Measuring Faraday's Constant & Avogadro's Number via Potassium Iodide Electrolysis, Study notes of Law

Download Measuring Faraday's Constant & Avogadro's Number via Potassium Iodide Electrolysis and more Study notes Law in PDF only on Docsity! Experiment 21A FV 04/20/11 FARADAY’S LAW MATERIALS: DPDT (double pole/double throw) switch, digital ammeter, J-shaped platinum electrode and holder, Hubbell plug with attached leads, carbon electrode, electrical lead, 10 mL graduated cylinder, 100 mL beaker, 400 mL beaker, buret, thermometer, starch-KI paper, starch solution, 0.20 M KI, 1 M H2SO4, 0.0200 M Na2S2O3. PURPOSE: The purpose of this experiment is to determine values for the Faraday constant and Avogadro’s number. LEARNING OBJECTIVES: By the end of this experiment, the student should be able to demonstrate the following proficiencies: 1. Construct an electrolytic cell from a diagram. 2. Determine the number of moles of products formed in a redox reaction from experimental data. 3. Determine the total charge that has passed through an electrolytic cell. 4. Calculate values for the Faraday constant and Avogadro’s number from experimental data. DISCUSSION: Forcing an electrical current through an electrolytic cell can cause a nonspontaneous chemical reaction to occur. For example, when direct current is passed through a solution of aqueous potassium iodide, KI, the following reactions occur at the electrodes: anode: 2 I− (aq) → I2 (aq) + 2 e− (oxidation) cathode: 2 H2O (l) + 2 e− → H2 (g) + 2 OH− (aq) (reduction) overall redox: 2 I− (aq) + 2 H2O (l) → I2 (aq) + H2 (g) + 2 OH− (aq) Electrons can be treated stoichiometrically like the other chemical species in these reactions. Thus, the number of moles of products formed is related to the number of moles of electrons that pass through the cell during the electrolysis. Iodine is formed at the anode in this electrolysis and dissolves in the solution upon stirring.1 Hydrogen gas is formed at the cathode and will be collected in an inverted graduated cylinder by displacement of water. In this reaction, because the same number of electrons must pass through each electrode, the number of moles of iodide ion oxidized at the anode must equal the number of moles of water reduced at the cathode (i.e., in redox equations, electrons gained = electrons lost). Thus, equimolar quantities of hydrogen gas (H2) and molecular iodine (I2) will be produced by the electrolysis. 1Iodine reacts with water according to the equilibrium below. I2 (aq) + H2O (l) ↔ HIO (aq) + H+ (aq) + I− (aq) For this reason, the pH of the solution must be adjusted to insure that I2 is the predominant species in solution. For a more detailed explanation see: a) I. M. Kolthoff and R. Belcher, Volumetric Analysis, New York, Interscience (1957), Vol. 3, pp 214-215 and b) W. C. Bray and E. L. Connolly, J. Am. Chem. Soc. 33 (1911), 1485. E21A-1 The current, or rate of flow of electricity, is measured in amperes, A. The ampere is the SI unit of current and corresponds to 1 coulomb of charge flowing for 1 second. Therefore, the total charge passing through the circuit, in coulombs, is equal to the product of the current in amperes and the time of current flow in seconds. C = A × t (in sec) In this experiment the following quantities will be obtained: (1) the total charge that has passed through the cell obtained from the average current and the elapsed time; (2) the number of moles of H2 determined from the volume of gas collected; and (3) the number of moles of I2 found by titration of the iodine with sodium thiosulfate (Na2S2O3). The number of moles of electrons that have passed through the cell can be obtained from the moles of hydrogen or the moles of iodine using stoichiometry. The total charge in coulombs that has passed through the cell can be obtained from the average current data. The value of the Faraday constant (F) can then be calculated from the total charge used in the electrolysis and the number of moles of electrons. Remember that 1 F is the electric charge, in coulombs, on 1 mole of electrons. Avogadro’s number can also be determined using the following procedure. The value of Avogadro’s number is the number of units in one mole. For the electrolysis, the required quantities for this calculation are: (1) the number of electrons and (2) the moles of electrons flowing through the electrolytic cell. As before, the moles of electrons can be determined stoichiometrically. The number of electrons can be determined from the total charge used, knowing that the charge on a single electron is 1.60 x 10-19 coulombs. E21A-2 Name Section Partner Date DATA SECTION Experiment 21A Report all data with the proper number of significant figures and units. Part A. Electrolysis and Quantity of Electrical Charge Used. Trial 1 Initial Time _____________ Final Time _____________ Current Average current (A) = ______________________ Total elapsed time (s) = _____________________ Observations of reaction: ________________________________________________________________________ Trial 2 Initial Time _____________ Final Time _____________ Current Average current (A) = ______________________ Total elapsed time (s) = _____________________ E21A-5 Part B. Moles of H2 formed. Trial 1 Trial 2 Volume of H2 (g) collected _________________ _________________ Temperature of the solution _________________ _________________ Barometric pressure _________________ _________________ Vapor pressure of H2O (g) at ______ °C _________________ _________________ Pressure of Dry H2 (g) _________________ _________________ Part C. Moles of I2 formed. Initial buret reading _________________ _________________ Final buret reading _________________ _________________ Volume of 0.0200 M Na2S2O3 used _________________ _________________ E21A-6 DATA TREATMENT Experiment 21A Show your work for all calculations. Include the proper number of significant figures and units in your final answers. Try to complete these calculations before you leave lab. Part A. Electrolysis and Quantity of Electrical Charge Used. (A.1) Using the average current and the total elapsed time for the electrolysis, calculate the total electrical charge (i.e., number of coulombs) that passed through the cell during the electrolysis. Trial 1: ___________________________ Trial 2: ___________________________ Part B. Moles of H2 formed. (B.1) From the volume of H2 gas collected and the pressure of the dry H2, use the Ideal Gas Law to calculate the number of moles of hydrogen gas formed during the electrolysis. Trial 1: ___________________________ Trial 2: ___________________________ Part C. Moles of I2 formed. (C.1) From your Na2S2O3 titration data and the reaction stoichiometry, calculate the number of moles of iodine formed during the electrolysis. titration reaction: I2 (aq) + 2 Na2S2O3 (aq) → 2 NaI (aq) + Na2S4O6 (aq) ↑ ↑ electrolysis titrant product Trial 1: ___________________________ Trial 2: ___________________________ E21A-7 QUESTIONS Experiment 21A 1. Explain why the moles of electrons calculated in (D.1) and (D.2) should be approximately equal. 2. The moles of electrons that passed through the cell during the electrolysis were calculated from two different measurements in (D.1) and (D.2). Which method of measurement might have more sources of experimental error? Explain. (In your explanation, consider the types of glassware and the techniques involved.) 3. How would the calculated value of the Faraday be affected (larger or smaller) if the pressure of the hydrogen gas were not corrected for the presence of the water vapor? Support your answer (mathematically or otherwise). E21A-10 Name ___________________________________________________ Section _________ Date ___________ Pre-Lab Exercise Experiment 21A 1. In this experiment, you will be studying the electrolysis reaction occurring in an aqueous solution of potassium iodide: 2 I− (aq) + 2 H2O (l) → I2 (aq) + H2 (g) + 2 OH− (aq) a. Using Table 18.1 in the textbook, determine the standard cell potential for this reaction. b. Is this reaction spontaneous as written (under standard conditions)? YES NO c. If the electrolysis process was carried out for 10 minutes at 120 mA, what was the total charge (in C) that passed through the cell? 2. Hydrogen gas is collected during the electrolysis of aqueous potassium iodide. If 8.00 mL of gas was collected over water at standard temperature and pressure, how many moles of hydrogen gas were collected? The vapor pressure of water at this temperature is 23.8 mm Hg. Assume ideal gas behavior. (For a review of collecting a gas over water, see page 200 of your textbook.) E21A-11