Hydrogen

Hydrogen ($Z = 1$, configuration $1s^1$) is unique — it shows similarities with both alkali metals (Group 1) and halogens (Group 17). Its position in the periodic table is debated.

Resemblance with Alkali Metals:

• Electronic configuration: $ns^1$
• Forms unipositive ion (H⁺)
• Forms halides (HCl like NaCl)
• Reducing nature

Resemblance with Halogens:

• One electron short of noble gas configuration
• Forms diatomic molecule (H₂)
• Forms uninegative ion (H⁻, hydride)
• Combines with metals and non-metals

Unique Features:

• Smallest atom
• H⁺ is just a bare proton (no electrons) — different from any other cation
• Lower IE than expected for halogens
• Higher IE than alkali metals

Isotopes of Hydrogen:

Heavy Water (D₂O): $D_2O$ (deuterium oxide). Properties differ slightly from $H_2O$:

• Higher BP ($101.4°$C vs $100°$C)
• Higher MP ($3.8°$C vs $0°$C)
• Higher density ($1.106$ g/mL vs $1.000$)
• Used as moderator in nuclear reactors (slows neutrons without absorbing them)
• Toxic in large amounts to organisms

Ortho and Para Hydrogen:

• Ortho: nuclear spins parallel (75% at room T)
• Para: nuclear spins antiparallel (25% at room T)
• At low T (below 20 K), all converts to para (more stable)

Laboratory Preparation:

• Action of dilute HCl or H₂SO₄ on Zn:

$$\text{Zn} + 2\text{HCl} \to \text{ZnCl}_2 + \text{H}_2$$ $$\text{Zn} + \text{H}_2\text{SO}_4 \to \text{ZnSO}_4 + \text{H}_2$$

• (Pure H₂): Action of NaOH on Al or Zn:

$$2\text{Al} + 6\text{NaOH} \to 2\text{Na}_3\text{AlO}_3 + 3\text{H}_2$$

• Action of pure water on Na (violent — not preferred):

$$2\text{Na} + 2\text{H}_2\text{O} \to 2\text{NaOH} + \text{H}_2$$

Industrial Preparation:

Physical Properties:

• Colorless, odorless, tasteless, non-toxic gas
• Lightest gas (M = 2)
• BP $-253°$C, MP $-259°$C
• Slightly soluble in water; very soluble in metals like Pd (used for purification)

Chemical Properties:

1. Combustion: $2H_2 + O_2 \to 2H_2O + \text{heat}$ (used in rockets; produces water vapor only — clean fuel)
2. With halogens: $H_2 + X_2 \to 2HX$. Reactivity: F > Cl > Br > I.
3. With nitrogen (Haber): $N_2 + 3H_2 \rightleftharpoons 2NH_3$ (high P, T, Fe catalyst)
4. With metals (hydrides): $2\text{Na} + H_2 \to 2\text{NaH}$
5. Reducing agent: Reduces metal oxides ($\text{CuO} + H_2 \to \text{Cu} + \text{H}_2\text{O}$)
6. Hydrogenation: Catalytic addition to unsaturated C=C and C=O (Ni, Pd, Pt catalysts)

Uses of Hydrogen:

• Hydrogenation of oils → vanaspati ghee
• Ammonia synthesis (Haber)
• Production of methanol ($\text{CO} + 2H_2 \to \text{CH}_3\text{OH}$)
• Rocket fuel (with O₂)
• Atomic hydrogen torch (welding metals; very high temperature)
• Hydrogen as fuel (zero emissions; fuel cell)

Hydrides: Binary compounds of hydrogen with other elements. Classified by bonding character.

Three Major Types:

Ionic Hydrides:

• Formed by most reactive metals (Group 1 and heavier Group 2)
• Electrolysis liberates H₂ at anode (proves H⁻)
• React violently with water: $\text{NaH} + \text{H}_2\text{O} \to \text{NaOH} + H_2$

Covalent Hydrides are further classified:

Metallic Hydrides:

• Found in d- and f-block metals
• H₂ adsorbed in interstitial spaces of metal lattice
• Composition often non-stoichiometric (e.g., $TiH_{1.7}$)
• Used in H₂ storage technology

Some metals do NOT form stable hydrides:

• 7-11 groups of d-block (called "hydride gap": Cr, Mn, Fe, Co, Ni, Cu, Zn, etc.)
• This is where d-block has incomplete d-orbital filling/cohesion issues

Properties Comparison of Group 16 Hydrides:

H₂O has anomalously high BP due to H-bonding.

Water (H₂O): Universal solvent. High BP due to H-bonding. Max density at $4°$C (unique).

Properties:

• Amphoteric (acts as acid and base): $H_2O + HCl \to H_3O^+ + Cl^-$; $H_2O + NH_3 \to NH_4^+ + OH^-$
• Auto-ionization: $2H_2O \rightleftharpoons H_3O^+ + OH^-$, $K_w = 10^{-14}$ at $25°$C
• Hydration of ions (high dielectric constant ≈ $80$)
• Strong reducing properties in some reactions

Hardness of Water: Inability of water to form lather with soap. Caused by dissolved Ca²⁺ and Mg²⁺ ions (also Fe²⁺, sometimes).

Types:

Clark's process (for temporary hardness): $Ca(HCO_3)_2 + Ca(OH)_2 \to 2CaCO_3\downarrow + 2H_2O$

Ion exchange (permutit): $Na_2P + Ca^{2+} \to CaP + 2Na^+$. Regenerated with NaCl solution.

Calgon process: Sodium hexametaphosphate $Na_6P_6O_{18}$ binds Ca²⁺ and Mg²⁺.

Soap action with hard water: $2C_{17}H_{35}COONa + CaSO_4 \to (C_{17}H_{35}COO)_2Ca\downarrow + Na_2SO_4$ (scum)

Units of Hardness: ppm (mg CaCO₃ per L) or degrees Clark.

Hydrogen Peroxide (H₂O₂):

Preparation:

• $BaO_2 + H_2SO_4 \to BaSO_4 + H_2O_2$
• $2NH_4HSO_4 \xrightarrow{\text{electrolysis}} (NH_4)_2S_2O_8 \xrightarrow{H_2O} 2NH_4HSO_4 + H_2O_2$
• Auto-oxidation of 2-ethyl anthraquinol (industrial)

Structure: Non-planar; "open book" structure. O-O bond is single ($\sim$ $1.49$ Å). H-O-O-H dihedral angle $\sim 111°$ in gas, $\sim 90°$ in solid.

Properties:

• Colorless, viscous liquid (sometimes pale blue in pure form)
• Miscible with water
• Unstable; decomposes to $H_2O + O_2$ (catalyzed by MnO₂, Pt, light)

Concentration Expressions:

• % by mass (e.g., 30%)
• Volume strength: Volume of O₂ released at STP per volume of H₂O₂ solution. "$20$ V" means 1 L of H₂O₂ releases 20 L O₂.
• $\text{Volume strength} = 5.6 \times N$
• $\text{Volume strength} = 11.2 \times M$

Chemical Properties:

1. As oxidizing agent: $H_2O_2 + 2I^- + 2H^+ \to I_2 + 2H_2O$
2. As reducing agent: $H_2O_2 + Cl_2 \to 2HCl + O_2$; also reduces KMnO₄ in acidic medium
3. Acidic nature: Weak diprotic acid; $K_{a1} = 1.6 \times 10^{-12}$
4. Disproportionation: $2H_2O_2 \to 2H_2O + O_2$

Uses:

• Antiseptic (3% in dental rinses)
• Bleaching agent (paper, hair, textiles)
• Rocket propellant ($90\%$)
• Synthesis of organic compounds (epoxidation, hydroxylation)
• Pollution control (removes H₂S)