Hydrogen

Position of Hydrogen in Periodic Table

Hydrogen is the first element in the periodic table. It is placed in no specific group due to its dual nature: it can lose an electron to form H⁺ (like alkali metals) or gain an electron to form H⁻ (like halogens).

Similarities with Alkali Metals (Group I)

One electron in outer shell (1s¹).
Forms monovalent H⁺ ion like Li⁺, Na⁺.
Valency of 1.
Oxide H₂O stable like Li₂O, Na₂O.
Good reducing agent.

Similarities with Halogens (Group VIIA)

Diatomic like F₂, Cl₂.
Forms anion H⁻ by gaining one electron like F⁻, Cl⁻.
Stable inert gas configuration.
One electron short of duplet like halogens short of octet.
IE of H (1312 kJ/mol) similar to halogens.

Differences: High IE compared to alkali metals, small H⁺ size, forms stable hydrides only with strongly electropositive metals due to low electron affinity (72.8 kJ/mol).

Placed in Group I or VII due to anomalous behavior.

Discovery and Occurrence

Discovered by Henry Cavendish in 1766, named by Lavoisier. 9th most abundant element in Earth's crust.

Exists as diatomic H₂, triatomic as Hyzone. Systematic name of water is oxidane.

Preparation of Dihydrogen

By Action of Water with Metals

Active metals (Na, K) at room temp: 2M + 2H₂O → 2MOH + H₂

Less active (Ca, Zn, Mg, Al) on heating: e.g., 2Al + 3H₂O → Al₂O₃ + 3H₂

Fe, Ni, Co, Sn with steam: 3Fe + 4H₂O → Fe₃O₄ + 4H₂

By Water on Hydrides

NaH + H₂O → NaOH + H₂

CaH₂ + 2H₂O → Ca(OH)₂ + 2H₂

By Metals with Alkalies

Zn + 2NaOH → Na₂ZnO₂ + H₂

2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂

By Acids on Metals

Fe + 2HCl → FeCl₂ + H₂

By Electrolysis of Acidified Water

2H₂O → 2H₂ (cathode) + O₂ (anode)

Laboratory Method

Zn + H₂SO₄ (dil) → ZnSO₄ + H₂

Pure Hydrogen

Mg + H₂SO₄ → MgSO₄ + H₂

Electrolysis of warm aq. Ba(OH)₂

NaH + H₂O → NaOH + H₂

2Al + 2KOH + 2H₂O → 2KAlO₂ + 3H₂

Commercial Production

Bosch process: C + H₂O → CO + H₂; CO + H₂O → CO₂ + H₂

Lane’s process: 3Fe + 4H₂O → Fe₃O₄ + 4H₂

Electrolysis of water

From hydrocarbons: CH₄ + H₂O → CO + 3H₂

By-product of brine electrolysis

Physical Properties of Dihydrogen

Colourless, tasteless, odourless gas. Slightly soluble in water. Highly combustible.

Property	Value
Atomic radius (pm)	37
Ionic radius of H⁻ (pm)	210
Ionisation energy (kJ/mol)	1312
Electron affinity (kJ/mol)	-72.8
Electronegativity	2.1

Chemical Properties of Dihydrogen

Stable, dissociates above 2000 K. Bond energy 435.9 kJ/mol.

With Metals

Forms hydrides: 2Na + H₂ → 2NaH; Ca + H₂ → CaH₂

Interstitial hydrides with transition metals (occlusion).

With Non-Metals

H₂ + O₂ → 2H₂O

N₂ + 3H₂ → 2NH₃

H₂ + F₂ → 2HF (dark)

Reactivity: F₂ > Cl₂ > Br₂ > I₂

With Unsaturated Hydrocarbons

CH₂=CH₂ + H₂ → CH₃-CH₃

HC≡CH + 2H₂ → CH₃-CH₃

Hydrogenation of oils to fats.

Uses of Dihydrogen

Reducing agent
Hydrogenation of oils
Rocket fuel (liquid H₂)
Synthetic petrol
Compounds like NH₃, CH₃OH, urea
Oxy-hydrogen torch (2500°C), atomic hydrogen torch (4000°C)

Different Forms of Hydrogen

Atomic Hydrogen

From dissociation of H₂ at 4000-4500°C via electric arc. Extremely reactive, stable for fraction of second.

H₂ → 2H (ΔH = 435.9 kJ/mol)

Nascent Hydrogen

Newly born H in reaction mixture, more reactive than ordinary H.

E.g., Zn + H₂SO₄ → [2H] (nascent) reduces KMnO₄, while molecular H₂ does not.

Ortho and Para Hydrogen

Based on nuclear spin: Ortho (same direction, 3:1 at room temp), Para (opposite, stable at 0 K).

Ratio varies with temp: 1:1 at liquefaction, 3:1 at room temp. Pure para at 20 K, max 75% ortho.

Hydrides

Binary compounds MHₓ or MₘHₙ.

Saline/Ionic

s-block: e.g., NaH, CaH₂. Rock-salt structure, covalent for BeH₂, MgH₂.

Stability: LiH > NaH > KH > RbH > CsH; CaH₂ > SrH₂ > BaH₂.

Metallic/Interstitial

d/f-block: Non-stoichiometric, e.g., ZrHₓ (1.30≤x≤1.75). Good conductors.

Molecular/Covalent

p-block: e.g., CH₄, NH₃, H₂O, HF.

Stability decreases down group: NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃.

Increases with EN: CH₄ < NH₃ < H₂O < HF.

Types: Electron rich (H₂O, NH₃, HF), precise (CH₄), deficient (B₂H₆).

Isotopes of Hydrogen

Name	Symbol	Atomic No.	Mass No.	Abundance	Nature
Protium	¹H	1	1	99.985%	Non-radioactive
Deuterium	²H or D	1	2	0.015%	Non-radioactive
Tritium	³H or T	1	3	10⁻¹⁵%	Radioactive

Property	H₂	D₂	T₂
Mol. mass	2.016	4.028	6.03
M.p. (K)	13.8	18.7	20.63
B.p. (K)	20.4	23.9	25.0
Heat of fusion (kJ/mol)	0.117	0.197	0.250
Heat of vap. (kJ/mol)	0.994	1.126	1.393
Bond energy (kJ/mol)	435.9	443.4	446.9

Isotopic effect: Quantitative differences in chemical properties due to mass differences, e.g., H₂ + Cl₂ 13.4 times faster than D₂ + Cl₂.

Water

Oxide of hydrogen, 65% of body, principal constituent of Earth's surface.

Structure

Angular/bent due to lone pairs, H-O-H angle 104.5°, dipole moment 1.84 D. In ice, tetrahedral with H-bonds, lower density than water (floats). Max density 1 g/cm³ at 4°C.

Heavy Water (D₂O)

Discovered by Urey. By-product of H₂ electrolysis. Uses: Moderator in nuclear reactors, reaction mechanisms, preparation of D compounds.

Constant	H₂O	D₂O
Mol. mass	18.015	20.028
Max density (g/cm³)	1.000	1.106
M.p. (K)	273.2	276.8
B.p. (K)	373.2	374.4
Heat of fusion (kJ/mol)	6.01	6.28
Heat of vap. (kJ/mol)	40.66	41.61
Heat of formation (kJ/mol)	-285.9	-294.6
Ionisation constant	1.008×10⁻¹⁴	1.95×10⁻¹⁵

Chemical Properties

Dissociation: 2H₂O ⇌ H₃O⁺ + OH⁻ (K_w = 10⁻¹⁴ at 298 K)

Amphoteric: Acts as acid/base.

Oxidising/reducing: e.g., 2Na + 2H₂O → 2NaOH + H₂ (oxidising); 2F₂ + 2H₂O → 4HF + O₂ (reducing)

Hydrolytic: SO₃ + H₂O → H₂SO₄; Mg₃N₂ + 6H₂O → 3Mg(OH)₂ + 2NH₃

Hydrates: [Ni(H₂O)₆](NO₃)₂; CuSO₄·5H₂O

Hard and Soft Water

Soft: Lathers with soap (distilled, rain).

Hard: Does not lather (bicarbonates, chlorides, sulphates of Ca/Mg).

Temporary: Bicarbonates, removed by boiling or Clark's method (lime).

Permanent: Chlorides/sulphates, removed by washing soda, permutit (ion exchange).

Hydrogen Peroxide

Discovered by Thenard.

Preparation

Lab: Na₂O₂ + H₂SO₄ → Na₂SO₄ + H₂O₂

BaO₂·8H₂O + H₂SO₄ → BaSO₄ + H₂O₂ + 8H₂O

Industrial: Electrolysis of 50% H₂SO₄ → H₂S₂O₈ → H₂O₂

Redox: 2-Ethylanthraquinol + O₂ → 2-Ethylanthraquinone + H₂O₂

Physical Properties

Pale blue syrupy liquid, freezes -0.5°C, density 1.4. Diamagnetic, highly associated via H-bonds. Better polar solvent than H₂O but strong autooxidant. Dipole 2.1 D.

Chemical Properties

Decomposition: 2H₂O₂ → 2H₂O + O₂ (ΔH = -196 kJ)

Oxidising: In neutral/acidic/alkaline media, e.g., 2KI + H₂O₂ → 2KOH + I₂

Reducing: H₂O₂ + O₃ → H₂O + 2O₂

Bleaching: H₂O₂ → H₂O + O (oxidises colouring matter)

Acidic: K_a = 1.55×10⁻¹², forms salts like NaHO₂, Na₂O₂

Addition: CH₂=CH₂ + H₂O₂ → HO-CH₂-CH₂-OH

Structure

Non-linear, non-planar, open book. O-O peroxy linkage. Gas: 94.8° dihedral, 111.5° H-O-O; Solid: 90.2° dihedral, 101.9° H-O-O.

Concentration and Storage

Concentrate by evaporation, vacuum desiccation, distillation. Stored in wax-coated glass/plastic/teflon with stabilizers like acid, glycerol, H₃PO₄.

Uses

Bleaching delicate items
Restoring lead paintings
Aerating agent for sponge rubber
Antiseptic (perhydrol)
Sodium perborate/percarbonate for detergents
Antichlor
Oxidant for rocket fuel
Detection of Ti, V, Cr
Production of epoxides, pharmaceuticals
Pollution control

Important Points & Tips

Key JEE/NEET Points

Dual position of H
Preparation methods
Ortho/para forms
Hydride types
Isotopes and effects
Water hardness removal
H₂O₂ properties/uses

Do's

Distinguish ortho/para

Recall hydride stability

Understand H₂O₂ reactions

Don'ts

Confuse nascent/atomic

Forget heavy water uses

Ignore isotopic effects

JEE Main Weightage

Typically 1-2 questions. Focus on properties, preparation, isotopes, hydrides, H₂O₂.

Weightage Medium