Electrolysis the faraday and avogadro's number

Student name: Cosumnes River College Due date: Tuesday, Dec 15, 2020, by 11:59 PM Prof. Hoang Chem401 Lab: Faraday’s Constant and Avogadro’s Number INTRODUCTION Zn/Cu Voltaic Cell: In a voltaic cell consisting of Zn and Cu electrodes as shown in Fig. 1, Zn is the stronger reducing agent than Cu and hence will be oxidized as it reduces Cu 2+ ion to metallic Cu. The Zn electrode is the negatively-labelled anode and the Cu electrode is the positively-labelled cathode. Electron current flows spontaneously through an external circuit from the Zn electrode to the Cu electrode. Figure 1 Zn/Cu Electrolytic Cell: When a battery is wired to the external circuit that connects the Zn and Cu electrodes, battery power can be applied to cause a current to flow in reverse from the Cu electrode to the Zn electrode as shown in Fig. 2. This is now an electrolytic cell. e‒ e‒ Zn2+ + 2 e‒→ Zn(s) Figure 2 Cu(s) → Cu2+ + 2 e‒ 1 In an electrolytic cell, the current flow is nonspontaneous and requires a continuous input of an external source of power to occur. Unlike a voltaic cell, the anode of an electrolytic cell is labelled as positive electrode and the cathode as negative electrode. Electrolysis has many applications, including plating of metals on surfaces and production of pure elements from mining ores or other compounds. As in a voltaic cell, oxidation occurs at the anode and reduction occurs at the cathode in an electrolytic cell. The amount of reaction that occurs at the electrodes is directly proportional to the number of electrons transferred. Michael Faraday (1791-1867) determined that the mass of a substance produced or consumed at the electrodes during electrolysis is proportional to the quantity of charge [Charge (C) = Current (A) x Time (sec)] that has passed through the circuit. A faraday is defined as the total charge (in Coulombs) on 1 mole of electrons and is equal to 96,485 C. Hence, the charge on a single electron is 1.602 x 10−19 C. In this electrolysis experiment, you will determine (1) the experimental value of the Faraday’s Constant by measuring the amount of charge required to consume a known mass of copper on an electrode. From the mass of copper consumed (or electrolyzed), you will also be able to determine (2) the experimental value of the Avogadro’s number (NA). You will use a copper strip and a zinc strip as the electrodes. These are placed in a beaker of dilute sulfuric acid (H2SO4) as shown in Fig 3. The strong acid dissociates into ions in solution and thus allows a current to be conducted between the electrodes. A Direct-Current (DC) Power Supply is used as a source of power to drive the electrolytic cell. Using the DC power supply, an electric current of specific amperage can be set. DC power supply e‒ anode cathode Figure 3 At the anode, the following oxidation reaction takes place: Cu(s) → Cu2+(aq) + 2 e‒ By determining the average current used in the reaction, along with the knowledge that all copper ions formed are the 2+ cations, you will calculate the number of atoms in one mole of copper (i.e. experimental Avogadro’s number, NA) and compare this value with the accepted Avogadro’s number (NA = 6.022 x 1023). PROCEDURE A description of the procedure and sample calculations are given below to help you understand how the data are generated and used to achieve the objectives of this lab. 1. Two dry metal strips, one is made of copper and the other of zinc, are cleaned by scrubbing their surface with steel wool to remove any surface oxidation. The Cu dry metal strip is placed in an analytical balance and its mass recorded. 2 2. The metal strips are connected to a DC Power Supply via wires with alligator clips as shown in Fig. 4. The Cu metal strip acts as the anode and is connected to the Power Supply at the positive (+) terminal. The Zn metal strip acts as the cathode and is connected to the Power Supply at the negative (‒) terminal.