Everything Science Grade 12

4.3 CHAPTER 4. ELECTROCHEMICAL REACTIONS

Activity: The movement of coloured ions

A piece of filter paperis soaked in an ammonia-ammonium chlorideso-

lution and placed on amicroscope slide. Thefilter paper is then connected

to a supply of electriccurrent using crocodileclips and connecting wire as

shown in the diagram below. A line of copperchromate solution is placed in

the centre of the filter paper. The colour of this solution is initially green-brown.

copper chromate (greenbrown)

start of reaction

negative ions positive ions

after 20 minutes

The current is then switched on and allowed torun for about 20 minutes.

After this time, the central coloured band disappears and is replacedby two

bands, one yellow and the other blue, which seem to have separated out from

the first band of copperchromate.

Results:

• The cell that is used tosupply an electric current sets up a potential dif-
  ference across the circuit, so that one of the electrodes is positive and the
  other is negative.


• The chromate (CrO^24 −) ions in the copper chromate solution are attracted
  to the positive electrode, while the Cu2+ions are attracted to thenegative
  electrode.

Conclusion:

The movement of ions occurs because the electric current in the outer cir-

cuit sets up a potential difference between the two electrodes.

Similar principles applyin the electrolytic cell, where substances that aremade of ions
can be broken down into simpler substances through electrolysis.