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E-6
EXPERIMENTAL METHOD
In Part I, you will examine the equilibrium shown below for the reaction between iron(III) (Fe
3+
) and
thiocyanate (NCS
-
) ions forming the iron(III) thiocyanate complex, ([Fe(NCS)]
2+
):
Fe
3+
(aq)
+ NCS
-
(aq)
[Fe(NCS)]
2+
(aq).
(8)
The equilibrium position for reaction (8) can be shifted to the right by adding more Fe
3+
(aq)
or
NCS
-
(aq)
. Conversely, the equilibrium can be shifted to the left by removing either Fe
3+
(aq)
or NCS
-
(aq)
from
solution. The concentrations of Fe
3+
(aq)
or NCS
-
(aq)
can be decreased by the addition of various reagents that
react with the either the free Fe
3+
(aq)
or NCS
-
(aq)
to form complex ions. For example, the hydroxide anion reacts
with free Fe
3+
(aq)
to form insoluble Fe(OH)
3(s)
. The equilibrium system described by reaction (8) responds to a
drop in [Fe
3+
] by shifting to the reactant side replacing the depleted Fe
3+
(aq)
until equilibrium is re-established.
Similarly, the [NCS
-
] can be decreased by adding Ag
+
(aq)
. The silver ion reacts with the thiocyanate anion to
form AgNCS
(s)
. The system described by reaction (8) responds to the decrease in [NCS
-
] by again shifting to
the reactant side to replace the depleted NCS
-
(aq)
until equilibrium is re-established.
In solution, [Fe(NCS)]²
+
(aq)
, has a rust (reddish) colour (in very dilute solutions, it may appear to be a lighter
gold or orange colour). The NCS
-
(aq)
, is colourless and Fe
3+
(aq)
is pale yellow. Hence, the colour of the solution
as well as the appearance of a precipitate can be used to monitor the position of equilibrium. If the rust colour
of the solution becomes more intense, this indicates an increase in the concentration of [Fe(NCS)]
2+
(aq)
complex
ion, caused by a shift in the position of equilibrium to the product side (i.e. to the right). However, if the rust
colour of the solution becomes less intense or a colour change occurs or if a precipitate forms, this would
indicate a decrease in the concentration of the [Fe(NCS)]
2+
(aq).
complex ion arising from a shift in the position of
equilibrium to the reactant side (i.e. to the left).
You will determine the effect of several reagents on the position of equilibrium and explain your observations in
terms of Le Châtelier's Principle. The equations for the following reactions will help you identify the
interacting equilibria occuring:
