Chemistry Question?

Question

I need some help on the following question (I have searched in many books and over the internet to no avail):

During electrolysis of molten aluminium chloride, the carbon anodes are burned away. Explain why this should happen and write balanced chemical equations for the reactions that take place.

10 Points for the Best Answer.

Answer 1

By electrolysis method Aluminium is made by Hall's Method. Hall's first electrolytic cells, or "pots," were of cast iron, 24 inches (~61 cm) long, 16 inches (~41 cm) wide and 20 inches (~51 cm) deep, with a 3-inch (~8-cm) baked carbon lining. Six to ten carbon anodes, 3 inches (~8 cm) in diameter and 15 inches (~38 cm) long when new, were suspended in the electrolyte (bath) from a copper busbar above. The pots held 300 to 400 lbs (~136 to ~181 kg). of cryolite bath in which alumina was dissolved. The pots were arranged so that they could be heated from below by a gas flame, but it was found that they were self sustaining by the electrical heat generated. It is interesting that present-day cells are qualitatively of the same design, although considerably larger and under more precise quality control . The overall cell reaction is:
alumina + carbon = aluminum + carbon dioxide
Aluminum reduction cells today are of two types: those with prebaked anodes and those with baked-in-place anodes (Soderberg). Both types of anodes are made of baked petroleum coke and coal-tar or petroleum pitch. Prebaked anodes are baked in brick-lined pits and the hydrocarbon off gases can be captured and burned. Soderberg anodes are baked by the heat generated in the cells, and the off gases are more difficult to collect. Environmental restrictions in more advanced countries have halted construction of Soderberg cell plants.
The components of a prebake anode cell are described as follows. A carbon lining contains the bath and a pool of molten aluminum on the bottom. Carbon anodes suspended from an anode bus conduct current into the cell. Anode rods carry the current from the anode bus through steel stubs cast with iron into holes in the tops of the anodes. Current is conducted out of the cell through steel collector bars to the cathode bus and on to the next cell. The aluminum metal pad is the cathode where aluminum is deposited from the bath. Oxygen from the alumina dissolved in the bath combines with the bottom surface of the carbon anode to form carbon dioxide. The anodes are consumed in the process and replacements are added at individual locations on a regular schedule. The anode butts are sent back to the anode plant to be ground and mixed into new anode paste to be pressed and baked. Aluminum is siphoned out of the cells on a daily basis into vacuum crucibles and sent to the cast house. The carbon lining is contained in a steel shell with a thermal insulation of alumina or insulating brick.
In operation, cryolite freezes on the sidewalls of the cells forming a "ledge" which protects the sidelining from severe attack by aluminum and molten cryolite. Cryolite also freezes over the top of the bath and forms a "crust" to support a top layer of alumina thermal insulation. Alumina is fed to the bath through holes punched in the crust. The carbon dioxide exits through holes in the crust and is collected under the hoods. The carbon dioxide and air leaking in is now ducted to dry scrubbers which remove fluorides from the gas stream. Fresh alumina contacting the gases removes the hydrogen fluoride and evaporated fluoride particulate. This alumina, fed to the cells, returns fluoride to the cells. The hydrogen fluoride comes from residual hydrocarbons in the anodes and trace water in the alumina and air humidity reacting with the fluoride bath.
Control of alumina concentration in the cells is accomplished by a slight underfeeding. When the alumina reaches a critical level the cell goes on anode effect, caused by a limiting rate of diffusion of alumina to the anode surfaces. The cell voltage then rises and some fluorocarbons are generated. A light bulb connected across the cell lights up with increased cell voltage as a signal for the operators to feed the cell with alumina and kill the anode effect. Cells now run a day or longer between anode effects. The ratio of sodium fluoride to aluminum fluoride in the cryolite bath changes over time and corrective additions are added based on laboratory analyses.

Answer 2

During electroysis of molten aluminium chloride you obtain aluminium at the Cathode and chlorine at the anode. At the high temperature involved the aluminium and carbon at rthe cathode will react together to form Aluminium carbide whilst at the anode the carbon will react to form carbon tetrachloride.

4Al + 3C ---------> Al4C3

C + 2Cl2 --------> CCl4

Other possibilities is that as the eletrolysis is carried out in an amosphere of oxygen that carbon dioxide or monoxide is formed

C + O2 -------> CO2

2C + O2 ---------> 2CO

Hope that helps.

Answer 3

Carbon Anodes