Group 14: Reactivity towards Acids & Alkalis

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p-Block Elements • Group 14

Reactivity towards Acids & Alkalis

The $HF$ exception, Metastannic Acid, and Lead's Passivation Traps.

By chemca Team • Updated Sep 2026

Moving down Group 14, the elements shift from non-metallic (Carbon) to metalloid (Silicon, Germanium) to metallic (Tin, Lead). This transition is perfectly mirrored in their reactions with acids and alkalis, featuring several highly-tested exceptions.

1. Carbon: The Resistant Non-Metal

Carbon ($C$) is completely unaffected by dilute acids, non-oxidizing acids, and alkalis. It only reacts with hot, concentrated oxidizing acids (like Conc. $HNO_3$ and Conc. $H_2SO_4$), which oxidize it to its maximum $+4$ state ($CO_2$).

$C_{(s)} + 4HNO_3 \text{ (hot, conc.)} \rightarrow CO_{2(g)} + 4NO_{2(g)} + 2H_2O_{(l)}$

$C_{(s)} + 2H_2SO_4 \text{ (hot, conc.)} \rightarrow CO_{2(g)} + 2SO_{2(g)} + 2H_2O_{(l)}$

2. Silicon: Alkalis and the $HF$ Exception

Silicon ($Si$) is a metalloid. It is unaffected by most common acids, including conc. $HNO_3$. However, it exhibits two very specific, highly-tested reactions.

A. Reaction with Alkalis:

Unlike Carbon, Silicon is readily attacked by hot aqueous alkalis to form Silicates and liberate Hydrogen gas.

$Si_{(s)} + 2NaOH_{(aq)} + H_2O_{(l)} \rightarrow Na_2SiO_{3(aq)} \text{ (Sodium Silicate)} + 2H_{2(g)}$

B. The $HF$ Exception:

While inert to other acids, Silicon reacts vigorously with Hydrofluoric Acid ($HF$). This is because Silicon forms extremely strong $Si-F$ bonds and can expand its octet using vacant 3d orbitals to form the highly stable hexafluorosilicate complex.

$Si + 6HF \rightarrow \mathbf{H_2SiF_6} \text{ (Fluorosilicic acid)} + 2H_2$

3. Tin ($Sn$): Amphoteric Nature & The $HNO_3$ Trap

Tin is an amphoteric metal. It dissolves in both acids and bases. With bases (like $NaOH$), it forms stannate ($[Sn(OH)_6]^{2-}$) and $H_2$ gas. But its reaction with Nitric Acid is a classic exam trap.

The Conc. $HNO_3$ Trap:

Usually, metals react with Nitric Acid to form metal nitrates. Tin does this with dilute $HNO_3$. However, with Concentrated $HNO_3$, Tin is oxidized to a hydrated solid oxide known as Metastannic Acid ($H_2SnO_3$).

$Sn_{(s)} + 4HNO_3 \text{ (conc.)} \rightarrow \mathbf{H_2SnO_3}_{(s)} \text{ (Metastannic acid)} + 4NO_{2(g)} + H_2O_{(l)}$

Note: In metastannic acid, Tin is in the $+4$ oxidation state.

4. Lead ($Pb$): The Acid Passivation

Lead is also amphoteric, reacting with alkalis to form plumbates ($[Pb(OH)_6]^{2-}$). However, its reaction with common acids is limited by a phenomenon called passivation.

Passivation by $HCl$ and $H_2SO_4$:

When Lead is placed in dilute Hydrochloric Acid ($HCl$) or dilute Sulfuric Acid ($H_2SO_4$), it barely reacts. The initial reaction forms $PbCl_2$ and $PbSO_4$ respectively.

Because $PbCl_2$ and $PbSO_4$ are insoluble in cold water, they immediately form a protective, impermeable coating on the surface of the Lead. This film halts any further reaction (Passivation).

The Exception: Nitric Acid

Lead dissolves readily in dilute $HNO_3$ because Lead Nitrate, $Pb(NO_3)_2$, is highly soluble in water and does not form a protective film.

$3Pb_{(s)} + 8HNO_3 \text{ (dilute)} \rightarrow 3Pb(NO_3)_{2(aq)} + 2NO_{(g)} + 4H_2O_{(l)}$

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