EXTRACTION OFALUMINIUM

Aluminium, the commonest metal in the Earth's crust (8.3%), is usually found as the oxide (e.g., in the ore bauxite). Typical of main group metals, aluminium has a fairly low melting point (660°C), a low density (2.7 g cm-3), and forms (usually) white or colourless compounds in only one oxidation state (III). [.. K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > (H) > Cu > Hg > Ag ..]

1. Aluminium is extracted by the electrolytic reduction of purified molten bauxite; the ionic equations for the reactions occurring at the electrodes are:

(a) Label this diagram of an industrial electrolytic cell with: Graphite anode; Graphite cathode; Insulator; Molten aluminium; Molten electrolyte; and, Solid crust of electrolyte.

(b) To lower the melting point of aluminium oxide from about 1500°C to 1000°C, and so reduce energy costs, cryolite (Na3AlF6) is added to the bauxite.

Explain why (pure) aluminium oxide has a high melting point.

_______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ [3] (c)

Name one toxic gas that might arise from the thermal decomposition of cryolite.

__________________________________________________________ [1]

(d) The anode needs to be replaced periodically because the carbon is completely oxidized by the evolving dioxygen.

Name the toxic pollutant formed by partial oxidation of the carbon anode.

______________________ [1]

[Q = n × z × F and Q = I × t, where: Q, measured in coulombs (C), is the quantity of electricity; n is the number of moles of substance evolved at the electrode; z is the charge on the ion; F is a constant, with a value of 96500 C mol-¹; I, measured in amps (A), is the current; and t, measured in seconds (s), is the time.]

2. In industry, a typical electrolytic cell operates continuously at a current of 4000 kA.

Determine the mass (m) of aluminium that forms at the cathode every day - as follows.

Calculate the quantity of electricity (Q) used every 24 hours.

_______________________________________________________________________

Calculate the number of moles (n) of aluminium formed at the cathode. _______________________________________________________________________

And finally, calculate the mass (m) of aluminium formed at the cathode. _______________________________________________________________________ [6]

--------------------------------------------------------------------------------

3. Electrolytic reduction, which is often used for the extraction of the most reactive metals, requires expensive electrical energy: by contrast, chemical reduction with carbon uses a relatively cheap source of chemical energy.

Which method of generating electrical power is the cheapest and most environmentally acceptable?

_________________________ [1]

297 kJ of energy are required to produce 1 mole of aluminium metal by electrolysis, whereas only 26 kJ of energy are required to recycle 1 mole.

Calculate the percentage energy saved by recycling aluminium.

_______________________________________________________________________ [2]

4. Aluminium's high natural abundance, high resistance to corrosion, high thermal conductivity, low density, and low electrical resistance, has resulted in its widespread use (e.g., in window frames, cooking utensils, food and drink cans, aircraft, and electrical power lines). The metal surface is invariably covered by a very thin layer of oxide, because pure aluminium is rapidly oxidized by atmospheric oxygen; This layer is non-porous and protects the metal from further corrosion, providing it is not exposed to aqueous chloride ions.

(a) Paralleling aluminium's increased use in cooking utensils and in food containers, there has been an increase in the number of people who suffer from Alzheimer's disease. This parallel may indeed be a most unfortunate coincidence, rather than a correlation, but one partial explanation could be as follows. Food is often preserved in brine and cooked with salt; so, should the protective layer of aluminium oxide in the containers be removed by aqueous chloride ions, soluble aluminium ions could be formed, ingested, and then absorbed into the bloodstream.

Construct an explanation, complete with two symbol equations, for the assertion that it could be hazardous to use aluminium foil when cooking salted meat in the presence of steam.

_______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________

[4] (b) When the thin layer of oxide is made thicker, by anodizing, then it can easily absorb variously coloured dyes. Shown below is a diagram of a typical electrolytic cell used to anodize aluminium objects. Oxygen gas is evolved at the anode from the electrolysis of an aqueous solution containing H1+(aq), OH1-(aq), and SO42-(aq) ions; the ionic equations for the reactions occurring at the electrodes are: The aluminium object immediately combines with this evolved oxygen gas to form more aluminium oxide on its surface. Suggest two reasons why aluminium is anodized.

________________________________________________

_________________________________________________ [2]