Exam Master Review Sheet - Hydrogen (JEE & NEET)

CHEMCA

EXAM MASTER REVIEW SHEET

Inorganic Chemistry: Hydrogen & Its Compounds

Comprehensive Review for JEE Main, Advanced & NEET

1 Isotopes & Spin Isomers

Isotope	Symbol	Composition	Key Remarks
Protium	$^1_1H$	$1p, 0n$	Most abundant (>99.98%)
Deuterium	$^2_1H$ (D)	$1p, 1n$	Used as moderator in reactors
Tritium	$^3_1H$ (T)	$1p, 2n$	Radioactive ($\beta^-$ emitter)

Nuclear Spin Isomers (Ortho vs Para)

Molecular hydrogen ($H_2$) exists in two forms based on the relative nuclear spins of the two hydrogen atoms.

Ortho-Hydrogen: Nuclear spins are Parallel ($\uparrow \uparrow$). More stable at room temperature.
Para-Hydrogen: Nuclear spins are Antiparallel ($\uparrow \downarrow$). More stable at low temperatures.

At 0 K: 100% Para | At Room Temp: 75% Ortho, 25% Para

2 Preparation of Dihydrogen ($H_2$)

Laboratory Methods

Acid on Metals: Active metals (Zn, Mg, Fe) + dilute acids.
$Zn + H_2SO_4 \to ZnSO_4 + H_2 \uparrow$
Alkali on Amphoteric Metals: Metals like Zn, Al, Sn, Pb react with boiling NaOH.
$Zn + 2NaOH \to \underbrace{Na_2ZnO_2}_{\text{Sod. Zincate}} + H_2 \uparrow$

Industrial Methods

Coal Gasification / Bosch Process:
$C(s) + H_2O(g) \xrightarrow{1270K} \underbrace{CO(g) + H_2(g)}_{\text{Water Gas/Syngas}}$
Water-Gas Shift Reaction: Increases $H_2$ yield. CO is removed by scrubbing with sodium arsenite.
$CO + H_2O(g) \xrightarrow[473K]{FeCrO_4} CO_2 + H_2$
High Purity $H_2$ (>99.95%): Electrolysis of warm aqueous Barium Hydroxide ($Ba(OH)_2$) between Nickel electrodes.

3 Classification of Hydrides

1. Ionic / Saline Hydrides

Formed by highly electropositive s-block elements (except Be, Mg). They are crystalline, non-volatile, and non-conducting in solid state.

$NaH + H_2O \to NaOH + H_2 \uparrow$

Conduct electricity in melt, liberating $H_2$ at the anode.

2. Covalent / Molecular

Formed by p-block elements. Volatile compounds with low b.p.

• Electron Deficient: Gr 13 ($B_2H_6$). Acts as Lewis acids.
• Electron Precise: Gr 14 ($CH_4$). Perfect octet.
• Electron Rich: Gr 15-17 ($NH_3, H_2O, HF$). Possess lone pairs, act as Lewis bases.

3. Metallic / Interstitial

Formed by d- and f-block elements. They are almost always non-stoichiometric (e.g., $LaH_{2.87}, TiH_{1.5-1.8}$).

Hydride Gap: Metals of Group 7, 8, and 9 do not form hydrides.

4 Water: Hardness & Heavy Water ($D_2O$)

Temporary Hardness

Due to presence of soluble Bicarbonates of Calcium and Magnesium ($Ca(HCO_3)_2, Mg(HCO_3)_2$).

Removal Methods:

Boiling: Soluble bicarbonates convert to insoluble carbonates/hydroxides.
Clark's Method: Calculated amount of Lime ($Ca(OH)_2$) is added to precipitate $CaCO_3$.

Permanent Hardness

Due to presence of soluble Chlorides and Sulphates of Calcium and Magnesium.

Removal Methods:

Washing Soda ($Na_2CO_3$): Precipitates $Ca/Mg$ as carbonates.
Calgon's Process: Sodium hexametaphosphate ($Na_6P_6O_{18}$) forms soluble complex with $Ca^{2+}/Mg^{2+}$, keeping them in solution but inactive.
Permutit / Zeolite Process: Hydrated sodium aluminium silicate ($Na_2Z$) exchanges its $Na^+$ ions for $Ca^{2+}/Mg^{2+}$.

Heavy Water ($D_2O$)

Prepared by exhaustive electrolysis of ordinary water. (Protium is liberated faster than Deuterium at the cathode).

Exchange Reactions:

Compounds containing labile hydrogen exchange it with Deuterium when treated with $D_2O$.

$NaOH + D_2O \to NaOD + HOD$
$HCl + D_2O \to DCl + HOD$

Crucial use: Used as a moderator in nuclear reactors to slow down fast-moving neutrons.

5 Hydrogen Peroxide ($H_2O_2$)

Structure & Storage

$H_2O_2$ has a non-planar "Open Book" structure. The dihedral angle differs in phases:

Gas Phase: $111.5^\circ$
Solid Phase (Crystal): $90.2^\circ$

Storage: Decomposes slowly on exposure to light ($2H_2O_2 \to 2H_2O + O_2$). It is stored in dark, wax-lined glass or plastic vessels (rough surfaces/alkali trace from glass catalyzes decomposition). Urea is added as a negative catalyst (stabilizer).

Chemical Properties (Dual Nature)

$H_2O_2$ acts as both an oxidizing and reducing agent in both acidic and alkaline media.

Oxidizing Action (Acidic): $PbS + 4H_2O_2 \to \underbrace{PbSO_4}_{\text{White}} + 4H_2O$

Reducing Action (Acidic): $2MnO_4^- + 6H^+ + 5H_2O_2 \to 2Mn^{2+} + 8H_2O + 5O_2$

Oxidizing Action (Basic): $Mn^{2+} + H_2O_2 \to Mn^{4+} + 2OH^-$

Volume Strength Calculations (Highly Tested)

The concentration of $H_2O_2$ is often expressed as "Volume Strength". For example, "10V $H_2O_2$" means that 1 Liter of this $H_2O_2$ solution will yield 10 Liters of $O_2$ gas at STP upon complete decomposition.

Molarity ($M$)

$ M = \frac{\text{Volume Strength}}{11.2} $

Normality ($N$)

$ N = \frac{\text{Volume Strength}}{5.6} $

($n$-factor for $H_2O_2$ = 2)

% Strength (w/v)

$ \% = \frac{17 \times \text{Vol. Str.}}{56} $

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Exam Master Review Sheet - Hydrogen