Abby Darnell

Determination of an Equilibrium Constant July 6, 2011

Chemistry 188 Summer 2011 - Lab TA: Luke McCormick

Introduction:

The purpose of this experiment was to measure the equilibrium constant for a reaction

involving the complexation of two species. Chemical equilibrium applies to reactions that can

occur in both directions and when the net change in the reactants and the products is zero then

the reaction has reached equilibrium. Both reactions are still occurring but they are constant and

balanced. An equilibrium constant (K) is “a number that is equal to the ratio of the equilibrium

concentrations of products to the equilibrium concentrations of reactants, each raised to the

power of its stoichiometric coefficient.” The equilibrium constant is based on the initial

concentration of the reaction, and when it changes the constant changes.

Experimental:

In setting up for the experiment, the spectrophotometer was set up and calibrated. 50.0

mL of KSCN solution were put into a 250 mL beaker. Then 10 mL of Fe(NO3)3 solution were

transferred to a clean 25 mL beaker. Portions of 1 mL of Fe(NO3)3 were pipetted into the KSCN

solution. After each addition the solution was thoroughly stirred then put into a cuvette. The

absorbance was measured at 445 nm then the contents of the cuvette were returned to the parent

solution. Ten subsequent 1 mL additions were performed and the absorbance was recorded for

each. This whole procedure was performed three times. Once all three trials were complete, the

calculations to find the equilibrium constant were done.

Results:

Fe(NO3)3

added mL

KSCN added Total Volume Bound &Free SCN-

Free Fe3+ Trial 1Absorbance

Trial 2Absorbance

Trial 3Absorbance

0 50 mL 50 mL 6.0e-6 0 0 0 0

1 50 mL 51 mL 1.18e-7 1.96e-6 0.066 0.043 0.098

2 50 mL 52 mL 1.15e-7 1.92e-6 0.192 0.179 0.195

3 50 mL 53 mL 1.13e-7 1.89e-6 0.260 0.217 0.392

4 50 mL 54 mL 1.11e-7 1.85e-6 0.260 0.306 0.395

5 50 mL 55 mL 1.09e-7 1.82e-6 0.307 0.388 0.421

6 50 mL 56 mL 1.07e-7 1.79e-6 0.223 0.459 0.543

7 50 mL 57 mL 1.05e-7 1.75e-6 0.326 0.554 0.584

8 50 mL 58 mL 1.03e-7 1.72e-6 0.305 0.603 0.618

9 50 mL 59 mL 1.02e-7 1.69e-6 0.282 0.629 0.639

10 50 mL 60 mL 1.0e-7 1.67e-6 0.246 0.688 0.691

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

50000

100000

150000

200000

250000

300000

350000

400000

450000

036000

97400

161000190000

222000

280000

325000355000

375000413000

Series2Linear (Series2)

Average Absorbance (445 nm)

Aver

age

A/Fe

3+

y=mx+b

Discussion:

The chemical reaction that was studied in this experiment was:

Fe3+ (aq) + SCN- (aq) FeSCN2+ (aq)

The strategy to find the equilibrium constant in this experiment was to do three trials of adding

Fe(NO3)3 to KSCN, 1 mL at a time to create a graph and find the slope of that graph. Using the

slope, the equilibrium of the reaction can be determined. When adding the Fe(NO3)3 to the

KSCN the color of the solution becomes more intense which shows that the concentration is

getting higher. The average of the trials are the variables chosen to plot because they provide the

most accurate data and results of the experiment. The plot of the variables is used to determine

Kc by the slope and the equation:

A=(−1Kc )¿

Rearranging the equation gives the result of the equilibrium constant which is what is needed.

The results in trials two and three are what was expected but the results for trial one were not

expected. In trial one as the concentration increased the absorbance did not increase each time.

For this reason trial one had to be thrown out so that the rest of the results would come out as

expected. Trial one is also a source of error this trial being thrown out caused less data to work

with and more room for inaccuracy. The cause of trial one’s inconsistency could have been

because incorrect amounts were added to the solution or because something was off with the

spectrophotometer. The impact of this error if it had gone unnoticed would have been that our

data was incorrect, which would have given an incorrect equilibrium constant.