Hydrogen and its Compounds – Comprehensive Notes for NEET & JEE Main

Hydrogen, the first element in the periodic table, is the most abundant element in the universe. Its unique electronic configuration (1s1) gives it properties resembling both alkali metals and halogens, making its chemistry diverse and essential for understanding various chemical phenomena. This chapter is important for NEET and JEE Main.

1. Position of Hydrogen in the Periodic Table

• Unique Position: Hydrogen’s electronic configuration (1s1) is similar to alkali metals (ns1), but its properties also resemble halogens (needs one electron to complete the duplet).
  • Similarities with Alkali Metals (Group 1):
    • Electronic configuration (ns1).
    • Forms unipositive ion (H+ like Li+, Na+).
    • Forms oxides, halides, sulfides.
    • Can act as a reducing agent.
  • Similarities with Halogens (Group 17):
    • Needs one electron to complete its duplet (like halogens need one electron for octet).
    • Forms hydride ions (H- like Cl-).
    • Has high ionization enthalpy.
    • Forms diatomic molecules (H2 like F2, Cl2).
    • Forms covalent compounds.
• Conclusion: Due to these dual characteristics, hydrogen is placed separately in the periodic table.

2. Isotopes of Hydrogen

Hydrogen has three isotopes:

• Protium (1H or H):
  • Contains 1 proton, 0 neutrons, 1 electron.
  • Most abundant isotope (99.985%).
• Deuterium (2H or D):
  • Contains 1 proton, 1 neutron, 1 electron.
  • Also known as heavy hydrogen.
  • Used to prepare heavy water (D2O).
• Tritium (3H or T):
  • Contains 1 proton, 2 neutrons, 1 electron.
  • Radioactive isotope (beta emitter) with a half-life of 12.33 years.
  • Used in radioactive labeling and nuclear fusion research.
• Note: The difference in mass between these isotopes is significant, leading to different physical and chemical properties (isotopic effect), particularly for reaction rates.

3. Preparation of Dihydrogen (H2)

• 1. Laboratory Preparation:
  • From Metals and Acids: Reaction of active metals (e.g., Zn, Mg) with dilute acids.
    • Zn (s) + 2HCl (aq) -> ZnCl2 (aq) + H2 (g)
    • Mg (s) + H2SO4 (aq) -> MgSO4 (aq) + H2 (g)
  • From Metals and Alkali: Reaction of certain metals (e.g., Zn, Al) with hot concentrated alkali.
    • Zn (s) + 2NaOH (aq) -> Na2ZnO2 (sodium zincate) + H2 (g)
    • 2Al (s) + 2NaOH (aq) + 6H2O (l) -> 2Na[Al(OH)4] (sodium tetrahydroxoaluminate(III)) + 3H2 (g)
• 2. Industrial Preparation (Large Scale):
  • a) Electrolysis of Acidified Water: Pure dihydrogen is obtained.
    • 2H2O(l)electrolysis, acidified water​2H2(g)+O2(g)
  • b) From Brine Electrolysis: Dihydrogen is a byproduct during the electrolysis of brine (concentrated NaCl solution).
    • 2NaCl(aq)+2H2O(l)electrolysis​2NaOH(aq)+Cl2(g)+H2(g)
  • c) From Hydrocarbons / Coke (Steam Reforming): Most common industrial method.
    • Syngas (Synthesis Gas): A mixture of CO and H2. Also called “water gas” if from coke.
      • From coke: C(s)+H2O(g)1270K,Nicatalyst​CO(g)+H2(g) (Water gas reaction)
      • From hydrocarbons (e.g., methane): CH4(g)+H2O(g)1270K,Nicatalyst​CO(g)+3H2(g)
    • Water Gas Shift Reaction: To increase the yield of H2, CO is further reacted with steam.
      • CO(g)+H2O(g)673K,IronChromatecatalyst​CO2(g)+H2(g)
      • CO2 is then removed by scrubbing with sodium arsenite solution or by dissolving in water under pressure.
  • d) From Coke (Bosch Process – older): C+H2O→CO+H2 followed by water gas shift reaction and CO2 removal.

4. Properties of Dihydrogen (H2)

• Physical Properties:
  • Colourless, odourless, tasteless gas.
  • Non-polar molecule, very low boiling point (20.3 K) and melting point (14 K) due to very weak London dispersion forces.
  • Lightest gas, highly flammable.
  • Insoluble in water.
• Chemical Properties:
  • Reactivity: Relatively unreactive at room temperature due to high H-H bond dissociation enthalpy (436 kJ/mol). Reacts readily at high temperatures or in the presence of catalysts.
  • 1. Reaction with Halogens (X2): Forms hydrogen halides. Reactivity F2 > Cl2 > Br2 > I2.
    • H2 (g) + F2 (g) -> 2HF (g) (Explosive even at low temp)
    • H2 (g) + Cl2 (g) -> 2HCl (g) (In presence of light)
  • 2. Reaction with Oxygen (O2): Highly exothermic, forms water.
    • 2H2(g)+O2(g)ignition or catalyst​2H2O(l) (Explosive mixture: oxyhydrogen flame)
  • 3. Reaction with Metals: Forms metal hydrides (ionic or covalent). Highly electropositive metals (Group 1, 2) form ionic hydrides.
    • 2Na(s)+H2(g)673K​2NaH(s)
    • Ca(s)+H2(g)673K​CaH2(s)
  • 4. Reaction with Non-metals: Forms covalent hydrides.
    • N2(g)+3H2(g)Haber’s process​2NH3(g)
    • C(s)+2H2(g)→CH4(g)
  • 5. Reducing Properties: Dihydrogen is a good reducing agent, especially at high temperatures.
    • Reduces metal oxides to metals: CuO(s)+H2(g)→Cu(s)+H2O(l)
    • Hydrogenation of unsaturated hydrocarbons (in presence of Ni, Pd, Pt catalysts):
      • R−CH=CH−R′+H2Ni, Pd or Pt​R−CH2−CH2−R′ (Used in vegetable oil hardening)
  • 6. Reaction with Organic Compounds (Hydroformylation): Synthesis of aldehydes.
    • R−CH=CH2+CO+H2→R−CH2−CH2−CHO

5. Hydrides

Binary compounds formed between hydrogen and other elements.

• 1. Ionic/Saline Hydrides:
  • Formed by s-block elements (Group 1 and Group 2, except Be, Mg which form polymeric covalent hydrides).
  • Formed by transfer of electron(s) from metal to hydrogen (H- ion).
  • Crystalline, non-volatile solids. High melting points.
  • Strong reducing agents.
  • React violently with water to produce H2 gas.
    • NaH(s)+H2O(l)→NaOH(aq)+H2(g)
• 2. Covalent/Molecular Hydrides:
  • Formed by p-block elements (and BeH2, MgH2 from s-block).
  • Covalent bonds between hydrogen and other atoms.
  • Volatile liquids or gases. Low melting/boiling points.
  • Based on number of electrons:
    • Electron-deficient: Group 13 (e.g., B2H6 – diborane). Lack sufficient electrons for conventional bonds.
    • Electron-precise: Group 14 (e.g., CH4, SiH4). Have exact number of electrons for conventional bonds.
    • Electron-rich: Group 15, 16, 17 (e.g., NH3, H2O, HF). Have lone pairs on the central atom. Electron-rich hydrides (like H2O, NH3, HF) form hydrogen bonds.
• 3. Metallic/Non-stoichiometric Hydrides:
  • Formed by d-block and f-block elements (except Group 7, 8, 9 – hydride gap).
  • Hydrogen occupies interstitial sites in the metal lattice.
  • Often non-stoichiometric (composition is variable, e.g., LaH2.87, TiH1.5-1.8).
  • Hard, often conductors.
  • Used for hydrogen storage.

6. Water (H2O)

• Structure: V-shaped (bent) molecule with bond angle of 104.5 degrees. Oxygen is sp3 hybridized. It has two bond pairs and two lone pairs.
• Properties:
  • Highly polar molecule due to bent shape and high electronegativity difference (O-H).
  • High melting and boiling points, high specific heat, high heat of vaporization, high surface tension, high viscosity (all due to extensive intermolecular hydrogen bonding).
  • Density Anomaly: Ice (solid water) is less dense than liquid water due to the open, cage-like hydrogen-bonded structure formed in ice. This is why ice floats on water.
  • Amphoteric Nature: Can act as both acid and base (Bronsted-Lowry and Lewis).
    • As acid: H2O + NH3 <=> OH- + NH4+
    • As base: H2O + HCl <=> H3O+ + Cl-
  • Self-ionization (Auto-protolysis): 2H2O<=>H3O++OH− (Ionic product of water, Kw).
• Hard and Soft Water:
  • Hard Water: Water that does not readily form lather with soap. Contains dissolved salts of Calcium (Ca2+), Magnesium (Mg2+), and sometimes Iron (Fe2+).
    • Temporary Hardness: Due to bicarbonates of Ca and Mg (Ca(HCO3)2,Mg(HCO3)2). Can be removed by boiling.
      • Ca(HCO3)2(aq)heat​CaCO3(s)+H2O(l)+CO2(g)
      • Clark’s Method: Addition of calculated amount of lime (Ca(OH)2).
        • Ca(HCO3)2(aq)+Ca(OH)2(aq)→2CaCO3(s)+2H2O(l)
    • Permanent Hardness: Due to chlorides and sulfates of Ca and Mg (CaCl2,MgSO4). Cannot be removed by boiling.
      • Methods of Removal:
        • Washing Soda Method: Addition of sodium carbonate.
          • CaCl2(aq)+Na2CO3(aq)→CaCO3(s)+2NaCl(aq)
        • Calgon Method: Using sodium hexametaphosphate, (Na6​P6​O18​), which forms soluble complex with Ca2+ and Mg2+.
          • 2Ca2++Na2[Na4(PO3)6]→Na2[Ca2(PO3)6]+4Na+
        • Ion-exchange Method (Zeolite/Permutit Process): Hard water passed through sodium aluminium silicate (zeolite, NaZ).
          • 2NaZ(s)+Ca2+(aq)→CaZ2(s)+2Na+(aq) (Zeolite can be regenerated by NaCl solution).
        • Synthetic Resins Method (De-ionization/De-mineralization): Water passed through cation exchange resin (removes cations) and then anion exchange resin (removes anions) to get de-ionized water. Most pure form of water.

7. Hydrogen Peroxide (H2O2)

• Structure: Non-planar (open book) structure. Peroxide linkage (-O-O-). Has two dihedral angles.
• Preparation:
  • Laboratory: By adding calculated amount of barium peroxide to ice-cold dilute sulfuric acid.
    • BaO2(s)+H2SO4(aq)→BaSO4(s)+H2O2(aq)
  • Industrial:
    • Autoxidation of 2-ethylanthraquinol: This is the modern method.
    • Electrolytic Method (Old): Electrolysis of 30% H2SO4 or ammonium sulfate solution to form peroxodisulfuric acid (H2S2O8), followed by hydrolysis.
      • 2HSO4−→H2S2O8+2e−
      • H2S2O8+2H2O→2H2SO4+H2O2
• Properties:
  • Colourless liquid, slightly acidic.
  • Decomposes slowly in light and presence of metals/bases. Stored in dark, plastic bottles with stabilizers (like urea).
  • Oxidizing Agent: (Strong oxidizing agent in both acidic and basic medium).
    • In acidic medium: H2O2+2H++2e−→2H2O (oxidizes Fe2+ to Fe3+, PbS to PbSO4).
    • In basic medium: H2O2+2e−→2OH− (oxidizes Mn2+ to MnO2).
  • Reducing Agent: (Can also act as a reducing agent towards strong oxidizing agents).
    • In acidic medium: H2O2→O2+2H++2e− (reduces KMnO4, Cl2).
    • In basic medium: H2O2+2OH−→O2+2H2O+2e− (reduces KMnO4).
  • Bleaching Agent: Acts as a bleaching agent due to the release of nascent oxygen.
    • H2O2→H2O+[O] (nascent oxygen)
• Uses:
  • Antiseptic (perhydrol, 30% H2O2 solution)
  • Bleaching agent for textiles, paper, hair, etc.
  • Oxidizing agent in chemical synthesis.
  • Propellant in rockets.
  • For restoring the colour of old lead paintings (by oxidizing black PbS to white PbSO4).

8. Hydrogen Economy

• Concept: A future scenario where hydrogen gas is the primary fuel source for energy generation, transportation, and industrial processes.
• Advantages:
  • Clean Fuel: Combustion of H2 produces only water (H2O), eliminating pollution (2H2+O2→2H2O).
  • High Energy Content: Higher energy per unit mass compared to fossil fuels.
  • Renewable: Can be produced from water.
• Challenges:
  • Production Cost: Currently, H2 production is expensive and often relies on fossil fuels.
  • Storage and Transportation: H2 is a gas, difficult to store and transport efficiently. Requires high pressure tanks or cryogenic temperatures.
  • Safety: Highly flammable.

This comprehensive set of notes on Hydrogen and its Compounds covers all essential concepts for NEET and JEE Main. Pay close attention to its unique position, isotopic differences, various preparation methods (especially industrial ones), dual chemical nature (oxidizing and reducing) of H2O2, and the different types of hydrides. Practice related chemical equations and conceptual questions thoroughly.