Determination of an Equilibrium Constant

Experiment (7) Determination of an Equilibrium Constant Some processes (chemical or physical) go in one direction and are irreversible, such as burning a piece of paper which produces, mainly, CO2 and H2O. Another example is the reaction between a strong acid (such as HCl) and astron base (such as NaOH) to give a salt and water (eq 1). A process that involves evaporating and condensation of a liquid is reversible. When the rate of condensation of the vapor above the liquid phase equals to the rate of evaporation of the liquid, the system is at equilibrium. The chemical reaction is at equilibrium when the rate at which the products are formed from reactants equals to the rate at which the reactants are formed from products. Chlorine (Cl2), bleach, is in equilibrium with OCl- in aqueous basic solution (eq 2). At equilibrium, the forward rate equals to the reverse one. When solutions containing Fe3+ ion and SCN- ion are mixed; they form the deep red color FeSCN2+ complex. reversible reaction as shown below: This reaction takes part in a The reaction is called in equilibrium and the equilibrium constant, Keq, is define by the equilibrium concentration (eq 4). In this experiment you are going to calculate Keq by determining the concentrations of Fe3+, SCN- and FeSCN2+ at equilibrium (equilibrium concentrations). 1. Equilibrium Concentration of FeSCN2+. This can be measured by a method which uses a spectrophotometer. (Your instructor will demonstrate for you how to operate the spectrophotometer). The principle of this technique is based on absorbing light by certain chemical substances. When light passes through solution of the red complex, FeSCN2+, the amount of light absorbed by the complex at a wavelength of 447 nm is linearly related to the concentration of FeSCN2+ in the solution according to Beer's law: Absorbance at 447 nm (A) = constant x [FeSCN2+]……………………. (5) If the amount of light absorbed (represented by Astd) by a standard solution of FeSCN2+ containing a known concentration of [FeSCN2+]std is measured then for any equilibrium mixture the concentration [FeSCN2+]eq in the equilibrium mixture can be calculated from the following relation: where Aeq is the absorbance of the equilibrium mixture and Astd is the absorbance of the standard. Note: The standard solution is a solution containing a large excess of Fe3+, which forces the equilibrium in eq. 3 to shift completely to the right, consuming all SCN-, and therefore [FeSCN2+]std = [SCN-]initial. 2. Equilibrium Concentration of Fe3+. For each one mole of FeSCN2+ produced, one mole of Fe3+ must be consumed. The equilibrium concentration of Fe3+ can be calculated as follow: 3. Equilibrium Concentration of SCN-. In a similar way: Example: In an experiment a STANDARD solution of FeSCN2+ was prepared by mixing 2.00 mL of 0.0045 M NaSCN with 8.00 mL of 0.20 M Fe(NO3)3 in 0.05 M HNO3 solution. Calculate the equilibrium constant for an equilibrium solution prepared by mixing 4.00 mL of 0.0030 M solution of NaSCN with 6.00 mL of 0.0100 M Fe(NO3)3 solution. The absorbance of the standard solution is 0.48 and the absorbance of the equilibrium solution was 0.36. 1. The Standard Solution Calculations: a) The initial SCN- concentration: b) FeSCN2+ concentration: 2- The Equilibrium Mixture Calculations: a) The initial Fe3+ concentration: b) The initial SCN- concentration: c) The equilibrium concentration of FeSCN2+: We can use equation (6) and [FeSCN2+]std (from part 1 of this exercise) to calculate d) The equilibrium concentration of SCN-: e) The equilibrium concentration of Fe3+: f) The Equilibrium Constant: Keq = 2.5 x 102 (2 significant figures) Procedure: The Quality of your results depends of the accuracy of which you measure the solution volumes. A. The Standard Solutions: 1. Clean two 10-mL volumetric flask (or test tubes) very well, rinse then with distilled water and let them drain. Into each volumetric flask (or test tube) add 1.0 mL of 0.0020 M NaSCN, 30 mL of 0.300 M Fe(NO3)3 and 6.0 mL of 0.500 M HNO3. Spectrophotometric Measurements: Note 1: Your instructor will demonstrate for you how to operate the spectrophotometer!!!! Note 2: Rinse the spectrophotometer cell twice with 1 mL of the solution, which you are going to measure its absorbance, before doing any absorbance measurement. These cells are very expensive, use and handle them carefully. 2. Make sure that the wavelength in the spectrophotometer is set at 447 nm. 3. Measure the absorbance for the blank solution (the solution that has only 0.500 M HNO3) and adjust the absorbance (A) to zero and the transmitance (T%) to 100%. 4. Determine the absorbance (A) of both standard solutions and record your results in a table in your report sheet. 5. If the two absorbance values are close enough (show these values to your instructor, less than 5% difference is acceptable) determine the average and record it in your report. If the two absorbance values are not close enough (difference is more than 5%) preparte another standard solution and repeat steps 1-4. 6. If the new (third) absorbance measurement is close to one of the other two absorbance values, reject one which is not close and take the average for the other two. If not (all the three absorbance values are not close), take the average value of the three measurement and record it in your report sheet. B. The equilibrium solution: 1. Clean four test tubes very well, rinse them with distilled water and let them drain. Label the test tubes by numbers from 1-4. The table below lists all the solutions and the quantities of the original solution to be added (from burets) to each test tube. NaSCN Fe(NO3)3 HNO3 0.0020 M 0.0050 M 0.500 M 1 3.00 mL 2.00 mL 5.00 mL 2 3.00 mL 1.50 mL 5.50 mL 3 3.00 mL 1.00 mL 6.00 mL 4 3.00 mL 0.50 mL 6.50 mL Test Tube 2. Measure the absorbance at 447 nm (A) from each solution by following the same procedure above (step 2-4 in part A). Record the results in a table in your report sheet. Report Sheet Student Name ……………………… Student No. ………………………..... Instructor Section No. ………………………….. ……………………… Partner Name ………………………. Date: ………………………….. A. The Standard: Trail 1 Trail 2 Trail 3 Test Tube Test Tube Test Tube Test Tube #1 #2 #3 #4 [FeSCN2+]std = [SCN-]in Absorbance (Astd) Average Astd = B. The Equilibrium Mixture: [Fe3+]in [SCN-]in Absorbance, A [FeSCN2+]eq [Fe3+]eq [SCN-]eq Keq (Attached a separate paper that shows one sample calculations of K for solution # 2) Average, Keq: (K1 + K2 + K3 + K4)/4 Standard Deviation,