Experiment 20 Chemical Equilibrium: Finding a Constant, Kc The purpose of this l

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Experiment 20 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(II) thiocyanate thcanoiron(III) When Fe3+ and SCN- are combined, equilibrium is established between these two ions and the FeSCN2+ ion. In order to calculate Kg for the reaction, it is necessary to know the concentrations of all ions at equilibrium: [FeSCN'g [SCN-leg, and [Fe eg 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 Ke is indeed constant In order to determine [FeSCN2+-leq you will use the Colorimeter shown in Figure 1. The FeSCN 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, Aeg to the absorbance of a standard solution, Astd, you can determine [FeSCN eg. The standard solution has a known FeSCN+ concentration. To prepare the standard solution, a very large concentration of Felt will be added to a small initial concentration of SCN- hereafter referred to as [SCN-1), The (Fe+] in the standard solution is 100 times larger than [Fe"] in the equilibrium mixtures. According to LeChatelier's principle, this high concentration fores 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+1std is assumed to be equal to [SCNli. Assuming [FeSCN2+] and absorbance are related directly (Beer's Law), the concentration of FeSCN+ for any of the equilibrium systems can be found by Astd

Explanation / Answer

To start with I will write here the given table for volume of each reactant used and absorbance

Absorbance for standerd = 0.002

[FeSCN2+] = 0.0002 M

Kc = [FeSCN2+]eq / [Fe3+]eq [SCN-]eq

Now according to the given formulas complete the table,

Trial 1 Kc = 0.0185 / ( -0.0175 * -0.0174) = 60.75

Trial 2 Kc = 0.0242 / ( -0.0232 * -0.0236) = 44.20

Trial 3 Kc = 0.0343 / ( -0.0333 * -0.0335) = 30.75

Trial 4 Kc = 0.0630 / ( -0.0620 * -0.0620) = 16.39

Average Kc = (60.75 + 44.20 + 30.75 + 16.39) / 4 = 38.02

( Note: It seems there is some problem in your absorbance value for standerd sample, thats why the equilibrium conentration for Fe3+ and SCN- are negative it should not be. All above calculations are based on your given data.)

Test tube number Fe(NO2)3 (ml) KSCN (ml) H2O (ml) Total (ml) Absorbance 1 5 2 3 10 0.185 2 5 3 2 10 0.242 3 5 4 1 10 0.343 4 5 5 0 10 0.630

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