56 Experiment 10 The Determination of an Equilibrium Constant Chemical reactions occur to reach a state of equilibrium. The equilibrium state can be characterized by quantitatively defining its equilibrium constant, K eq . In this experiment, you will determine the value of K eq for the reaction between iron (III) ions and thiocyanate ions, SCN – . Fe 3+ (aq) + SCN – (aq) → FeSCN 2+ (aq) The equilibrium constant, K eq , is defined by the equation shown below. ] SCN ][ Fe [ ] FeSCN [ 3 2 eq K To find the value of K eq , which depends only upon temperature, it is necessary to determine the molar concentration of each of the three species in solution at equilibrium. You will use a colorimeter to help you measure the concentrations (see Figure 1). The amount of light absorbed by a colored solution is proportional to its concentration. The red FeSCN 2+ solution absorbs blue light, and it will be analyzed at 470 nm (blue light). Figure 1 In order to successfully evaluate this equilibrium system, it is necessary to conduct three separate tests. First, you will prepare a series of standard solutions of FeSCN 2+ from solutions of varying concentrations of SCN – and constant concentrations of H + and Fe 3+ that are in stoichiometric excess. The excess of H + ions will ensure that Fe 3+ engages in no side reactions (to form FeOH 2+ , for example). The excess of Fe 3+ ions will make the SCN – ions the limiting reagent, thus all of the SCN – used will form FeSCN 2+ ions. The FeSCN 2+ complex forms slowly, taking at least one minute for the color to develop. It is best to take absorbance readings after a specific amount of time has elapsed, between two and four minutes after preparing the equilibrium mixture. Do not wait much longer than four minutes to take readings, however, because the mixture is light sensitive and the FeSCN 2+ ions will slowly decompose. In Part II of the experiment, you will analyze a solution of unknown [SCN – ] by using the same procedure that you followed in Part I. In this manner, you will determine the molar concentration of the SCN – solution. Third, you will prepare a new series of solutions that have varied concentrations of the Fe 3+ ions and the SCN – ions, with a constant concentration of H + ions. You will use the results of this test to accurately evaluate the equilibrium concentrations of each species.
57 Experiment 10 OBJECTIVES In this experiment, you will Prepare and test standard solutions of FeSCN 2+ in equilibrium.
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