Silicon (Si): Chemical Properties & Reactions

Chemical Properties Overview

Silicon (Si) is a metalloid element in Group 14 of the periodic table, known for its semiconducting properties and ability to form strong covalent bonds.

Electronegativity: 1.90 (Pauling scale).
Bonding: Primarily forms covalent bonds, often exhibiting a valency of 4.
Oxidation States: Predominantly +4, with +2 being less common and stable.
Reactivity: Generally less reactive than metals but more reactive than carbon at high temperatures due to the strong Si-Si and Si-O bonds. A thin, protective layer of silicon dioxide (SiO₂) often renders it passive.

Action of Air and Oxygen

Silicon is relatively unreactive at room temperature but reacts with oxygen at elevated temperatures.

With Oxygen (Air): Silicon burns in oxygen at temperatures above 800 °C to form silicon dioxide. This oxide layer is protective and prevents further oxidation at moderate temperatures. Si(s) + O₂(g) --(Heat)--> SiO₂(s)

Action of Water and Steam

Silicon is remarkably inert towards water and steam under normal conditions.

With Water/Steam: Silicon does not react with water or steam even at high temperatures. This inertness is attributed to the formation of a stable, protective layer of silicon dioxide (SiO₂) on its surface, which prevents water molecules from reaching the underlying silicon atoms.

Action of Acids and Bases

Silicon exhibits selective reactivity with acids and reacts with strong bases.

Action of Acids

Non-oxidizing Acids (e.g., HCl, dilute H₂SO₄): Silicon is unreactive with non-oxidizing acids.
Oxidizing Acids (e.g., HNO₃): Silicon does not react with pure nitric acid. However, it reacts vigorously with a mixture of concentrated nitric acid and hydrofluoric acid. Hydrofluoric acid dissolves the protective SiO₂ layer and reacts with silicon to form fluorosilicic acid. 3Si(s) + 4HNO₃(aq) + 18HF(aq) → 3H₂SiF₆(aq) + 4NO(g) + 8H₂O(l)
Hydrofluoric Acid (HF): Hydrofluoric acid readily attacks silicon, a property utilized in semiconductor etching. Si(s) + 6HF(aq) → H₂SiF₆(aq) + 2H₂(g)

Action of Bases

Strong Hot Alkalis: Silicon reacts with hot, concentrated solutions of strong alkalis (e.g., NaOH, KOH) to form soluble silicates and liberate hydrogen gas. Si(s) + 2NaOH(aq) + H₂O(l) --(Heat)--> Na₂SiO₃(aq) + 2H₂(g)

Key Laboratory Test/Identification Reactions

The identification of silicon typically involves its compounds, particularly silicates (SiO₃²⁻ ions). Elemental silicon is not generally identified in routine qualitative analysis.

Identification of Silicates (SiO₃²⁻)

Acidification Test: When a soluble silicate solution (e.g., sodium silicate) is acidified with dilute hydrochloric acid, a gelatinous precipitate of silicic acid (H₂SiO₃) is formed. Na₂SiO₃(aq) + 2HCl(aq) → H₂SiO₃(s)↓ + 2NaCl(aq) The silicic acid precipitate (often represented as SiO₂·nH₂O) can dehydrate upon heating to form silica gel.
Ammonium Molybdate Test: Upon adding ammonium molybdate solution to a silicate solution acidified with dilute nitric acid and warming, a characteristic yellow colouration or precipitate forms. This is due to the formation of silicomolybdic acid or its ammonium salt, a heteropoly acid complex. Observation: Yellow colouration or precipitate. Key Reactants: Silicate ion, ammonium molybdate, dilute nitric acid. Product (General): Silicomolybdic acid (e.g., H₄[Si(Mo₁₂O₄₀)]) or its ammonium salt (e.g., (NH₄)₄[Si(Mo₁₂O₄₀)]). The exact stoichiometry of this complex reaction is often simplified in high school.