Exp2 - Equillibrium constant

Exp2 - Equillibrium constant - SPECTROPHOTOMETRIC...

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1 SPECTROPHOTOMETRIC DETERMINATION OF AN EQUILIBRIUM CONSTANT Experiment 2 INTRODUCTION 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) (1) iron(III) thiocyanate thiocyanoiron(III) red The equilibrium constant is given by K c = [FeSCN 2+ ] eq [Fe 3+ ] eq [ SCN - ] eq (2) 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. Usually, molecules (or ions) can only absorb at certain discreet wavelengths of light, which correspond to specific electronic transitions. The FeSCN 2 + ion produces solutions with a red color, indicating that the cyan light included in white light has been absorbed by FeSCN 2 + ion 1 . Since cyan is close to blue, the blue LED setting on the colorimeter will be 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, A eq , to the absorbance of a standard solution , A std , one can determine [FeSCN 2 + ] eq . The standard solution, having a known FeSCN 2 + concentration at equilibrium. Figure 1 Computer-interfaced colorimeter 1 See your textbook, section 24-7, p.1020-1021, for a review on colors.
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2 Beer’s Law relates the absorbance to the concentration of the absorbing species: A = a b c (3) where A : Absorbance a : Absorptivity constant for a given species, at a given wavelength (L mol 1 cm 1 ) b : Cell, path length (cm) c : Concentration of absorbing species, FeSCN 2 + (mol/L) For a given species, a and b are constants at a given wavelength. The absorbance, is therefore proportional to the concentration. 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 in the solution (see the 1:1 ratio of coefficients in equation (1)). The [Fe 3 + ] can be determined by: [Fe 3 + ] eq = [Fe 3 + ] o – [FeSCN 2 + ] eq (4) Because one mole of SCN - is used up for each mole of FeSCN 2 + ions produced, [SCN ] eq can be determined by: [SCN ] eq = [SCN ] o – [FeSCN 2 + ] eq (5) Knowing the values of [Fe 3 + ] eq , [SCN ] eq , and [FeSCN 2 + ] eq , you can use equation (2) to calculate the value of K c , the equilibrium constant. APPARATUS AND CHEMICALS
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This note was uploaded on 04/04/2011 for the course SCIENCE CHEM 120 taught by Professor Fenster during the Winter '11 term at McGill.

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Exp2 - Equillibrium constant - SPECTROPHOTOMETRIC...

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