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REDOX REACTIONS AND ELECTROCHEMISTRY INTRODUCTION: In a copper-zinc electrochemical cell, two separated half-cell reactions take place. Zinc atoms undergo oxidation to Zn 2+ ions: Zn (s) Zn 2+ + 2 e-

The electrons that are lost by the zinc atoms travel through the wire. These electrons are then available to reduce the copper ions to solid copper metal: Cu 2+ + 2 eCu (s)

In order for the reaction to continue for more than a few seconds, a salt bridge must be used. Without this aqueous connection between the two half-cells, the flow of electrons will stop. Positive charge builds up as the Zn 2+ ions go into solution and negative charge accumulates as the positive metal ions (Cu 2+ ) plate out on the electrode. A salt bridge provides a way for ions to move between the half-cells and ensure electrical neutrality. MATERIALS: balance 24- well microplate scoop toothpick cotton swab filter paper strips forceps voltmeter REAGENTS: copper strips magnesium ribbon strips zinc strips lead strips graphite electrodes 0.1 M CuSO4 0.1 M MgSO4 0.1 M ZnSO4 0.1M PbSO4 0.1 M KNO3 NH4Cl MnO2

PART I: DETERMINING THE VOLTAGE OF ELECTROCHEMICAL CELLS 1. 2. Obtain strips of Mg, Cu, Zn, and Pb. Obtain a 24-well microplate. Using 20 drops of solution per well (enough to cover the bottom of the well ), set up 6 pairs of wells containing : Cu 2+ --- Zn 2+ Cu 2+ --- Mg 2+ Cu 2+ --- Pb 2+ Mg 2+ --- Zn 2+ Mg 2+ --- Pb 2+ Zn 2+ --- Pb 2+ 3. 4. Place a piece of the metal in its own ionic solution. For example, in the first set, place Cu metal into the Cu 2+ solution and a piece of zinc into the zinc solution. Remove a strip of filter paper from the beaker of 1 M KNO3 solution in which it is soaking. Drape the strip over the wall that separates each pair of ionic solutions so that each end of the filter paper extends into the wells of the solutions.

5. 6.

Attach the voltmeter to the metal strips and read the voltage. If the meter does not read a positive voltage, reverse the hookup. Record the voltage of the electrochemical cell in your notebook. Keeping the voltmeter attached, slide a toothpick under the filter paper strip and lift it out of the solutions. Observe what happens to the voltage. Repeat the steps 1-4 for the other electrochemical cells. Discard the paper strips and toothpicks in the wastebasket. Remove the metal strips and rinse them and dry them and return them to the designated area. Rinse the microplate three times.


PART II: DETERMINING THE VOLTAGE OF A DRY CELL INTRODUCTION The dry cell consists of a zinc electrode and a graphite electrode in contact with a paste of ammonium chloride ( NH4Cl ) and manganese dioxide ( MnO2 ). The half-cell reactions associated with this dry cell are: Zn (s) Zn 2+ + 2 e(oxidation reaction) 2 Mn 3+ (aq) + 8 NH3 (aq) + 4 H2O (reduction reaction)

2 e- + 2 MnO2 (s) + 8 NH4 + (aq)

Electrons are liberated into the paste mixture by the oxidation of the metallic zinc electrode. Although the graphite rod serves as the positive electrode, the reduction reaction occurs within the paste. Electrons are then transferred though the inert graphite rod into the external circuit. Anytime there is not a solid metal involved in the redox reaction to act as the electrode, and inert electrode is used, such as graphite or platinum. 1. 2. 3. 4. 5. Using a piece of weighing paper, mass 0.5 grams of ammonium chloride, and 0.5 grams of manganese dioxide. Combine the two solids in one of the wells of the microplate. While mixing the components with a toothpick, add 4-6 drops of water to form a thick paste. Place a zinc strip and a graphite rod into the paste. Connect the voltmeter to the zinc and graphite electrodes, switching them if necessary to get a positive reading. Measure the voltage of the cell and record it on the report sheet. Remove the electrodes, wipe them with paper towels, rinse and dry them, and return them to the designated area. Use a metal scoop and remove as much of the paste as possible from the well. Wipe out the rest of the plate with a cotton swab under running water. Rinse the microplate and return it to the designated area. Discard the cotton swab and toothpicks in the wastebasket. Before leaving the laboratory, wash your hands thoroughly.



NAMES ___________________________

1. In each of the reactions, identify the substance that was oxidized, the substance that was reduced, the oxidizing agent, and the reducing agent, and the volage produced by the electrochemical cell. Zn/Cu Oxidized Mg/Cu Pb/Cu Mg/Zn Mg/Pb Zn/ Pb


Oxidizing agent

Reducing agent Experimental Voltage 2. Zn/Cu: Determine the voltage for each cell from the table of reduction potentials. Compare the theoretical voltage to the experimental voltage and calculate your percent error for each electrochemical cell.





Zn/ Pb:


Consider the Mg/Cu electrochemical cell. Draw a picture indicating the anode and the cathode, the path of the electrons and the path of the cations and anions. Indicate the oxidation reaction and the reduction reaction as well as the oxidizing and reducing agents. Then draw a line notation diagram.


What purpose does the filter paper soaked in KNO3 serve? Explain why removing the filter paper strip has the effect it does.


Of the metal ions used, which is the best oxidizing agent? Explain


Which of the metal ions is the least effective oxidizing agent? Explain


List the four metals in order of increasing ease of oxidation. Explain


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