Week of February 22 - February 25, 2010
The Final Report for experiments 12-090 - 15-015 is due this week.
Report Sheets for Experiment 16-077b and Experiment 17-044 are due next week (March 1, 2010).
Experiment 17-044: Determination of an Equlibrium Constant
Lab 17-044 report sheets are due the week of March 1st: Extra Report Sheet for experiment 17-044 are available in PDF format.
This week you will determine the equlibrium constant (at room temperature) for the reaction:
Fe3+(aq) + SCN- (aq) ----> FeSCN2+(aq) [1]
Briefly, known amounts of solutions containing known concentrations of Fe3+(aq) and SCN- (aq) ions will be mixed together, and then the absorbance of the resulting FeSCN2+(aq) ion will be measured. The experiment is divided into two sections:
In the first part of the experiment, you will prepare solutions with known concentrations of FeSCN2+(aq) by mixing solutions containing SCN-(aq)with a vast excess of Fe3+(aq) ion. This will drive reaction 1 all the way to the right (theoretically). Since the concentration of FeSCN2+(aq) in each of the solutions can be determined, the concentrations can be correlated to the measured absorbance values. And you guessed it -- you can generate a Standard Absorbance Curve for FeSCN2+(aq) at 450 nm (lmax).
In the second part of the experiment, you will prepare solutions in which neither Fe3+(aq) nor SCN-(aq) will be present in vast excess, so you cannot assume the reaction is forced all the way to the right. The concentration of FeSCN2+(aq) must be determined by measuring the absorbance of the solution at 450 nm and using the standard curve to determine the concentration of FeSCN2+(aq) in that solution. Knowing the concentration of FeSCN2+(aq) formed and the initial concentrations of Fe3+(aq) and SCN-(aq) ions, the concentrations of Fe3+(aq) and SCN-(aq) remaining in solution at equilibrium can be calculated.
How does this experiment allow us to determine an equlibrium constant?
The Law of Mass Action says that when a reaction has reached equilibrium then Q will be constant. At equilibrium, the reaction quotient becomes the equilibrium constant, and is abbreviated as “K”. At equilibrium, Q = K. For reaction 1:
Q = [FeSCN2+(aq)]/[Fe3+(aq)][SCN-(aq)]
Hence, knowing [FeSCN2+(aq)],[ Fe3+(aq)] and [SCN-(aq)] at equilibrium allows you to determine the value of Q (K) for each reaction mixture. Since you will prepare the solutions using different inital concentrations of Fe3+(aq) and SCN-(aq), so you will also be able to determine whether K is independent of starting conditions.
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