D3

1 November 13, 2023 Experiment D3: The Determination of an Equilibrium Constant Purpose To experimentally determine the quantitative value of the equilibrium constant, K c , for the reaction of thiocyanate ion, SCN (aq) , with iron (III) ion, Fe 3+ (aq) , giving rise to thiocyanatoiron (III), FeSCN 2+ (aq) , Fe 3+ (aq) + SCN - (aq) ⇌ FeSCN 2+ (aq) (1) Using the expression K c = ¿¿ Via a spectrophotometer that utilizes the specific wavelengths given off by the respective ion concentrations within the solution. Secondary objectives include becoming familiar with construction of a calibration curve and practice with conducting dilutions. Introduction This experiment deals with the principle of dynamic equilibrium in chemical reactions, such that at a fixed temperature, any given reaction in a closed system will reach a state of equilibrium. This is the state in which the rate of the formation of products is equal to the rate of the breakdown of products and will no longer change with time, denoted by the following, aA + bB ⇌ cC + dD Thus, in each chemical reaction, there exists an equilibrium constant, K c , which describes the relationship between the products and reactants at a given temperature when the reaction has reached equilibrium, K c = [ C ] c [ D ] d [ A ] a ¿¿ Where [A] and [B] are the equilibrium concentrations of the reactants and a and b are their respective coefficients of the balanced chemical equation, and [C] and [D] are the equilibrium concentrations of the products and c and d are their respective coefficients. Following Le Châtelier’s Principle, changes in

2 concentrations of reactants or products will result in subsequent changes in concentrations where the equilibrium constant will remain the same within a given reaction at a given temperature. This experiment looks at the equilibrium reaction between thiocyanate ions, SCN (aq) , with iron (III) ions, Fe 3+ (aq) , giving rise to thiocyanatoiron (III), FeSCN 2+ (aq) , Fe 3+ (aq) + SCN - (aq) ⇌ FeSCN 2+ (aq) With equilibrium constant formula, K c = ¿¿ The varying concentrations of each respective ion can be related to the colour change observed in the solution through spectrophotometry, which measures how much light is absorbed at a particular wavelength. The thiocyanate ion, SCN (aq) , is colourless, while the iron (III) ion, Fe 3+ (aq) , is yellow, and the resultant product, thiocyanatoiron (III), FeSCN 2+ (aq) , is red. Therefore, a calibration curve can be made using various dilutions of thiocyanatoiron (III). The colour variation in subsequent solutions can then be quantitatively related to the respective quantity of thiocyanatoiron (III) ions through the calibration curve by Beer-Lambert Law, A ∝ c where absorbance (A) is directly proportional to the concentration (c) of the solution. Procedure ref: Experiment D3 of the Chemistry 218 Laboratory Manual , pp. 65-76. Part A. Construction of a Calibration Curve 1. A mass of 4.03 g of iron (III) nitrate (nonahydrate) was used to prepare 50 mL of a 0.20 mol/L iron (III) nitrate solution. 2. A mass of 1.26 g of potassium thiocyanate was used to prepare 250 mL of a 5.18 x 10-2 mol/L potassium thiocyanate solution. This solution was further diluted taking 1.0 mL of the stock solution to 100 mL of distilled water, creating a 5.18 x 10-4 mol/L potassium thiocyanate solution.

3 3. Both 25 mL of the 0.20 mol/L iron (III) nitrate solution and 25 mL of the 5.18 x 10-4 mol/L potassium thiocyanate solution were then pipetted into a beaker and mixed for 15 minutes using a magnetic stirrer. 4. 25 mL of a 0.1 mol/L nitric acid solution was added into 6 separate beakers label A through F. 5. Following this, 25 mL of the mixed solution from step 3 was pipetted into beaker A and mixed well. 6. 25 mL of the solution from beaker A was transferred using a clean pipette to beaker B and mixed well. 7. Using the same procedure as listed in step 6, serial dilutions were made in beakers C through F subsequently. 8. A Spectronic 20 spectrophotometer was then blanked using a 0.10 mol/L solution of nitric acid at a wavelength of 445 nm from 0% to 100% transmittance. 9. The absorbance of each serial dilution, beakers A through F, was then measured in duplicate. Part B. Determination of the Equilibrium Constant 1. 5 mL of the 0.20 mol/L iron (III) nitrate solution from step 1 of Part A was transferred into a volumetric flask and 245 mL of 0.10 mol/L of nitric acid was added. 2. 25 mL of the solution from step 1 and 25 mL of 0.10 mol/L nitric acid was then added to an additional flask. 3. Then 25 mL of the solution from step 2 and 25 mL of 0.10 mol/L nitric acid was added to a new flask. 4. The procedure in step 3 was then repeated to dilute 2 additional times, giving a total of 4 serial dilutions. 5. Following this, 10 mL of the 5.18 x 10-4 mol/L potassium thiocyanate solution from step 2 of Part A was transferred into 5 separate beakers, labeled A through E. 6. Five reaction mixtures were then made by combining 10 mL of the solutions from step 1 and the 4 serial dilutions to the beakers A through E respectively. These solutions were mixed and left for 15 minutes. 7. Using the spectrophotometer, absorbance was recorded at 445 nm for each of the five reaction mixtures.

4 Results and Calculations Part A. Construction of a Calibration Curve 1. Preparation of a solution of iron (III) nitrate Mass of vial + iron (III) nitrate 18.0411 g Mass of empty vial 14.0065 g Mass of iron (III) nitrate (nonahydrate) 4.0346 g Concentration of the iron (III) nitrate solution 4.0346 g× 1 mol 403.95 g × 1 0.050 L = 0.20 mol / L 2. Preparation of a solution of potassium thiocyanate Mass of vial + potassium thiocyanate 9.9664 g Mass of empty vial 8.7079 g Mass of potassium thiocyanate 1.2585 g Concentration of the potassium thiocyanate solution 1.2585 g× 1 mol 97.181 g × 1 0.250 L = 5.18 × 10 − 2 mol / L Dilution 5.18 × 10 − 2 mol / L× 1 × 10 − 3 L 0.1 L = 5.18 × 10 − 4 mol / L 3. Calculate the concentration of Fe(SCN) 2+ in the solution formed from mixing 25 mL of your iron (III) nitrate solution with 25 mL of your potassium thiocyanate solution. Moles of iron (III) nitrate (Fe(NO 3 ) 3 *9H 2 O) 0.20 mol 0.050 L × 0.025 L = 0.10 mol Moles of potassium thiocyanate solution (KSCN) 5.18 × 10 − 4 mol 0.050 L × 0.025 L = 2.59 × 10 − 4 mol←limiting reagent Concentration of Fe(SCN) 2+ CV product = CV limiting reagent