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Experiment- Finding the Equilibrium Constant, Kc, for the Formation of Iron (III) Thiocyanate Chemicals: 0.200 M Fe(NO3)3 (aq), 2.00x10-3 M NaSCN (aq), 2.00x10-3 M Fe(NO3)3 (aq), 0.10 M HNO3 (aq) Equipment: Spec 20 spectrophotometer, cuvettes, 10mL pipettes, 5mL pipettes For any reaction in which the backwards or reverse reaction will occur, an equilibrium is established when the reaction rate of the reverse reaction equals the rate of the forward reaction. Once the equilibrium state has been reached, the concentrations of all products and reactants will remain constant until there is a change in the reaction conditions. The presence of an equilibrium state is shown as: For any equilibrium, the ratio of the product and reactant concentrations is described by the Equilibrium Constant, Kc: The value of Kc can be calculated from the initial concentrations of the reactants as long as the concentration at equilibrium can be measured for at least one reactant or product. Sample Calculation: A mixture of 1.000 x 10-3 mol of H2 (g) and 2.000 x 10-3 mol of I2 (g) is placed in a 1.000 L container and allowed to reach equilibrium. If the concentration of HI (g) at equilibrium is 1.87 x 10-3 M, what is the value of Kc for the reaction? This problem can best be solved by creating an Initial, Change, Equilibrium or ICE table: The change in concentration of HI is +1.87 x 10-3 M. The change in concentration of both H2 and I2 can be found using stoichiometry: 1.87 x 10-3 M HI x 1 mol H2 2 mol HI = 0.935 x 10-3 M By plugging in the changes in concentration, the ICE table can be completed to show the equilibrium concentration for each species: The equilibrium constant expression for the reaction is: The value of Kc is found by inserting the equilibrium concentrations into the equilibrium constant expression: In today’s experiment you will combine solutions of known concentration of iron (III) nitrate and sodium thiocyanate in order to establish the following equilibrium: The equilibrium constant expression for this reaction is: Once equilibrium has been established, you will measure the equilibrium concentration of FeNCS2+ (aq) using spectrophotometry. You will then use stoichiometry and the initial concentrations of the reactants to find the equilibrium concentrations of the remaining species and calculate the value of Kc. Spectrophotometry The Beer-Lambert Law states that for solutions where the concentration is relatively low a graph of the concentration of the analyte (meaning the substance detected by the spectrophotometer) vs. absorbance (A) will produce a straight line. The instrument we will be using for these measurements is called a Spec 20, shown below: This instrument measures the intensity of light at a specific wavelength passing through a sample, the percent transmittance (%T). To calculate the amount of light absorbed by the sample (A) from %T the following relationship is used, A = 2 – log (%T) Setting 0%T (Note: Be sure the instrument has been switched on and warmed up for at least 15 minutes before setting 0%T and 100%T) With the sample chamber empty, meaning there is no cuvette in the chamber, use the On/Off knob on the front of the instrument to adjust the reading for %T to 0.0. Setting 100%T Fill the cuvette with 0.10 M HNO3 and place the cuvette in the sample chamber. Be sure the white line on the cuvette is lined up with the raised mark at the front of the sample chamber and close the lid to the sample chamber. Use the 100%T knob on the front of the instrument to adjust the reading for %T to 100.0. Remove the cuvette and verify that the reading returns to 0.0%T. If the reading does not return to 0.0, repeat the steps above to confirm that the scale has been set for 0%T and 100%T. Once set-up is complete, rinse the cuvette with distilled water. Recording Measurements Each time a measurement is to be recorded, rinse the cuvette by filling it with the solution to be tested then discarding in waste. Refill the cuvette with the solution to be tested, place the cuvette in the sample chamber (again making sure the white line on the cuvette is aligned with the mark at the front of the sample chamber), close the camber lid and read the %T from the display. Procedure Waste Disposal: All waste from today’s experiment will be disposed of in Inorganic Waste. Preparation of a Standard Curve In order to find the concentration of FeNCS2+ at equilibrium a series of solutions of known concentration of FeNCS2+ will be prepared and their absorption will be measured using the Spec20. When the absorption (A) of these solutions is plotted against concentration a straight line (the Standard Curve) should be produced. Prepare five standard solutions according to the table below. Solutions will be prepared in 25.00mL volumetric flasks. Use a volumetric pipette to measure the Fe(NO3)3. Use the burette provided by your instructor to measure the NaSCN. Add 0.10 M HNO3 to each flask until the volume reaches the 25.00mL mark. By using a large concentration of Fe(NO3)3 it is assumed that the equilibrium is forced to the right and all of the NaSCN will be converted to FeNCS2+. Solution mL 0.200 M Fe(NO3)3 mL 2.00x10-3 M NaSCN 1 10.00 0.00 2 10.00 1.00 3 10.00 2.00 4 10.00 3.00 5 10.00 4.00 Turn on the Spec20, set the wavelength to 447 nm, allow it to warm up for ~10 minutes and set 0%T and 100%T. Measure %T for each solution using the Spec20 and calculate the absorption, A, for each. Calculate the concentration of FeNCS2+ in each standard solution assuming that 100% of the NaSCN has been converted to FeNCS2+. Create a graph of absorption vs. concentration of FeNCS2+ in your lab notebook. Use a ruler to draw the best-fit line for the data, this is your Standard Curve. Finding Kc Prepare the equilibrium solution by adding 5.00mL of 2.00x10-3 M Fe(NO3)3 and 5.00mL of 2.00x10-3 M NaSCN to a 25.00mL volumetric flask. Dilute to the mark with 0.10 M HNO3. Allow the solution to stand for approximately 10 minutes to assure that there is time for equilibrium to be established. Measure the %T of the solution and calculate absorption. Using the Standard Curve, find the concentration of FeNCS2+ at equilibrium. Calculate the initial concentrations of the reactants and create an ICE table in your lab notebook to find Kc. Discussion Questions 1. Write the Keq expression for the equilibrium reaction shown below: 2. Given that Keq for the reaction shown below is 5.0x10-9 at 25oC, what are the concentrations of barium and carbonate in a saturated solution of BaCO3 at 25oC?

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