20kcequi - Experiment 20 Chemical Equilibrium: Finding a...

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Experiment 20 Chemical Equilibrium: Finding a Constant, K c The purpose of this lab is to experimentally determine the equilibrium constant, K c , for the following chemical reaction: Fe 3+ (aq) + SCN - (aq) ¬→ FeSCN 2+ (aq) iron(III) thiocyanate thiocyanoiron(III) When Fe 3+ and SCN - are combined, equilibrium is established between these two ions and the FeSCN 2+ ion. In order to calculate K c for the reaction, it is necessary to know the concentrations of all ions at equilibrium: [FeSCN 2+ ] eq , [SCN - ] eq , and [Fe 3+ ] 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 K c is indeed constant. In order to determine [FeSCN 2+] eq , you will use the colorimeter shown in Figure 1. The FeSCN 2+ 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 colorimeter measures the amount of blue light absorbed by the colored solutions (absorbance, A). By comparing the absorbance of each equilibrium system, A eq , to the absorbance of a standard solution, A std , you can determine [FeSCN 2+ ] eq . The standard solution has a known FeSCN 2+ concentration. Figure 1 To prepare the standard solution, a very large concentration of Fe 3+ will be added to a small initial concentration of SCN - (hereafter referred to as [SCN -] i . The [Fe 3+ ] in the standard solution is 100 times larger than [Fe 3+ ] 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 FeSCN 2+ is produced. Thus [FeSCN 2+ ] std is assumed to be equal to [SCN - ] i . Assuming [FeSCN 2+ ] and absorbance are related directly (Beer’s law), the concentration of FeSCN 2+ for any of the equilibrium systems can be found by: [FeSCN 2+ ] eq = X [FeSCN 2+ ] std Knowing the [FeSCN 2+ ] eq allows you to determine the concentrations of the other two ions at equilibrium. For each mole of FeSCN 2+ ions produced, one less mole of Fe 3+ ions will be found Chemistry with CBL 20 - 1
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Experiment 20 in the solution (see the 1:1 ratio of coefficients in the equation on the previous page). The [Fe 3+ ] can be determined by:
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This note was uploaded on 11/16/2011 for the course CHM 2046 taught by Professor Chamusco during the Summer '10 term at Santa Fe College.

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20kcequi - Experiment 20 Chemical Equilibrium: Finding a...

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