3_3 Reactions of Aluminium: Oxide Layer, Amphoteric Behaviour and Thermite
Aluminium sits high in the reactivity series, well above iron, yet aluminium window frames and drink cans last for years. The explanation is a layer of oxide only a few nanometres thick. Once you know when that layer protects the metal and when it fails, aluminium's reactions with air, acids and alkalis make sense. Its strong attraction for oxygen also makes it a powerful reducing agent.
Reaction with Air: The Protective Oxide
A freshly exposed aluminium surface reacts with oxygen at once to form a continuous film of aluminium oxide,
\mathrm{Al_2O_3}
. This film is unusually protective because it sticks tightly to the metal, has no gaps and is insoluble in water, so it cuts the metal off from further attack. The film is a form of passivation. It can be thickened deliberately by electrolysis, a process called anodizing, which can also take up coloured dyes.Powdered aluminium has so much surface that the film cannot protect it. Heated to about 800 °C in air, the powder burns with an intense white light, forming the oxide and some aluminium nitride. This flash was once used in photography and is still used in fireworks:
\begin{aligned} \mathrm{4Al + 3O_2} &\rightarrow \mathrm{2Al_2O_3} \\ \mathrm{2Al + N_2} &\rightarrow \mathrm{2AlN} \end{aligned}
Chloride ions break down the oxide film locally, so salt water causes pitting corrosion in aluminium. Marine equipment therefore uses special alloys and coatings.
Reaction with Non-Metals
Hot aluminium combines directly and exothermically with halogens, sulphur, nitrogen, phosphorus and carbon. With chlorine it forms aluminium chloride. Like boron, aluminium is electron deficient in
\mathrm{AlCl_3}
, so in the vapour two molecules join through bridging chlorine atoms. Each chlorine donates a lone pair to the other aluminium atom:Cl[Al](Cl)Cl.Cl[Al](Cl)Cl>>Cl[Al-]1(Cl)[Cl+][Al-](Cl)(Cl)[Cl+]1Amphoteric Behaviour: Acids and Alkalis
Aluminium is amphoteric, which means it reacts with both acids and bases and releases hydrogen in each case. With hydrochloric acid it forms aluminium chloride. The reaction starts slowly while the oxide film is being dissolved, then speeds up:
\mathrm{2Al + 6HCl \rightarrow 2AlCl_3 + 3H_2}
Hot concentrated sulphuric acid oxidizes aluminium and is itself reduced to sulphur dioxide, while dilute sulphuric acid reacts only slowly. Nitric acid of any concentration hardly attacks aluminium, because it is an oxidizing acid that keeps rebuilding the oxide film. This passivation is why concentrated nitric acid can be transported in aluminium tanks.
Sodium hydroxide solution dissolves the oxide film and then the metal itself, forming the soluble tetrahydroxoaluminate ion and hydrogen gas. A metal that dissolves in alkali is unusual, and this behaviour marks aluminium's position on the borderline between metals and non-metals:
[Al].[Al].[OH-].[OH-].O.O.O.O.O.O>>O[Al-](O)(O)O.O[Al-](O)(O)O.[H][H].[H][H].[H][H]The Thermite Reaction
Because aluminium oxide is so stable (its enthalpy of formation is about −1676 kJ/mol), aluminium can strip oxygen from the oxides of less reactive metals. In the thermite reaction, a mixture of aluminium powder and iron(III) oxide is ignited with a magnesium fuse:
\mathrm{2Al(s) + Fe_2O_3(s) \rightarrow Al_2O_3(s) + 2Fe(l)} \qquad \Delta H \approx -850\ \text{kJ/mol}
The heat released raises the temperature well above 2000 °C, so the iron forms as a liquid. Molten thermite iron is used to weld railway rails on site. The same idea, called aluminothermy, is used to extract metals such as chromium and manganese from their oxides when carbon reduction would leave unwanted carbides.
Why These Properties Make Aluminium Useful
- Low density (about a third that of iron) combined with strength in alloys makes it ideal for aircraft, vehicles and ships.
- It conducts heat and electricity well, so it is used in overhead power cables and heat exchangers.
- The oxide film makes it corrosion resistant and non-toxic for food packaging, cans and cooking foil.
- It reflects heat and light well, so it is used for insulation and in reflective storage tanks.
- Its strong pull on oxygen makes it a useful reducing agent and a deoxidizer that removes dissolved oxygen from molten steel.
Tips for remembering
- Reactive metal, protective skin: aluminium is very reactive, but a thin, tough oxide film forms instantly and keeps air, water and nitric acid away.
- Amphoteric means it dissolves both ways: aluminium gives H₂ with hydrochloric acid and with sodium hydroxide.
- Thermite is a fight for oxygen: aluminium takes the oxygen from iron oxide because Al₂O₃ is far more stable, and the energy released melts the iron.