2+ ]std
Atrial#
[FeSCN2+ ]std
Astd
….Background Information
Mass Action Expression
FeSCN ]
[
=
[Fe ][SCN ]
[FeSCN ]
[Fe ] • [SCN ]
2+
kf
+3
2+
=
−
3+
3+
eq1
[ ]
= Fe3+
i
[
[
€
#% 3+
Fe
$
[ ]
i
[
- FeSCN2 +
eq1
[FeSCN ]
2+
- FeSCN2 +
]
[
= FeSCN2 +
[SCN ]
,
eq1
eq1
-
eq1
[
std
•
A1
std
eq
-
i
]
2+
eq1
&(
'
•
std
= SCN -
] A
&(#%
] '$[SCN ] - [FeSCN ]
FeSCN2 +
kf =
-
eq1
At equilibrium for trial #1 :
[Fe ]
eq1
]
i
A1
Astd
[
- FeSCN2 +
]
eq1
Pictorial Flowchart
GogglesVenier with Colorimetry -
Test tubes
Volumetric Pipets
Set up
GogglesVernier with Colorimetry-
Test tubes
Volumetric Pipets
Procedure
Absorption Spectra
Excel Data