Experiment 5 - EXPERIMENT 5 Spectroscopic Determination of...

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1 EXPERIMENT 5 Spectroscopic Determination of an Equilibrium Constant Objective: In this experiment the equilibrium constant for a reaction will be determined. The concentration of one of the reaction species will be monitored spectroscopically. The other concentrations will be determined stoichiometrically. The equilibrium constant will be determined from a number of experimental trials, each trial having different initial reactant concentrations. Introduction: All chemical reactions occur so as to approach a state of equilibrium. Once chemical equilibrium is attained the concentrations of all reactants and products remain constant with time. The equilibrium state of a chemical reaction is characterized by an equilibrium expression. For the general reaction a A + b B c C + d D (1) the equilibrium expression has the form K = C [ ] c D [ ] d A [ ] a B [ ] b (2) The reactant and product concentrations in the equilibrium expression are those at equilibrium, each raised to a power equal to its stoichiometric coefficient in the balanced chemical equation. K is a temperature-dependent constant called the equilibrium constant. By convention, the units are omitted when reporting equilibrium constants. In this experiment the value of the equilibrium constant for the reaction between iron(III) (Fe 3+ ) and thiocyanate ion (SCN ), Fe 3+ ( aq ) + SCN ( aq ) FeSCN 2+ ( aq ) (3) (Orange) will be determined. The equilibrium expression for this reaction is K = FeSCN 2 + [ ] Fe 3 + [ ] SCN " [ ] (4) To calculate the value of K it is necessary to determine the concentration of each of the species in the solution at equilibrium. In this experiment, the equilibrium FeSCN 2+ concentration will be measured spectrophotometrically, taking advantage of the fact that FeSCN 2+ is the only
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Experiment 5: Spectroscopic Determination of an Equilibrium Constant 2 highly colored species in the solution. From the determined FeSCN 2+ equilibrium concentration and the known initial concentrations of Fe 3+ and SCN , the stoichiometry of the reaction can be used to calculate the equilibrium concentrations of Fe 3+ and SCN . To spectrophotometrically determine the equilibrium concentration of FeSCN
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This note was uploaded on 06/10/2011 for the course CHEM 2090 taught by Professor Zax,d during the Spring '07 term at Cornell University (Engineering School).

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Experiment 5 - EXPERIMENT 5 Spectroscopic Determination of...

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