Magnesium (Mg): Chemical Properties & Reactions

Chemical Properties Overview

Magnesium (Mg) is an alkaline earth metal, positioned in Group 2 and Period 3 of the periodic table.

• Reactivity Series Position: Mg is an active metal, placed above hydrogen in the reactivity series. This indicates its strong tendency to displace hydrogen from acids and water (under specific conditions).
• Electronegativity: It has a low electronegativity (Pauling scale: 1.31), reflecting its metallic character and tendency to readily lose its two valence electrons to form stable Mg²⁺ ions.
• General Reactivity: Magnesium is a fairly reactive metal. It tarnishes slowly in moist air due to the formation of a thin, protective layer of magnesium oxide and hydroxide. It burns with a dazzling white flame when ignited in air or oxygen.

Action of Air and Oxygen

Magnesium reacts readily with air upon heating.

1. Reaction with Oxygen (Combustion)

Magnesium ribbon burns in oxygen with a brilliant white flame to form magnesium oxide.

2Mg(s) + O₂(g) → 2MgO(s)

2. Reaction with Nitrogen

When magnesium burns in air, it also reacts with nitrogen to form magnesium nitride, though this reaction is slower than with oxygen.

3Mg(s) + N₂(g) → Mg₃N₂(s)

Magnesium nitride reacts with water to form magnesium hydroxide and ammonia:

Mg₃N₂(s) + 6H₂O(l) → 3Mg(OH)₂(s) + 2NH₃(g)

Action of Water and Steam

Magnesium reacts with water and steam, with the vigor of the reaction depending on the temperature.

1. Reaction with Cold Water

Magnesium reacts very slowly with cold water to form magnesium hydroxide and hydrogen gas. The reaction is slow because the insoluble Mg(OH)₂ formed coats the surface of the metal, preventing further reaction.

Mg(s) + 2H₂O(l) → Mg(OH)₂(s) + H₂(g)

2. Reaction with Boiling Water

The reaction is faster with boiling water as the Mg(OH)₂ layer is less adherent at higher temperatures, and the increased kinetic energy promotes the reaction.

Mg(s) + 2H₂O(l) → Mg(OH)₂(s) + H₂(g)

3. Reaction with Steam

Magnesium reacts vigorously with steam (at red heat) to form magnesium oxide and hydrogen gas. Here, the Mg(OH)₂ product decomposes to MgO at high temperatures.

Mg(s) + H₂O(g) → MgO(s) + H₂(g)

Action of Acids and Bases

1. Reaction with Dilute Acids

Magnesium reacts readily with dilute acids like hydrochloric acid and sulfuric acid to produce magnesium salts and hydrogen gas.

• With Dilute Hydrochloric Acid (HCl):

  Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

• With Dilute Sulfuric Acid (H₂SO₄):

  Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)

• With Dilute Nitric Acid (HNO₃): Magnesium is a strong reducing agent. With very dilute nitric acid, it produces ammonium nitrate.

  4Mg(s) + 10HNO₃(dilute) → 4Mg(NO₃)₂(aq) + NH₄NO₃(aq) + 3H₂O(l)

  With moderately dilute nitric acid, it often produces nitrous oxide (N₂O) or nitric oxide (NO).

  4Mg(s) + 10HNO₃(moderately dilute) → 4Mg(NO₃)₂(aq) + N₂O(g) + 5H₂O(l)

  3Mg(s) + 8HNO₃(moderately dilute) → 3Mg(NO₃)₂(aq) + 2NO(g) + 4H₂O(l)

2. Reaction with Concentrated Acids

• With Concentrated Sulfuric Acid (H₂SO₄): Magnesium reduces concentrated sulfuric acid to sulfur dioxide.

  Mg(s) + 2H₂SO₄(conc) → MgSO₄(aq) + SO₂(g) + 2H₂O(l)

• With Concentrated Nitric Acid (HNO₃): Magnesium reduces concentrated nitric acid to nitrogen dioxide.

  Mg(s) + 4HNO₃(conc) → Mg(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l)

3. Reaction with Bases (Alkalis)

Magnesium metal does not react with cold or hot solutions of strong alkalis like sodium hydroxide (NaOH) or potassium hydroxide (KOH). This is because Mg(OH)₂ is a weak base and does not dissolve in excess alkali.

Key Laboratory Test/Identification Reactions for Mg²⁺ Ions

Magnesium ions (Mg²⁺) are identified in salt analysis using the following tests:

1. Reaction with Sodium Hydroxide (NaOH)

When NaOH solution is added to a solution containing Mg²⁺ ions, a white precipitate of magnesium hydroxide is formed. This precipitate is insoluble in excess NaOH.

Mg²⁺(aq) + 2NaOH(aq) → Mg(OH)₂(s) + 2Na⁺(aq)

Mg²⁺(aq) + 2OH⁻(aq) → Mg(OH)₂(s) (White precipitate)

2. Reaction with Ammonium Hydroxide (NH₄OH)

• In the absence of Ammonium Chloride (NH₄Cl): If NH₄OH is added to a solution of Mg²⁺, a white precipitate of Mg(OH)₂ forms.

  Mg²⁺(aq) + 2NH₄OH(aq) → Mg(OH)₂(s) + 2NH₄⁺(aq)

• In the presence of Ammonium Chloride (NH₄Cl): No precipitate of Mg(OH)₂ forms when NH₄OH is added in the presence of NH₄Cl. This is due to the common ion effect: NH₄Cl suppresses the dissociation of NH₄OH, thereby reducing the concentration of OH⁻ ions below the solubility product (Ksp) of Mg(OH)₂, preventing its precipitation. This is a crucial distinction from Group IIIB cations (Fe³⁺, Al³⁺, Cr³⁺) which precipitate under these conditions.

3. Phosphate Test (Magnesium Ammonium Phosphate Test)

This is a definitive test for Mg²⁺ ions, carried out in an ammoniacal medium (NH₄OH/NH₄Cl buffer). When disodium hydrogen phosphate (Na₂HPO₄) is added to a solution containing Mg²⁺ ions in the presence of NH₄Cl and NH₄OH, a white crystalline precipitate of magnesium ammonium phosphate is formed.

Mg²⁺(aq) + NH₄⁺(aq) + HPO₄²⁻(aq) → MgNH₄PO₄(s) + H⁺(aq)

Alternatively, simplified:

Mg²⁺(aq) + NH₄OH(aq) + Na₂HPO₄(aq) → MgNH₄PO₄(s) + 2Na⁺(aq) + H₂O(l)

(White crystalline precipitate, often appears slowly)

4. Magneson Reagent Test (Adsorption Indicator Test)

Magneson reagent (4-(p-nitrophenylazo)resorcinol) is an organic dye. In the presence of Mg²⁺ ions and an alkaline medium (e.g., NaOH), Mg(OH)₂ is formed, which adsorbs the Magneson dye to produce a characteristic blue precipitate or “lake.”

Mg²⁺(aq) + 2OH⁻(aq) → Mg(OH)₂(s)

Mg(OH)₂(s) + Magneson Reagent → Blue Precipitate / Lake