practical 8

• describe a method for setting up a standard cell.

  1. clean piece of copper and zinc with emergy paper or fine grade sandpaper
  2. degrease the metals using cottonwool and propanone
  3. place copper in solution of 1 mol dm-3 copper sulfate
  4. place zinc in solution of 1 mol dm-3 zinc sulfate
  5. lightly plug end of the U tube with cotton wool and fill with 2 mol dm-3 sodium chloride and plug the other end with cotton wool
  6. join the two beakers, with each end of U tube in different solutions
  7. connect half cells with crocodile clips and wires connected to a voltmeter to read of potential difference
• why do the metals have to be cleaned with emergy paper?

  to remove the oxide layer and ensure they react in their appropriate redox reaction
• why do the metals have to be degreased?

  removes grease as this could prevent the cell working properly
• why is a salt bridge necessary?

  allows the movement of ions between electrodes to complete the circuit, this prevents the build-up of charge.

  solution used must be inert so cannot react with electrolytes or ions in solution but it must conduct electricity
• describe a method for measuring comparative electrode potentials of different metals

  1. clean metals with emergy paper
  2. connect the positive terminal of the voltmeter to the metal using clips and leads
  3. place a piece of filter paper, moistened in sodium chloride solution, on top of the metal
  4. connect the voltmeter to a second metal using clips and leads
  5. hold the metal against the filter paper and note the voltage
  6. repeat for different metals
• why does the metal need to be held against the filter paper to record the voltage?

  the metals must be in direct contact with the salt bridge for charge to flow