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Chemical Equilibrium and Santa Monica College

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Chemistry 12

Santa Monica College

Determination of Kc for a Complex Ion Formation
Objectives
• • Find the value of the equilibrium constant for formation of FeSCN2+ by using the visible light absorption of the complex ion. Confirm the stoichiometry of the reaction.

Background
In the study of chemical reactions, chemistry students first study reactions that go to completion. Inherent in these familiar problems—such as calculation of theoretical yield, limiting reactant, and percent yield—is the assumption that the reaction can consume all of one or more reactants to produce products. In fact, most reactions do not behave this way. Instead, reactions reach a state where, after mixing the reactants, a stable mixture of reactants …show more content…

These values can be determined from a reaction table ('ICE' table), as shown in Table 1:
Table 1: Reaction ICE Table Fe3+(aq) Initial Concentration Change in Concentration Equilibrium Concentration [Fe3+]i –x [Fe ]i – x
3+

+

SCN–(aq) [SCN–]i –x [SCN ]i – x
–

!

FeSCN2+(aq) 0 +x x = [FeSCN2+]

The reaction "ICE" table demonstrates the method used in order to find the equilibrium concentrations of each species. The values that come directly from the experimental procedure are found in the shaded regions. From these values, the remainder of the table can be completed.

The initial concentrations of the reactants in Table 1—that is, [Fe3+] and [SCN–] prior to any reaction—can be found by a dilution calculation based on the values from Table 2 found in the procedure. Once the reaction reaches equilibrium, we assume that the reaction has shifted forward by an amount, x. Notice from Table 1 that the value of x is simply equal to the equilibrium concentration of FeSCN2+(aq), or that x = [FeSCN2+] at equilibrium. The equilibrium value of [FeSCN2+] is determined spectroscopically using Beer’s Law. Its initial value in the table is zero because no FeSCN2+(aq) is added to the initial solution. Finally, the equilibrium concentrations of the reactants, Fe3+(aq) and SCN–(aq), are found by subtracting the equilibrium concentration of FeSCN2+(aq) from the initial concentrations of Fe3+(aq) and SCN–(aq), as shown in the table above. Once all the equilibrium values are

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