Reactivity towards Acids & Alkalis
Amphoteric Aluminum, the Conc. $HNO_3$ Trap, and Boron's Resistance.
The chemical behavior of Group 13 elements towards acids and bases clearly demonstrates their transition from non-metallic (Boron) to amphoteric (Aluminum, Gallium) and finally to purely metallic (Indium, Thallium). This section contains some of the most frequently asked reactions in inorganic chemistry.
1. Boron: The Resistant Non-Metal
Boron ($B$) is a non-metal and is highly unreactive due to its tight crystalline lattice. It does not react with non-oxidizing acids (like dilute $HCl$ or dilute $H_2SO_4$) or with boiling alkalis under normal conditions.
Reaction with Strong Oxidizing Acids:
Boron can only be attacked by strong, hot oxidizing acids like Concentrated Nitric Acid ($HNO_3$) or Concentrated Sulfuric Acid ($H_2SO_4$), which oxidize it to Boric Acid ($H_3BO_3$).
With Alkalis: While resistant to aqueous alkalis, Boron reacts with fused (molten) alkalis to form borates:
$2B + 6NaOH \text{ (fused)} \rightarrow 2Na_3BO_3 + 3H_2$
2. Aluminum & Gallium: The Amphoteric Metals
Aluminum ($Al$) and Gallium ($Ga$) are amphoteric, meaning they dissolve in both strong acids and strong bases, liberating Hydrogen gas ($H_2$) in both cases!
Reaction with Acids (e.g., dilute $HCl$):
Aluminum easily dissolves in dilute $HCl$, forming Aluminum Chloride and releasing Hydrogen gas.
Reaction with Alkalis (e.g., aqueous $NaOH$):
Because of its amphoteric nature, Aluminum also dissolves in strong aqueous alkalis to form a soluble complex called Sodium tetrahydroxoaluminate(III).
3. The Ultimate JEE Trap: Aluminum and Conc. $HNO_3$
Since Aluminum reacts easily with $HCl$ and $H_2SO_4$, one might assume it reacts violently with Concentrated Nitric Acid ($HNO_3$), which is a very strong acid and powerful oxidizing agent. This is a trap.
Why does this happen?
Concentrated $HNO_3$ is such a strong oxidizing agent that it immediately oxidizes the surface of the Aluminum metal to form a microscopic, tough, and impermeable layer of Aluminum Oxide ($Al_2O_3$).
This oxide layer acts as a shield, preventing the acid from reaching the pure metal underneath. The reaction stops almost instantly, rendering the metal unreactive or "passive." Because of this, Aluminum containers are often used to store and transport Concentrated Nitric Acid!
4. Indium & Thallium: Purely Metallic Behavior
As we move to the bottom of the group, metallic character is fully established.
- With Acids: Both Indium ($In$) and Thallium ($Tl$) react with non-oxidizing acids (like dilute $H_2SO_4$) to liberate Hydrogen gas, typical of true metals.
- With Alkalis: Unlike Aluminum and Gallium, Indium and Thallium are NOT amphoteric. They are purely basic metals, and therefore, they do not react with alkalis.
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