Required Practical 8

• how do you set up an electrochemical cell?

  1. Clean a piece of copper and a piece of zinc using emery paper or fine grade sandpaper.

  2. Degrease the metal pieces using some cotton wool and propanone. Grease could prevent the cell working as efficiently.

  3. Place the copper into a 100 cm3 beaker with about 50 cm3 of 1.0 mol dm-3 CuSO4 solution. This is a half-cell. 4. Place the zinc into a 100 cm3 beaker with about 50 cm3 of 1.0 mol dm-3 ZnSO4 solution. This is a half-cell. 5. Lightly plug one end of a plastic U-tube with cotton wool and fill the tube with the solution of 2.0 mol dm-3 sodium chloride solution (NaCl), then plug other end.

  6. Join the two beakers with U-tube so that the plugged ends are in the separate beakers.

  7. Connect the Cu(s) |Cu2+ (aq) and Zn(s) |Zn2+ (aq) half-cells by connecting the metals using crocodile clips and leads. Include a voltmeter in the circuit in order to read off the voltage.
• why are salt bridges used?

  ● Allow the movement of ions between electrodes and to complete the circuit.

  ● Must not react (inert) with the electrolyte or ions in solution and must conduct electricity.

  ● Platinum is often used.

  ● Alternatively, a strip of filter paper saturated with NaCl solution can be used instead of a U-tube.
• How do you compare electrode potentials of different metals?

  1. Clean a piece of copper using emery paper or fine grade sandpaper.

  2. Connect to the positive terminal of a voltmeter using a crocodile clip and one of the leads.

  3. Cut a piece of filter paper to about the same area as the copper and moisten with sodium chloride solution. Place it on top of the metal.

  4. Connect a second lead to the voltmeter and use the crocodile clip to connect the lead to a piece of a different metal.

  5. Hold the metal against the filter paper. Record the voltage reading, including the sign. Cell potential can be positive or negative.

  6. Repeat with different metals and compare the electrode potentials.
• Why may a graphite or platinum electrode be used?

  They are very unreactive- i.e. will not react with the half cell solutions and will not affect the voltmeter readings. Usually used in ion/ion half cells.
• Why do you need to file/sand away the outer layer of the metal?

  It removes the oxide layer on the outside of the metal