Aluminium — chemistry, alloys; alumina, alums: Question bank

Before we begin, three words you must already own (built in the parent note, restated so nothing is assumed):

True or false — justify

Al does not react with air at all, which is why it never corrodes.

False. Al does react with

O_{2}

eagerly; it forms

A l_{2} O_{3}

which is a dense, adhering skin (passivation) that then blocks further attack — so it looks unreactive but is actually very reactive underneath. See Passivation and Corrosion.

Concentrated

H N O_{3}

reacts faster with Al than dilute HCl because it is the stronger oxidiser.

False. The very fact that conc.

H N O_{3}

is a powerful oxidiser makes it passivate Al — it instantly thickens the oxide skin, rendering Al inert. That is why conc.

H N O_{3}

is shipped in Al tankers.

A l_{2} O_{3}

being amphoteric means it is neutral (neither acidic nor basic).

False. Amphoteric is the opposite of neutral — it means it can act as both an acid and a base depending on the partner: base-like with HCl, acid-like with NaOH.

A l_{2} C l_{6}

all six Cl atoms are equivalent.

False. There are 2 bridging Cl (each bonded to both Al atoms) and 4 terminal Cl (each bonded to one Al). The bridging Al–Cl bonds are longer and weaker than the terminal ones.

Aqueous

A l C l_{3}

solution is neutral because it is just a salt.

False. It is acidic: the small, high-charge-density

A l^{3 +}

polarises its coordinated water molecules so strongly that they release

H^{+}

(hydrolysis). This is a Fajans Rules consequence.

An alum must contain the same metal element in its two cation slots.

False. The two cations must differ in charge (

+ 1

and

+ 3

), not in identity. Chrome alum

K C r (S O_{4})_{2} \cdot 12 H_{2} O

pairs

K^{+}

with

C r^{3 +}

— two different elements.

The thermite reaction works because Fe is a better reducing agent than Al.

False. It is the reverse: Al is the better reducing agent here because

A l_{2} O_{3}

is more thermodynamically stable than

F e_{2} O_{3}

, so Al pulls the oxygen away from iron. See Thermodynamics of Reduction (Ellingham).

Adding another metal to pure Al always makes it heavier and denser, which is the point of alloying.

False. The point is strength, not weight. Alloying atoms distort the lattice and block dislocation slip, making Al harder and stronger while keeping the density low — that is why aircraft use Duralumin.

A l C l_{3}

conducts electricity well when molten because it is an ionic salt.

False. In the vapour/melt it exists as covalent

A l_{2} C l_{6}

molecules (electron-deficient Al pulls the bonds covalent), so molten

A l C l_{3}

is a poor conductor — evidence of its covalent character.

Spot the error

A l C l_{3}

dimerises because Al already has a complete octet and wants to share extra electrons."

Error. Backwards. Monomeric

A l C l_{3}

has only 6 electrons around Al (electron-deficient); it dimerises so a Cl lone pair from the other molecule fills its empty orbital and completes the octet. It's a Lewis acid seeking electrons, not donating them.

2 A l + 2 N a O H \to 2 N a A l O_{2} + H_{2}

, and the water is optional."

Error. Water is essential and the aluminate formed in aqueous NaOH is

[A l (O H)_{4}]^{-}

, not the anhydrous

A l O_{2}^{-}

. Correct:

2 A l + 2 N a O H + 6 H_{2} O \to 2 N a [A l (O H)_{4}] + 3 H_{2}

"Al reacts with base, therefore Al is a base."

Error. Reacting with a base makes Al behave as an acid in that reaction. Reacting with both acids and bases is what makes it amphoteric — not a base.

"Potash alum is

K A l (S O_{4})_{2} \cdot 24 H_{2} O

Error. The formula unit

K A l (S O_{4})_{2}

carries 12 waters, not 24. The 24 belongs to the doubled form

K_{2} S O_{4} \cdot A l_{2} (S O_{4})_{3} \cdot 24 H_{2} O

, which contains two such units.

"Alum purifies water by killing germs (it is a disinfectant)."

Error. Alum coagulates, it does not disinfect. Its

A l^{3 +}

forms gelatinous

A l (O H)_{3}

floc that traps suspended dirt and sinks — clarification, not sterilisation. (Chlorine, not alum, kills germs.)

"In

N a [A l (O H)_{4}]

the aluminium is

s p^{2}

and has coordination number 3."

Error. Aluminate

[A l (O H)_{4}]^{-}

is tetrahedral: Al is $s p^{3}$ with coordination number 4 (see Coordination Compounds).

"Duralumin is Al + Ni + Co and is used for magnets."

Error. That describes Alnico. Duralumin is Al + Cu + Mg + Mn, used for aircraft and light-strong parts.

Why questions

Why does the same high charge density of

A l^{3 +}

explain both the covalent

A l C l_{3}

and the acidic aqueous solution?

A high-charge-density cation polarises whatever surrounds it: it distorts Cl's electron cloud (→ covalent bond) and distorts coordinated water's O–H bonds (→ releases

H^{+}

). One cause, two effects — the heart of Fajans Rules.

Why does Al keep reacting with NaOH but stop reacting quietly in plain air?

In air the

A l_{2} O_{3}

skin seals the metal and stays put. In NaOH, the

O H^{-}

dissolves that oxide skin into aluminate, continually exposing fresh Al, so the reaction does not self-limit.

Why is passivation useful rather than a nuisance, given Al is chemically reactive?

The oxide armour is dense, thin and tightly adhering, so it protects the bulk metal from further attack — this is exactly why soda cans, window frames and

H N O_{3}

tankers survive despite Al's reactivity.

Why must an alum pair a

+ 1

cation with a

+ 3

cation specifically?

To balance charge against the two

S O_{4}^{2 -}

(total

- 4

(+ 1) + (+ 3) + 2 (- 2) = 0

. Two like-charged cations could not give this neutral, well-defined double-salt lattice.

Why does molten cryolite matter for Al but is not what makes Al amphoteric?

Cryolite lowers the melting point for extraction (electrolysis) — a separate practical fact. Amphoterism comes from

A l^{3 +}

's charge density and has nothing to do with how Al is manufactured.

Edge cases

What happens if you drop Al in very dilute, cold HCl versus warm concentrated HCl — does the trend hold?

With dilute HCl it dissolves giving

H_{2}

; the reaction is limited mainly by how fast acid strips the oxide skin, so warming/concentrating speeds it up. The direction (dissolves, releases

H_{2}

) is unchanged — HCl does not passivate the way

H N O_{3}

does.

A l (O H)_{3}

amphoteric the same way the metal Al is, and what is the boundary case?

Yes:

A l (O H)_{3}

dissolves in acid (as

A l^{3 +}

salt) and in strong base (as

[A l (O H)_{4}]^{-}

). The boundary case is neutral water, where it is nearly insoluble — the gelatinous floc that clarifies water.

At the extreme of a very small, very highly charged cation, does Fajans predict more or less ionic character, and does

A l^{3 +}

sit at that extreme?

Smaller size + higher charge → more polarising → more covalent character.

A l^{3 +}

sits near that extreme among common

+ 3

ions, which is why its compounds show marked covalency (e.g.

A l_{2} C l_{6}

Consider the limiting case where the two cations of an alum are identical in charge (say two

+ 3

ions) — is it still an alum?

No. Without a

+ 1/ + 3

pairing you cannot build the

M^{+} M^{' 3 +} (S O_{4})_{2} \cdot 12 H_{2} O

framework; you'd get a different sulphate salt, not an alum.

What is the degenerate case of "alloying" — adding zero other metal — and why does it fail the purpose?

Zero additive is just pure Al: soft, with freely moving dislocations. No lattice distortion means no strengthening, so it fails the whole reason alloys exist.

If Al's oxide layer were loose and flaky (like iron's rust) instead of dense, what would change?

Passivation would fail — fresh metal stays exposed and Al would corrode away continuously, exactly like iron rusting. The dense, adhering nature of

A l_{2} O_{3}

is the crucial property, contrast this in Passivation and Corrosion.

Recall One-line self-check

If someone says "Al is unreactive," how do you correct them in one sentence? ::: Al is highly reactive but wears a dense $A l_{2} O_{3}$ armour (passivation) that hides that reactivity.

Connections

Parent: Aluminium
Fajans Rules — the polarising-power engine behind covalency and hydrolysis
Lewis Acids and Bases — why the $A l C l_{3}$ monomer grabs a lone pair
Passivation and Corrosion — dense vs flaky oxide films
Thermodynamics of Reduction (Ellingham) — why thermite runs
Coordination Compounds — the tetrahedral aluminate ion
Group 13 Elements — where these traps generalise across the group