Chemistry · Chemistry of the elements · Grade 9

Aluminium and its compounds

Why does light aluminium not rust, yet dissolve in both acid and alkali?

The situation

An aluminium pot in the kitchen does not rust for years, and an aircraft body is made of an aluminium alloy — almost three times lighter than iron. Yet this same metal dissolves in both acid and alkali, giving off hydrogen, which is why it is not recommended to keep sour or salty food in an aluminium vessel for long. To settle muddy water from an irrigation ditch, its salt — alum — is used. Why does an active metal not rust in air, and why does its hydroxide react with both acid and alkali?

Topic

Aluminium and its compounds. Aluminium pan and aeroplane: a thin oxide film, a metal that dissolves in both acid and alkali, and alum that clarifies water

Key ideas

Oxide layer
Amphoteric hydroxide
The thermite process

Formulas

2 Al + 2 NaOH + 6 H_{2} O \overset{}{\to} 2 Na [Al (OH)_{4}] + 3 H_{2}

In alkali the oxide film dissolves and the metal displaces hydrogen from water: 2 mol Al → 3 mol H₂ (the same amount in acid).

AlCl_{3} + 3 NaOH \overset{}{\to} Al (OH)_{3} ↓ + 3 NaCl Al (OH)_{3} + NaOH \overset{}{\to} Na [Al (OH)_{4}]

Al(OH)₃ is amphoteric: it precipitates at 3 mol OH⁻ per 1 mol Al³⁺ and dissolves at the fourth OH⁻.

Fe_{2} O_{3} + 2 Al \overset{\overset{}{t}}{\to} 2 Fe + Al_{2} O_{3}

Aluminothermy: ΔH = −851.5 kJ, the mixture heats up to 2500 °C — iron is released in the liquid state.

2 Al_{2} O_{3} + 3 C \overset{\overset{}{electrolysis}}{\to} 4 Al + 3 CO_{2} ↑

The Hall–Héroult process: Al₂O₃ dissolved in molten cryolite (≈ 960 °C); ≈ 2980 A·h per 1 kg of Al, in practice 13–15 kW·h.

Al^{3 +} + H_{2} O \overset{}{⇌} AlOH^{2 +} + H^{+}

Aluminium salts hydrolyse — the solution is acidic: in 0.1 mol/L AlCl₃, pH ≈ 3.0.

Theory

Aluminium: position and physical properties (§25)

Aluminium Al is element 13, period 3, group IIIA: $[Ne] 3 s^{2} 3 p^{1}$ — 3 electrons in the outer shell, and always +3 in compounds. It is the most abundant metal in the Earth's crust (≈ 8 % by mass, third place after oxygen and silicon), but because of its reactivity it occurs only in compounds: bauxite Al₂O₃·nH₂O (the main ore), corundum Al₂O₃, kaolin (clay), feldspars, cryolite Na₃AlF₆. Aluminium is silvery white and light (ρ = 2.70 g/cm³, ≈ 2.9 times lighter than iron), melts at 660.3 °C, and is soft and ductile: it can be rolled into foil about 0.01 mm thick. Its electrical conductivity is ≈ 63 % that of copper, but an aluminium wire of the same mass conducts twice as well as a copper one — which is why overhead power lines are made of aluminium.

The oxide film and chemical properties (§26, lab work 8)

Aluminium is an active metal ( $E^{\circ}$ = −1.66 V), but in air it is instantly covered with a thin, dense oxide film of Al₂O₃ a few nanometres thick (≈ 1–5 nm). The film lets neither water nor air through, so aluminium does not react with water and does not rust. If the film is damaged, the metal reacts even with water: $2 Al + 6 H_{2} O \overset{}{\to} 2 Al (OH)_{3} + 3 H_{2}$ . In acid first the film and then the metal dissolves — in the experiment gas begins to appear after a delay of a few seconds: $2 Al + 6 HCl \overset{}{\to} 2 AlCl_{3} + 3 H_{2}$ . In alkali both the amphoteric oxide and the metal dissolve: $2 Al + 2 NaOH + 6 H_{2} O \overset{}{\to} 2 Na [Al (OH)_{4}] + 3 H_{2}$ . In both cases 2 mol Al gives 3 mol H₂: 27 mg Al → 33.6 mL H₂ (STP). Cold concentrated nitric and sulfuric acids passivate aluminium (the film thickens) — which is why concentrated nitric acid is carried in aluminium tanks. As a powder or foil, aluminium burns: $4 Al + 3 O_{2} \overset{}{\to} 2 Al_{2} O_{3}$ .

Aluminothermy and the production of aluminium

Aluminium has a very strong affinity for oxygen and displaces many metals from their oxides — aluminothermy: $Fe_{2} O_{3} + 2 Al \overset{}{\to} 2 Fe + Al_{2} O_{3}$ , ΔH = −851.5 kJ. The thermite mixture (≈ 3 : 1 by mass) heats up to 2500 °C and the iron runs out as a liquid — the welding of railway rails is based on this (H. Goldschmidt, 1893). Chromium and manganese are obtained in the same way: $Cr_{2} O_{3} + 2 Al \overset{}{\to} 2 Cr + Al_{2} O_{3}$ . Aluminium itself is obtained only by electrolysis (the Hall–Héroult process, 1886): Al₂O₃ (melting point 2072 °C) is dissolved in molten cryolite at ≈ 950–980 °C; the cathode is the liquid aluminium at the bottom of the bath, and the anodes are carbon blocks, which burn in the oxygen released and turn into CO₂. For 1 kg of Al a charge of $Q = \frac{m}{M} z F$ ≈ 2980 A·h is needed; at a voltage of 4.0–4.2 V, 13–15 kW·h of electrical energy is used in practice. Recycling saves ≈ 95 % of this energy. Uzbekistan has no primary aluminium (electrolysis) plant.

Alloys and uses (lab work 9)

Pure aluminium is soft, so articles are often made from its alloys: duralumin (Al + ≈ 4.4 % Cu, 1.5 % Mg, 0.6 % Mn; ρ ≈ 2.78 g/cm³) is, after heat treatment, several times stronger than pure aluminium — aircraft skin and ribs are made from it; silumin (Al + ≈ 12 % Si) casts well — engine and pump housings; magnalium (Al + Mg) resists sea water. In the lab work the colour, lustre and hardness of the samples are inspected and their density is measured: $ρ = m / V$ , the volume from the rise of the water level in a measuring cylinder. Aluminium is used in kitchen vessels, foil, drinks cans, window profiles, electric wires and car parts; its powder in “silver” paint and in pyrotechnics.

Compounds: Al₂O₃, Al(OH)₃ and salts (§27, labs 10–11)

Al₂O₃ is a white, very hard, water-insoluble amphoteric oxide: $Al_{2} O_{3} + 6 HCl \overset{}{\to} 2 AlCl_{3} + 3 H_{2} O$ , $Al_{2} O_{3} + 2 NaOH + 3 H_{2} O \overset{}{\to} 2 Na [Al (OH)_{4}]$ ; its natural crystal is corundum, and its coloured varieties are ruby (Cr³⁺ impurity) and sapphire. Al(OH)₃ is a white gelatinous precipitate, formed when alkali is added drop by drop to a salt solution: $Al^{3 +} + 3 OH^{-} \overset{}{\to} Al (OH)_{3}$ . It is amphoteric: it dissolves in acid ( $Al (OH)_{3} + 3 HCl \overset{}{\to} AlCl_{3} + 3 H_{2} O$ ) and in an excess of alkali ( $Al (OH)_{3} + OH^{-} \overset{}{\to} [Al (OH)_{4}]^{-}$ , the tetrahydroxoaluminate ion). So it is convenient to make Al(OH)₃ with aqueous ammonia — the precipitate does not dissolve in an excess of it. Aluminium salts hydrolyse in water: the aqua ion gives up H⁺ ( $Al^{3 +} + H_{2} O \overset{}{⇌} AlOH^{2 +} + H^{+}$ ) — the solution is acidic and turns litmus red (pH ≈ 3.0 in 0.1 mol/L AlCl₃). Alum $KAl (SO_{4})_{2} \cdot 12 H_{2} O$ is a double salt; with the hydrogencarbonates in water it forms flakes of Al(OH)₃ ( $Al^{3 +} + 3 HCO_{3}^{-} \overset{}{\to} Al (OH)_{3} + 3 CO_{2}$ ), the flakes stick clay particles together and settle, and the water clears. Aluminium sulfate and alum are used in water treatment as coagulants; Al(OH)₃ is in medicines that reduce stomach acidity.

Safety and the model

Safety: wear safety goggles and gloves when working with acid and alkali; if alkali gets in the eye, rinse at once with plenty of water. Do not point the mouth of a test tube at yourself or others. Hydrogen forms an explosive mixture with air — do not light a flame nearby. The thermite reaction is shown only by the teacher, in the open air or in a fume cupboard, over sand and from a distance: liquid iron spatters, do not look straight at the flame. The model: reaction rates are given qualitatively (film — a delay, granule — a shrinking sphere) and time is shortened; the gas volume is brought to STP and is exact by the equation. The amount of precipitate follows stoichiometry: the final state depends on the moles added. The pH of salt solutions comes from the charge balance (25 °C, the first hydrolysis step of the aqua ion, lg K = −4.97). Water clearing is a simplified model (1 s ≈ 1 min). The electrolysis cell is an unscaled mock-up, with a voltage of 4.2 V and a current efficiency of 94 % assumed.

Did you know?

The cheap electrolytic production of aluminium was discovered in 1886 by two people unaware of each other — Charles Hall in the USA and Paul Héroult in France. Curiously, both were born in 1863 and both died in 1914.
Uzbekistan has no primary aluminium (electrolysis) plant: aluminium profiles and wires are made from imported metal or from recycled metal. Projects to obtain alumina (Al₂O₃) from the kaolin clay of the Angren deposit are being studied.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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Chromium and manganese: coloured ions — Grade 9
Iron and its compounds — Grade 9
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