Aluminum
- Aluminum forms many minerals. Bauxite is an important aluminum mineral.
- Aluminum is purified through the Bayer process and the Hall-Héraoult process.
- Gallium, in gallium arsenide, is an important light-emitting semiconductor.
note 1
- Aluminum is the most abundant metal found in the
lithosphere. Only oxygen (a nonmetal) and silicon (a
metalloid) are more common. - Aluminum forms many minerals, several of which include other metals.
- Bauxite is composed of aluminum oxides and other oxides (such as ferric oxide and silicon dioxide). Bauxite is an important source of aluminum.
- The Bayer process is used to purify aluminum from bauxite. In the first step of the Bayer process, bauxite is heated with hydroxide (OH – ). The iron precipitates out, and the silicon dioxide and aluminum enter solution.
- In the second step, the solution is cooled. The aluminum hydroxide precipitates out, leaving the silicon hydroxides in solution.
- Heating aluminum hydroxide yields alumina (Al 2 O 3 ).
Alumina is used to make aluminum metal. - Because of its large heat of formation ( H f ), aluminum
metal requires a huge amount of energy to produce. - Before 1886, aluminum chloride was reduced with a
potassium amalgam: - AlCl 3 (s) + 3K(Hg) x (l) -> 3KCl(s) + Al(Hg) x (l)
- This reaction required a great deal of heat, but allowed the aluminum to be separated from the mercury by distillation.
- In 1886, Hall and Héraoult discovered that alumina dissolves in cryolite (Na 3 AlF 6 ) at 950°C. This allows electrolysis of alumina to be carried out at a much lower temperature than the melting point of alumina (950°C versus 2050°C).
note 2
- Aluminum has many household and industrial uses, such as aluminum cans, drain cleaners, catalytic converter supports, and aluminum foil.
- Even though aluminum is an excellent reducing agent,
aluminum cans and foil do not oxidize to a great extent.
Aluminum forms a thin coating of Al 2 O 3 , which binds to the aluminum and protects it from further oxidation through a process called passivation. This protective layer is not like rust (ferric oxide), which flakes off iron after forming. - Gallium arsenide (GaAs) is a semiconductor that gives off light.
- Gallium arsenide is commonly found in light-emitting diodes (LEDs).
- Aluminum is used to dope gallium arsenide diodes, giving different colors.
Which of the following substances does not contain aluminum?
Apatite
Which of the following describes the role of cryolite in the Hall-Héroult process?
Cryolite serves as the electrolyte and solvent in the reaction.
Which formula for an aluminum compound is incorrectly matched with its application?
AlCl3 / flocculating agent
Although aluminum is a strong reducing agent, aluminum cans do not “rust”. Why?
Aluminum oxide creates a protective coating.
Arrange the steps of the Bayer process of alumina production (from bauxite) in the correct order:
- Al(OH)4- -> Al(OH)3 + OH-
- Al(OH)3 + OH- -> Al(OH)4-; Al(O)OH + OH- + H2O -> Al(OH)4-
- Al(OH)3 (heated) -> Al2O3 + H2O
- AlCl3 + 3K(Hg)x(l) -> 3KCl(s) + Al(Hg)x(l)
2, 1, 3
Which of the following is the most abundant metal in the lithosphere?
Aluminum
All of the following compounds are amphoteric except:
CaO
Which of the following aluminum ores contains aluminum and no other metals?
Corundum
Which of the following statements regarding the Group 13 elements is false?
They form only 3+ cations
Which would not have been a use of aluminum prior to 1886?
Cookware such as pots and pans