Exp14 - Experiment 14 DETERMINATION OF AN EQUILIBRIUM...

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Experiment 14 14—1 DETERMINATION OF AN EQUILIBRIUM CONSTANT IN AQUEOUS SOLUTION I. Learning Objectives… To become familiar with the concept of equilibrium by determination of an equilibrium constant for a reaction in solution. II. Background Information If you have ever seen the beautiful stalactites and stalagmites that form in limestone caves, you must surely have wondered how they were formed. The key aspect of the formation of these primarily calcium carbonate natural wonders is the reversibility of chemical reactions. Calcium carbonate occurs in underground deposits as a remnant from ancient oceans. When water that contains dissolved CO 2 seeps through these deposits, they slowly dissolve as a result of the following reaction: CaCO 3(s) +CO 2(g) +H 2 O (l) Ca 2+ aq +2HCO 3 - (aq) [1] When the water saturated with Ca(HCO 3 ) 2 reaches a cave, the reverse reaction occurs, liberating gaseous CO 2 and very slowly depositing solid CaCO 3 as stalactites and stalagmites. We write the expression for the reversible equation reaction as shown below with a double arrow to indicate that the reaction proceeds in both directions simultaneously. CaCO 3(s) +CO 2(g) +H 2 O (l) Ca 2+ aq +2HCO 3 - (aq) [2] All chemical equilibria are dynamic and are constantly proceeding in both directions. The system reaches a state of equilibrium when the rate of the reaction in the forward
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14—2 direction is equal to the rate of the reaction in the reverse direction. The equilibrium expression for this equilibrium with equilibrium constant K c is: K c = Ca 2 + HCO 3 2 CaCO 3 [ ] CO 2 [ ] H 2 O [ ] [3] For the general equilibrium equation (4), where A, B, C, and D are the chemical species involved and a, b, c, and d are their coefficients in the balanced chemical equation, the equilibrium constant expression is aA + bB cC + dD [4] K eq = C [ ] c D [ ] d A [ ] a B [ ] b [5] The numerator in equation 5 is the product of the molar concentrations of the products raised to the powers of their respective coefficients. The denominator is the product of the molar concentrations of reactants raised to the powers of their respective coefficients. In order to determine the value of the equilibrium constant, K eq , we need to know the equilibrium concentration of all the species involved. In this experiment you will determine the value of K eq for the following equilibrium: Fe 3+ (aq) + SCN - (aq) FeNCS 2+ (aq) yellow colorless dark red [6] If we determine the equilibrium FeNCS 2+ concentration in solution of known initial concentrations of Fe 3+ and SCN - , then by simple stoichimetry we may calculate the final equilibrium concentrations of all three species and hence the value of the equilibrium constant, K eq . This is illustrated in sample calculation 1.
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14—3 Sample Calculation 1.
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