Chemical Equilibrium: Finding a Constant, Kc The purpose of this lab is to experimentally determine the equilibrium constant, Kc, for the following chemical reaction: Fe3+(aq) + SCN–(aq) FeSCN2+(aq) iron(III) thiocyanate thiocyanoiron(III) When Fe3+ and SCN- are combined, equilibrium is established between these two ions and the FeSCN2+ ion. In order to calculate Kc for the reaction, it is necessary to know the concentrations of all ions at equilibrium: [FeSCN2+]eq, [SCN–]eq, and [Fe3+]eq. You will prepare four equilibrium systems containing different concentrations of these three ions. The equilibrium concentrations of the three ions will then be experimentally determined. These values will be substituted into the equilibrium constant expression to see if Kc is indeed constant. In order to determine [FeSCN2+]eq, you will use the Colorimeter shown in Figure 1. The FeSCN2+ion produces solutions with a red color. Because the red solutions absorb blue light very well, the blue LED setting on the Colorimeter is used. The computer-interfaced Colorimeter measures the amount of blue light absorbed by the colored solutions (absorbance, A). By comparing the absorbance of each equilibrium system, Aeq, to the absorbance of a standardsolution, Astd, you can determine [FeSCN2+]eq. The standard solution has a known FeSCN2+ concentration. Figure 1 To prepare the standard solution, a very large concentration of Fe3+ will be added to a small initial concentration of SCN–(hereafter referred to as [SCN–]i. The [Fe3+] in the standard solution is 100 times larger than [Fe3+] in the equilibrium mixtures. According to LeChatelier's principle, this high concentration forces the reaction far to the right, using up nearly 100% of the SCN–ions. According to the balanced equation, for every one mole of SCN–reacted, one mole of FeSCN2+is produced. Thus [FeSCN2+]stdis assumed to be equal to [SCN–]i. Assuming [FeSCN2+] and absorbance are related directly (Beer's Law), the concentration of FeSCN2+for any of the equilibrium systems can be found by: [FeSCN2+]eq = Aeq AstdX [FeSCN2+] std
Knowing the [FeSCN 2 + ] eq
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