The Chemistry of Aluminum

Physical Properties

Atomic Number: 13
Atomic Weight: 25.98
Electron Configuration: 1s22s22p63s23p1
Crystal Structure: Face-Centered Cubic
Density: 2.70 g/cm3
Melting Point: 650.4oC
First Ionization Energy: 577 kJ/mol
Second Ionization Energy: 1816 kJ/mol
Third Ionization Eneergy: 2744 kJ/mol
Ionic Radius: 54 pm
Standard Reduction Potential: -1.66 V (Al3+ + 3e- = Al)

Properties of Elemental Aluminum

Aluminum is a shiny, hard metal. It is the most abundant metallic element in the Earth's crust; however it is very reactive and is never found in elemental form. Aluminum compounds are widely distributed; a few minerals include beryl, cryolite, garnets, and spinels.

Aluminum is widely used in modern society. Just a few uses include the construction industry (door and window frames, siding), the manufacture of aircraft, automobile parts, and in food packaging (aluminum foil, aluminum cans). Larger electrical cables are made almost entirely out of aluminum; despite the fact that it is not as good a conductor as copper this is offset by the lighter weight.

Occurrence and Isolation

Aluminum is obtained from bauxite. Bauxite is essentially what is left of aluminum-contianing clay minerals ather the more soluble components have been leached away by weathering. Before it can be used bauxite ore must first be purified, a process known as the Bayer process. The bauxite is first treated with hydroxide; the aluminum forms a soluble complex ion, allowing it to be separated from the other compoents of the ore. This is acidified slightly, producing hydrated aluminum oxide, which is heated to drive off the water of hydration. Next, the aluminum metal is obained electrolysis, a process known as the Hall process. The aluminum could be obtained by eelctrolysis of molten aluminum oxide. However, the melting point of aluminum oxide is prohibitively high (over 2000 degrees Celsius). Instead, it is dissolved in cryolite, Na3AlF3, the mixture having a much lower melting point than pure aluminum oxide. The cells are operated usign carbon (graphite) electrods, producing aluminum metal at the cathode and carbon dioxide at the anode (from oxidation of the electrode material). Since aluminum is a +3 ion, three moles of electrons must be passed for evert mole of alumimun produced. The producion of aluminum is a major conusmer of electricity.

Aluminum Oxide

The only oxide of aluminum is alumina, Al2O3. The crystalline form of aluminum oxide is known as corundum. Structually, it is a hexagonal close-packed array of oxide ions with the aluminum ions in two-thirds of the octahedral holes. It is a very a hard material, and and can scratch most other minerals. It is widely used as an abrasive; sandpaper often contains aluminum oxide.

Several gemstones are based on corundum; examples are ruby and sapphire. Corundum itself is colorless; the colors of these gemstones are due to the traces of transition metals. The red color of rubies is to to the presence of chromium(III) in the lattice. The blue color of sapphire is due to the presence of iron and titanium; one Fe2+ and one Ti+4 can replace two Al3+ ions, therefore maining charge balance. Although sapphires are most known for their blue color, they can also be several other colors, including green, yellow, and pink.

Reactions of Aluminum

Aluminum is actually a very reactive metal. However, it quickly devlops a thin oxide coating that protects the surface of the metal and prevents further reaction. The coating is thin enough as not to dull the surface of the metal and is therefore not noticeable. In fact, anodized aluminum is simply aluminum to which has a very thick oxide coating has been electrolytically applied.

Nevertheless aluminum does undergo a number of reactions. Like most of the reactive metals, aluminum will react with hydrochloric acid to yield hydrogen gas.

2Al + 6HCl = 2Al3+ + 6Cl- + 3H2

Aluminum reacts with hydroxide solutions to liberate hydrogen gas. At first this reaction might seem strange. Several other metals react wtih water to produce hydrogen gas, but not with hydroxide. In this case the hydroxide reacts with the aluminum oxide layer as shown below.

Al2O3 + 2OH- +3H2O = 2[Al(OH)4]-

Next, the aluminum reacts to displace hydrogen from water like the other reactive metals; this is the reaction that liberates hydrogen gas.

2Al + 6H2O = 2Al3+ + 3H2 + 6OH-

This reaction could be written several different ways. Aluminum hydroxide is insoluble, so this might be written Al(OH)3. Also, in an excess of hydroxide, this could react with additional hydroxide to form [Al(OH)4]-.

Many commercial drain cleaners consist of bits of aluminum mixed with lye. The strongly basic solution helps to dissolve grease, and the bubbling produced by the hydrogen gas helps to break up the clog.

Aluminum also reacts with the halogens. For example, aluminum undergoes a spectacular reaction wtih elemental bromine. This reaction is quite spectacular, producing fire, smoke, and sometimes throwing up bits of molten aluinun.

External Links

An interesting article about aluminum
Figure 1.
Figure 2. Aluminum electrical cable
Figure 3. Blue sapphires
Photo courtesy of R. Weller, Cochise College
Figure 4. The reaction between aluminum and bromine