Tuesday, 20 January 2015

6.2.1 - Laboratory Activity : Electrolysis of molten compounds

Laboratory Activity 6.2.1:
Electrolysis of molten compounds
Aim: To investigate the electrolysis of molten lead(II) bromide
» Carbon rod
» Lead (II) bromide powder

» Crucible
» Battery
» Connective wires
» Bunsen burner
» Clamps
» Clay triangle
» Ammeter

1. The apparatus is set us as shown in the following diagram.
2. The crucible is filled until half full with lead(II) bromide powder.
3. The crucible is placed on top of a tripod stand and heated strongly until the lead(II) bromide powder melts.
4. The rheostat is adjusted and electric current is passed through the molten salt for 10 minutes.
5. All observation are recorded.
6. The current is switched off and the liquid is poured out from the crucible carefully. Observations are recorded.

Anode (+) Cathode (-)
Brown vapour released Grey solid formed

This video shows the electrolysis of molten lead(II) bromide.

Lead (II) bromide comprises of Pb2+ and Br- ions which are held together by strong ionic bonds.

When lead(II) bromide melts, Pb2+ and Br- ions are released from their bonds and become freely moving particles as shown in the following equation:
PbBr2(s) → Pb2+(l) + 2Br-(l)

During electrolysis, Pb2+ ions are attracted to the cathode while Br- ions the move the anode.

At the anode, Br- ions lose their electrons:
2Br-(l) → Br2(g) + 2e-
A brown gas, bromine is formed.

At the cathode, Pb2+ ions receive electrons to form lead, a shinning grey metal.
Pb2+(l) + 2e- → Pb(s)

The overall equation is:
Pb2+(l) + 2Br-(l) → Pb(s) + Br2(g)

When lead(II) bromide is electrolysed, it decomposes to produce a grey metal, lead and a brown gas, bromine.

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