Aluminium and its compounds

The metal that was once rarer than gold

Aluminium is the most abundant metal in the Earth's crust, and the third most abundant element after oxygen and silicon, at about 8 per cent by mass. Yet it was costly until the electrolytic method of Hall and Heroult, which dates from 1886. It never occurs free, because it holds on to oxygen so tightly. A cheap way to separate it came only when electricity was cheap.

The ores are bauxite, $\text{Al}_{2}\text{O}_{3}\cdot \text{xH}_{2}\text{O}$, which is the main ore, corundum, $\text{Al}_{2}\text{O}_{3}$, and cryolite, $\text{Na}_{3}\text{AlF}_{6}$.

From the ore to the metal

The extraction has two stages.

Stage 1. The Bayer process makes pure alumina. Bauxite contains iron(III) oxide and silica as well. It is digested in concentrated sodium hydroxide at 473 to 523 K and 35 to 36 bar. Alumina dissolves, because it is amphoteric, and iron oxide stays behind:

The solution is filtered. Carbon dioxide is passed in, or the solution is seeded with crystals of hydrated alumina, and hydrated alumina comes out: $2\text{Na}[\text{Al}(\text{OH})_{4}] + \text{CO}_{2} \to \text{Al}_{2}\text{O}_{3}\cdot \text{xH}_{2}\text{O} + 2\text{NaHCO}_{3}$. It is dried and heated strongly, and it gives pure $\text{Al}_{2}\text{O}_{3}$.

Stage 2. The Hall-Heroult process makes the metal. Alumina melts at about $2345\ \text{K}$, which is too high to be practical. It is dissolved in molten cryolite with some calcium fluoride. The mixture melts near $1230\ \text{K}$ and conducts electricity. Cryolite lowers the melting point and makes the melt a good conductor.

The cell is a steel tank lined with carbon, which is the cathode. Graphite rods dipped from above are the anode.

• Cathode: $\text{Al}^{3+} + 3\text{e}^{-} \to \text{Al}$. Molten aluminium collects at the bottom and is tapped off.
• Anode: $\text{C} + \text{O}^{2-} \to \text{CO} + 2\text{e}^{-}$ and $\text{C} + 2\text{O}^{2-} \to \text{CO}_{2} + 4\text{e}^{-}$. The graphite is used up and must be replaced.

Electrical cost. Aluminium needs three electrons per atom, so a good plant uses between $13\ \text{kWh}$ and $15\ \text{kWh}$ for each kilogram, mostly because of the large charge. The sum is made by Faraday's law: $m = \dfrac{M\,Q}{3F}$.

Change the current. The mass made in a day changes in the same proportion, but the energy for one kilogram stays the same. Now change the voltage. That is what moves the energy per kilogram.

Properties and reactions

Aluminium is a silvery, light metal (density $2.70\ \text{g/cm}^3$). It is a good conductor, and it is malleable and ductile.

• Oxide film. In air a layer of $\text{Al}_{2}\text{O}_{3}$ forms at once and protects the metal. Concentrated nitric acid makes the film thicker, and the metal becomes passive, which means that it no longer reacts.
• Acids. $2\text{Al} + 6\text{HCl} \to 2\text{AlCl}_{3} + 3\text{H}_{2}$. Hot concentrated sulphuric acid gives sulphur dioxide: $2\text{Al} + 6\text{H}_{2}\text{SO}_{4} \to \text{Al}_{2}(\text{SO}_{4})_{3} + 3\text{SO}_{2} + 6\text{H}_{2}\text{O}$.
• Alkalis. $2\text{Al} + 2\text{NaOH} + 6\text{H}_{2}\text{O} \to 2\text{Na}[\text{Al}(\text{OH})_{4}] + 3\text{H}_{2}$.
• Thermite. Aluminium has a great affinity for oxygen and takes it from iron(III) oxide: $2\text{Al} + \text{Fe}_{2}\text{O}_{3} \to \text{Al}_{2}\text{O}_{3} + 2\text{Fe}$. The heat melts the iron, and the reaction is used to weld rails.
• Alloys. Duralumin (Al with about 4 per cent Cu, and Mg and Mn) is as strong as mild steel and much lighter.

Compounds of aluminium

Aluminium oxide and hydroxide are amphoteric. They react with both acids and bases:

• $\text{Al}_{2}\text{O}_{3} + 6\text{HCl} \to 2\text{AlCl}_{3} + 3\text{H}_{2}\text{O}$ and $\text{Al}_{2}\text{O}_{3} + 2\text{NaOH} + 3\text{H}_{2}\text{O} \to 2\text{Na}[\text{Al}(\text{OH})_{4}]$.
• $\text{Al}(\text{OH})_{3} + 3\text{HCl} \to \text{AlCl}_{3} + 3\text{H}_{2}\text{O}$ and $\text{Al}(\text{OH})_{3} + \text{NaOH} \to \text{Na}[\text{Al}(\text{OH})_{4}]$.

Aluminium hydroxide forms as a white, jelly-like solid when ammonia or alkali is added to a solution of an aluminium salt: $\text{Al}^{3+} + 3\text{NH}_{3} + 3\text{H}_{2}\text{O} \to \text{Al}(\text{OH})_{3} + 3\text{NH}_{4}^{+}$. It dissolves in excess sodium hydroxide, but not in excess ammonia.

Aluminium chloride. The anhydrous salt is made by passing dry chlorine or dry hydrogen chloride over the hot metal: $2\text{Al} + 3\text{Cl}_{2} \to 2\text{AlCl}_{3}$. It is a covalent compound. It fumes in moist air, because it is hydrolysed: $\text{AlCl}_{3} + 3\text{H}_{2}\text{O} \to \text{Al}(\text{OH})_{3} + 3\text{HCl}$. In the vapour and in non-polar solvents it exists as a dimer, $\text{Al}_{2}\text{Cl}_{6}$. Each aluminium has four chlorine neighbours around it in a tetrahedron, and two chlorine atoms bridge the two aluminium atoms. Each bridge uses a lone pair of chlorine and an empty orbital of aluminium. These bridges are different from those in diborane: they have two electrons for two atoms. Aluminium chloride is a Lewis acid and is a catalyst in Friedel-Crafts reactions.

In water the aluminium ion exists as $[\text{Al}(\text{H}_{2}\text{O})_{6}]^{3+}$, which is acidic: $[\text{Al}(\text{H}_{2}\text{O})_{6}]^{3+} + \text{H}_{2}\text{O} \rightleftharpoons [\text{Al}(\text{H}_{2}\text{O})_{5}(\text{OH})]^{2+} + \text{H}_{3}\text{O}^{+}$. A solution of an aluminium salt turns blue litmus red.

Alum. Potash alum, $\text{K}_{2}\text{SO}_{4}\cdot \text{Al}_{2}(\text{SO}_{4})_{3}\cdot 24\text{H}_{2}\text{O}$, is written for short as $\text{KAl}(\text{SO}_{4})_{2}\cdot 12\text{H}_{2}\text{O}$. It is a double salt: it gives the ions of both salts in water. It is used to clear muddy water, because the aluminium hydroxide that forms carries the dirt down with it. The name alum is used for any salt with the formula $\text{M}(\text{I})\text{M}(\text{III})(\text{SO}_{4})_{2}\cdot 12\text{H}_{2}\text{O}$.

Lithium aluminium hydride. $4\text{LiH} + \text{AlCl}_{3} \to \text{LiAlH}_{4} + 3\text{LiCl}$ in ether. It is a strong reducing agent.

Practise with new numbers

Try it in Python

The program works out the aluminium made and the energy used in a cell, from the current, the voltage and the time.

Common mistakes

• Using one mole of electrons for each mole of aluminium. The ion is 3+.
• Saying that cryolite is the ore. The ore is bauxite. Cryolite is added to lower the melting point.
• Thinking that Al(OH)$_3$ dissolves in excess ammonia. It dissolves in excess alkali only.
• Drawing Al$_2$Cl$_6$ with three-centre bonds. The bridges have two electrons for two atoms.
• Forgetting that the anode is consumed. Graphite burns to carbon monoxide and carbon dioxide.

Remember these

• Ores: bauxite, cryolite and corundum. Bayer process first, Hall-Heroult electrolysis next.
• Cathode: $\text{Al}^{3+} + 3\text{e}^{-} \to \text{Al}$. The graphite anode is consumed.
• Aluminium, its oxide and its hydroxide dissolve in acids and in alkalis.
• Anhydrous $\text{AlCl}_{3}$ is covalent and a dimer, and a Lewis acid. In water the ion is $[\text{Al}(\text{H}_{2}\text{O})_{6}]^{3+}$, which is acidic.
• Potash alum is $\text{KAl}(\text{SO}_{4})_{2}\cdot 12\text{H}_{2}\text{O}$. It clears water.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why is cryolite added to alumina before electrolysis?

2. What is the charge, in coulombs, needed to make 1.00 mol of aluminium?

3. A white jelly forms when ammonia is added to aluminium sulphate solution. What happens when more ammonia is added? What happens with more sodium hydroxide?

4. Aluminium chloride is a solid with a low melting point and fumes in moist air. What does this tell us about its bonding?

References

• OpenStax, Chemistry 2e, Chapter 18 (Representative Metals, Metalloids and Nonmetals), the section on the occurrence and preparation of the representative metals.
• NCERT Chemistry for Class 11, the chapter The p-Block Elements (older edition), and for Class 12 the chapter on the general principles of isolation of elements.
• Atkins and Shriver, Inorganic Chemistry, the chapter on the group 13 elements.