Splitting melts with electricity

Science · Matter & Materials · ages 15-16

Name ______________________ Date ____________

Molten lead bromide is electrolysed using inert electrodes. What forms at the cathode?
- Bromine gas
- Lead metal
- Oxygen gas
Why must lead bromide be melted before electrolysis?
- Melting adds the missing electrons
- Melting stops the electrodes reacting
- Melting frees the ions to migrate
During electrolysis, negative ions move to the positive anode.
Circle one: True False
Which half equation shows bromine forming at the anode during electrolysis of molten lead bromide?
- Br2 + 2e- -> 2Br-
- Pb2+ + 2e- -> Pb
- 2Br- -> Br2 + 2e-
Which half equation shows lead forming at the cathode during electrolysis of molten lead bromide?
- 2Br- -> Br2 + 2e-
- Pb2+ + 2e- -> Pb
- Pb -> Pb2+ + 2e-
The halves Pb2+ + 2e- -> Pb and 2Br- -> Br2 + 2e- combine. What is the overall reaction?
- 2Pb + Br2 -> 2PbBr
- Pb + Br2 -> PbBr2
- PbBr2 -> Pb + Br2
Molten sodium chloride contains Na+ and Cl- ions. It is electrolysed using inert electrodes. What forms at each electrode?
- Chlorine at the cathode, sodium at the anode
- Sodium at the cathode, chlorine at the anode
- Hydrogen at the cathode, oxygen at the anode
A student writes the cathode half as Pb2+ -> Pb + 2e-. Which change gives the correct cathode reaction?
- Move the electrons left because lead ions gain them.
- Move the lead atoms left because oxidation occurs there.
- Change Pb2+ to Pb+ because electrons add positive charge.

Answer key

For grown-ups. Fold this page away before handing over the rest.

Lead metal · Pb2+ ions reach the negative cathode and gain electrons, forming lead metal.
Melting frees the ions to migrate · The ions must be able to move through the material to carry charge. Melting frees them from fixed positions in the lattice.
True · Negative ions are attracted to the positive electrode, which is the anode in electrolysis.
2Br- -> Br2 + 2e- · Two bromide ions lose two electrons to form Br2. Both sides have two bromine atoms and total charge -2.
Pb2+ + 2e- -> Pb · Reduction uses electrons. Pb2+ gains two electrons, giving a neutral lead atom and charge 0 on each side.
PbBr2 -> Pb + Br2 · Adding the halves cancels two electrons. Pb2+ and two Br- ions represent the starting PbBr2, giving PbBr2 -> Pb + Br2.
Sodium at the cathode, chlorine at the anode · Na+ gains electrons at the cathode to form sodium. Cl- loses electrons at the anode to form chlorine. There is no water in this melt.
Move the electrons left because lead ions gain them. · The cathode reaction is reduction. Putting two electrons on the left gives Pb2+ + 2e- -> Pb, with charge 0 on each side.