Important Chemical Compounds of Hydrogen

Introduction to Major Compounds of Hydrogen

Hydrogen, being the lightest and most abundant element in the universe, forms a vast array of compounds. For high school chemistry, the most frequently tested compounds are its oxides (water and hydrogen peroxide) and some crucial hydrides like hydrogen chloride. This guide focuses on their preparation, properties, and significant chemical reactions.

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1. Water (H₂O)

Chemical Formula and Common Name

• Chemical Formula: H₂O
• Common Name: Water

Laboratory Preparation (Synthesis)

Water is generally obtained purified from natural sources. Its synthesis from elements demonstrates its formation.

• Equation: 2H₂(g) + O₂(g) → 2H₂O(l)

Properties

• Physical Properties:
  • Colourless, odourless, tasteless liquid.
  • Melting Point: 0 °C (273.15 K)
  • Boiling Point: 100 °C (373.15 K)
  • Maximum Density: 1 g/cm³ at 4 °C.
  • High specific heat and latent heat of vaporisation due to hydrogen bonding.
• Chemical Properties:
  • Amphoteric Nature: Acts as both an acid and a base.
    • As an acid: H₂O + NH₃ ⇌ OH⁻ + NH₄⁺
    • As a base: H₂O + H₂S ⇌ H₃O⁺ + HS⁻
  • Redox Reactions: Can act as both an oxidising and a reducing agent.
    • Oxidising (with electropositive metals): 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
    • Reducing (with highly electronegative non-metals at high temp): C(s) + H₂O(g) → CO(g) + H₂(g) (Water gas shift)
  • Hydrolysis: Reaction with many ionic and covalent compounds.
    • Covalent: P₄O₁₀(s) + 6H₂O(l) → 4H₃PO₄(aq)
    • Ionic: Na₂CO₃(s) + 2H₂O(l) → H₂CO₃(aq) + 2NaOH(aq) (Partial hydrolysis leading to basic solution)

Exam-Relevant Reactions

1. Reaction with Active Metals:
   • 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
   • Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)
2. Reaction with Halogens (Hot water):
   • 2F₂(g) + 2H₂O(l) → 4HF(aq) + O₂(g)
3. Hydrolysis of Carbides (e.g., Calcium carbide):
   • CaC₂(s) + 2H₂O(l) → Ca(OH)₂(aq) + C₂H₂(g) (Ethyne formation)
4. Hardness of Water:
   • Temporary Hardness (due to bicarbonates of Ca & Mg):
     • Removal by boiling: Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)
     • Removal by Clark’s process: Ca(HCO₃)₂(aq) + Ca(OH)₂(aq) → 2CaCO₃(s) + 2H₂O(l)
   • Permanent Hardness (due to chlorides and sulphates of Ca & Mg):
     • Removal by washing soda: CaCl₂(aq) + Na₂CO₃(s) → CaCO₃(s) + 2NaCl(aq)
     • Removal by Ion-exchange (Permutit/Zeolite process): Na₂Z(s) + Ca²⁺(aq) → CaZ(s) + 2Na⁺(aq) (where Z is zeolite)

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2. Hydrogen Peroxide (H₂O₂)

Chemical Formula and Common Name

• Chemical Formula: H₂O₂
• Common Name: Hydrogen Peroxide

Preparation Processes

1. From Barium Peroxide (Old Method):
   • Equation: BaO₂(s) + H₂SO₄(aq) → BaSO₄(s) + H₂O₂(aq)
   • BaSO₄ is insoluble and can be filtered off. Requires ‘hydrated’ barium peroxide BaO₂·8H₂O and cold dilute sulphuric acid to prevent decomposition.
2. Electrolytic Process (Peroxodisulphate Process):
   • Electrolysis of 50% H₂SO₄ or (NH₄)₂SO₄ in H₂SO₄.
   • Equations:
     • 2H₂SO₄(l) → H₂S₂O₈(l) + H₂(g) (at anode)
     • H₂S₂O₈(l) + 2H₂O(l) → 2H₂SO₄(l) + H₂O₂(l) (Hydrolysis of peroxodisulphuric acid)
3. Auto-oxidation of 2-ethylanthraquinol (Industrial Method):
   • The most important industrial method. 2-ethylanthraquinol is oxidised by air to 2-ethylanthraquinone and H₂O₂. The quinone is then catalytically hydrogenated back to quinol.

Properties

• Physical Properties:
  • Pure H₂O₂ is a pale blue, syrupy liquid.
  • Miscible with water in all proportions.
  • Density: 1.44 g/cm³ at 20 °C (higher than water).
• Chemical Properties:
  • Decomposition: Unstable and decomposes into water and oxygen, especially in presence of light, heat, or catalysts (metals, metal oxides).
    • Equation: 2H₂O₂(l) → 2H₂O(l) + O₂(g)
  • Oxidising Agent: In both acidic and basic media.
    • Acidic: PbS(s) + 4H₂O₂(aq) → PbSO₄(s) + 4H₂O(l) (Turns black PbS white)
    • Basic: 2Fe²⁺(aq) + H₂O₂(aq) → 2Fe³⁺(aq) + 2OH⁻(aq)
  • Reducing Agent: In both acidic and basic media. (Less common, usually with strong oxidising agents).
    • Acidic: 2KMnO₄(aq) + 3H₂SO₄(aq) + 5H₂O₂(aq) → K₂SO₄(aq) + 2MnSO₄(aq) + 8H₂O(l) + 5O₂(g)
    • Basic: 2K₃[Fe(CN)₆](aq) + 2KOH(aq) + H₂O₂(aq) → 2K₄[Fe(CN)₆](aq) + 2H₂O(l) + O₂(g)

Exam-Relevant Reactions

1. Bleaching Action (Oxidising property): Due to the liberation of nascent oxygen.
   • H₂O₂(aq) → H₂O(l) + [O]
   • Colouring matter + [O] → Colourless matter
2. Detection of H₂O₂ (Titanium Sulphate Test):
   • Ti(SO₄)₂(aq) + H₂O₂(aq) + 2H₂O(l) → TiO₂(SO₄)₂(aq) + 2H₂SO₄(aq)
   • Forms a yellow/orange complex (peroxotitanic acid), H₂TiO₄.

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3. Hydrogen Chloride (HCl)

Chemical Formula and Common Name

• Chemical Formula: HCl
• Common Name: Hydrogen Chloride (gas), Hydrochloric Acid (aqueous solution)

Laboratory Preparation

Hydrogen chloride gas is commonly prepared by the action of concentrated sulphuric acid on common salt.

• Equation: NaCl(s) + H₂SO₄(conc)(l) → NaHSO₄(s) + HCl(g) (at < 200°C)
  • At higher temperatures (above 200°C), the reaction is: 2NaCl(s) + H₂SO₄(conc)(l) → Na₂SO₄(s) + 2HCl(g)

Properties

• Physical Properties:
  • Colourless gas with a pungent odour.
  • Extremely soluble in water, forming hydrochloric acid.
  • Non-flammable.
• Chemical Properties:
  • Acidic Nature: Dissolves in water to form a strong acid.
    • HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq)
  • Reaction with Metals: Reacts with active metals above hydrogen in the reactivity series to produce hydrogen gas.
    • Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
  • Reaction with Metal Oxides/Hydroxides (Bases): Forms salt and water.
    • MgO(s) + 2HCl(aq) → MgCl₂(aq) + H₂O(l)
    • NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
  • Reaction with Carbonates/Bicarbonates: Liberates carbon dioxide gas.
    • Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
  • Reaction with Nitrates (e.g., Silver Nitrate): Forms a white precipitate of silver chloride.
    • AgNO₃(aq) + HCl(aq) → AgCl(s) + HNO₃(aq) (Used as a test for chloride ions)
  • Aqua Regia Formation: Concentrated HCl reacts with concentrated HNO₃ in a 3:1 molar ratio to form aqua regia, which can dissolve noble metals like gold and platinum.
    • Equation: HNO₃(conc) + 3HCl(conc) → NOCl(g) + 2H₂O(l) + 2[Cl] (nascent chlorine)

Exam-Relevant Reactions

1. Test for HCl gas (Ammonia test):
   • NH₃(g) + HCl(g) → NH₄Cl(s) (White dense fumes of ammonium chloride)
2. Laboratory preparation equation (as above).
3. Reactions demonstrating acidic properties (with metals, bases, carbonates).
4. Reaction to form Aqua Regia.

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Comparative Properties Summary

Property	Water (H₂O)	Hydrogen Peroxide (H₂O₂)	Hydrogen Chloride (HCl)
State (Pure)	Liquid	Liquid	Gas
Nature	Amphoteric (acid and base)	Oxidising and Reducing agent	Strong Acid (in aqueous solution)
Decomposition	Stable	Unstable, decomposes to H₂O + O₂	Stable gas, dissolves in water
Solubility	Excellent solvent	Miscible with water	Extremely soluble in water (forms HCl acid)
Oxidation State of H	+1	+1	+1
Key Use	Universal solvent, hydration	Bleaching agent, antiseptic, oxidiser/reducer	Strong acid, pickling of steel, production of chlorides
Hydrogen Bonding	Strong intermolecular H-bonding	Stronger H-bonding than water	Weak H-bonding in liquid state, none as gas