Cliffs AP Chemistry, 3rd Edition

Experiment 10 C: Determination of the Equilibrium Constant, Kc, for a Chemical Reaction

Background: Many reactions do not go to completion. Rather, they reach an intermediate state
in which both reactants and products are present simultaneously. When the concentrations of
all species remain constant at a particular temperature over time, a state of equilibrium has
been achieved. The equilibrium constant, Kc, relates the concentrations of products to reactants
at equilibrium. Kcvalues significantly greater than 1 indicate that the products are favored,
while Kcvalues significantly less that 1 indicate that the reactants are more predominant.

In this experiment the equilibrium between iron (III) ions, Fe3+, and thiocyanate ions, SCN–,
will be investigated. The equilibrium equation is:

Fe^32 ()aq+-+SCN()aqEFeSCN()aq+

From the equation, an equilibrium expression can be created:

K

Fe SCN

FeSCN

c 3

2

= +-

+

77

7

AA

A

The FeSCN2+ion is red. The degree of color is proportional to the concentration. The concen-
tration of FeSCN2+will be found through the use of a calibrated spectrophotometer whose most
optimal wavelength was determined in which a known concentration of FeSCN2+is made up
and the wavelength varied for maximum absorbance. Since the FeSCN2+is red, the wavelength
of the light in the spectrophotometer should be the complement color and appear blue-violet.
Then, with the spectrophotometer set at this optimal wavelength, the absorbances of various
known concentration of FeSCN2+will be recorded and graphed. From this graph the concentra-
tion of the FeSCN2+at equilibrium will be extrapolated through Beer’s law.

Scenario: A student determined that the optimal wavelength for the absorbance of FeSCN2+
experiment was 445 nm. Then the student prepared samples of known concentrations of
FeSCN2+ranging from 4.0 × 10 –5M to 1.4 × 10 –4M. The samples were then examined by
means of a spectrophotometer and their transmittances recorded. From the transmittance, the
absorbance was calculated and graphed. Next, he mixed 5.0 mL of 2.0 × 10 –3M Fe(NO 3 ) 3 with
5.0 mL of 2.0 × 10 –3M KSCN. This solution was then analyzed in the spectrophotometer and
through extrapolation, he was able to determine that the concentration of FeSCN2+at equilib-
rium was 1.3 × 10 –4M.

Analysis:

1. How many moles of Fe3+and SCN–were initially present?
   .'^3 + 5 +
   '
   ..
   1

0 0050 20 10

10 10

L sol n

L sol n

# # - moles Fe^3 = # - moles Fe^3

.' 5

'

..
1

0 0050 20 10

10 10

L sol n

L sol n

# # --^3 moles SCN = # --moles SCN

Part III: AP Chemistry Laboratory Experiments