Electrolysis of Some Aqueous Solutions by gmj10717

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									Electrolysis of Some Aqueous Solutions
                         When you recharge batteries for your cell phone or music player
                         you pass an electric current into the battery. If the applied voltage
                         is sufficiently large, a redox reaction will occur. This redox
                         reaction is a non-spontaneous reaction and will only occur when a
                         certain minimum voltage is applied.

                         This principle is used to produce elements that are not found in
                         nature e.g. if an electric current is passed through molten sodium
                         chloride, sodium and chlorine are produced.

                                 2NaCl(l)      2Na(l) + Cl2(g)

                         This is a very simple reaction because the only chemical entities
                         present in molten sodium chloride are sodium and chloride ions.
                         However, when a current is passed through aqueous solutions of
                         ionic compounds the entities present are the anions and cations but
                         also water. In addition, the electrodes are sometimes inert e.g.
                         carbon or platinum but at other times may themselves react. In this
                         lab we will examine the electrolysis of some aqueous solutions.

Problem                  To predict and then identify the products formed by the electrolysis
                         of the following aqueous solutions:

                         0.5 mol/L sodium sulphate solution with platinum electrodes
                         0.5 mol/L sodium chloride solution with platinum electrodes
                         0.5 mol/L copper II sulphate solution with carbon electrodes




Electrolysis of Some Aqueous Solutions      Page 1 of 11                                11/5/06
Prediction               For each of the following give the:

                         a)      anode reaction
                         b)      cathode reaction
                         c)      net cell reaction
                         d)      cell potential
                         e)      diagnostic tests that may be used to identify the products

                         Electrolysis of Na2SO4(aq) with platinum electrodes




                         Electrolysis of NaCl(aq) with platinum electrodes




Electrolysis of Some Aqueous Solutions    Page 2 of 11                                 11/5/06
                         Electrolysis of CuSO4(aq) with carbon electrodes




Design                   0.5 mol/L solutions of sodium sulphate and sodium chloride will each be
                         electrolyzed using inert platinum electrodes in a Hoffman cell. A 0.5
                         mol/L solution of copper II sulphate will be electrolyzed in a U-tube using
                         carbon electrodes. Prior to the reaction occurring, each solution will be
                         tested with red and blue litmus as a control. Each cell will operate for
                         approximately 10 minutes after which time the products of those reactions
                         will be tested using common diagnostic tests. A 6V battery will be used to
                         provide a direct current.

Materials                U-tube                                       carbon electrodes
                         red and blue litmus paper                    three 6V battery
                         two carbon electrodes                        nine connecting wires
                         multimeter                                   two Hoffman Cell
                         four utility clamps                          wooden splints
                         four test tubes                              test tube rack
                         250 mL 0.5 mol/L Na2SO4(aq)                  250 mL 0.5 mol/L NaCl(aq)
                         250 mL 0.5 mol/L CuSO4(aq)                   three clamp stands
                         four platinum electrodes                     bromothymol blue
                         four rubber stoppers

Procedure                Electrolysis of sodium sulphate solution

                         1. Insert platinum electrodes into the open ends of a Hoffman cell.

                         2. Clamp the cell vertically and open the valves

                         3. Pour 0.5mol/L sodium sulphate solution into the cell until the outer
                            arms are full. Close the valves.



Electrolysis of Some Aqueous Solutions    Page 3 of 11                                11/5/06
                         4. Set the multimeter to read direct current voltage and then use a red
                            connecting wire to connect the positive terminal of the battery to the
                            red, positive terminal of the multimeter.

                         5. Use a black connecting wire to connect the common or negative
                            terminal of the multimeter to the negative terminal of the battery.

                         6. Record the voltage of the battery.

                         7. Connect one terminal of the Hoffman cell to the negative terminal of
                            the battery.

                         8. Connect the other electrode to an open switch and then to the positive
                            terminal of the battery.

                         9. Record the connections on the cell diagram and also label the anode,
                            cathode and direction of electron flow.

                         10. As a control, test the cathode solution with blue and red litmus. Record
                             the observations.

                         11. Repeat the control tests at the anode and record the observations.

                         12. Close the switch and allow the cell to operate for approximately 10
                             minutes.

                         13. Record your observations.

                         14. Use a test tube to collect the gas produced at the cathode. Stopper the
                             test tube and place it in a test tube rack.

                         15. Remove the stopper and quickly test the gas by holding a burning
                             splint in the mouth of the test tube. Record your observations.

                         16. Test the solution at the cathode with blue and red litmus, recording
                             your observations.

                         17. As before, collect and stopper the gas at the anode.

                         18. Test this gas by placing a glowing splint in the test tube. Record your
                             observations.

                         19. Test the solution at the anode with blue and red litmus. Record your
                             observations.




Electrolysis of Some Aqueous Solutions    Page 4 of 11                                 11/5/06
                         20. Open the switch and then place a 250 mL baker under each electrode,
                             remove the electrode and empty the cell. Discard the solution in a
                             waste beaker.

                         21. Add a few drops of bromothymol blue to a fresh sample of sodium
                             sulphate solution and refill the cell as before. Remember to replace the
                             electrodes.

                         22. Close the switch and allow the cell to operate for 10 minutes, Record
                             your observations.

                         23. Empty the cell as before and discard the solution in a waste beaker.

                         Electrolysis of sodium chloride solution

                         24. Fill a clean Hoffman cell with sodium chloride solution.

                         25. Use the multimeter to measure the applied voltage. Record this
                             voltage.

                         26. Use the black connecting wire to connect the negative (-) terminal of
                             the battery to the electrode on the right.

                         27. Use the red connecting wire to connect the other electrode to an open
                             switch and then the positive (+) terminal of the battery.

                         28. Record the connections on the cell diagram and also label the anode,
                             cathode and direction of electron flow.

                         29. Close the switch and allow the cell to operate for approximately 10
                             minutes. Record your observations.

                         30. Collect the gas at the cathode and test it with a burning splint. Record
                             your observations.

                         31. Test the solution at the cathode with blue and red litmus. Record your
                             observations.

                         32. Test the anode solution with blue and red litmus. Record your
                            observations.

                         33. Disconnect the battery, empty the cell and discard the solution in a
                             waste beaker.




Electrolysis of Some Aqueous Solutions    Page 5 of 11                                  11/5/06
                         Electrolysis of copper II sulphate solution

                         34. Fill the U-tube with copper II sulphate solution to approximately 2 cm
                             to the top.

                         35. Clamp the U-tube vertically and clamp a copper electrode in each side
                             of the U-tube.

                         36. Use a red connecting wire to connect the positive terminal of the
                             battery to the electrode on the right side of the U-tube.

                         37. Use black connecting wires to connect the negative terminal of the
                             battery to an open switch and then to the electrode on the left side of
                             the U-tube.

                         38. Use the multimeter to measure and record the applied voltage. The red
                            connecting wire to the positive terminal of the battery, the black to the
                            negative.

                         39. Test each solution with blue and red litmus paper. Record this and
                             other observations of the cell.

                         40. Close the switch and allow the cell to operate for approximately 10
                             minutes. Record any observations.

                         41. After 10 minutes, examine the electrodes and record any observations.

                         42. Test the solution at each electrode with blue and red litmus. Record the
                             observations.

                         43. Disconnect the cell, return the electrodes and discard the solution in a
                             waste beaker.




Electrolysis of Some Aqueous Solutions    Page 6 of 11                                  11/5/06
Evidence                 Electrolysis of sodium sulphate solution.




                         Applied Voltage          =              V


                                                           Cathode   Anode



                            Control Tests




                            After 10 minutes




Electrolysis of Some Aqueous Solutions      Page 7 of 11              11/5/06
                         Electrolysis of sodium chloride solution.




                         Applied Voltage          =              V


                                                           Cathode   Anode



                            Control Tests




                            After 10 minutes




Electrolysis of Some Aqueous Solutions      Page 8 of 11              11/5/06
                         Electrolysis of copper II sulphate solution




                         Applied voltage          =              V

                                                           Cathode     Anode



                            Control Tests




                            After 10 minutes




Electrolysis of Some Aqueous Solutions      Page 9 of 11                11/5/06
Analysis                 1. Use the following table to compare your predictions with the results of
                            your diagnostic tests.


                                Predicted Products                      Actual Products
        Cell                  Anode           Cathode               Anode           Cathode


Sodium Sulphate




Sodium Chloride




Copper II Sulphate




                         2. Describe those cases where your observations did not agree with your
                            prediction or where you were unable to verify your prediction. In each
                            case, does this indicate a weakness in the experimental design or in the
                            method used to predict the products?




Electrolysis of Some Aqueous Solutions   Page 10 of 11                                11/5/06
Conclusion               Write anode, cathode and cell reactions for each electrolysis based on the
                         products identified by the diagnostic tests. Also, give the cell potential in
                         each case.




Evaluation               Were you able to completely identify the products of these reactions? If
                         not, how could the design be improved?




Electrolysis of Some Aqueous Solutions    Page 11 of 11                                  11/5/06

								
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