1. Position & Isotopes

Hydrogen is the first element (1s¹). It shows anomalous behavior because it resembles both Alkali Metals (Group 1) and Halogens (Group 17).

Resemblance with Alkali Metals: Electronic config (ns¹), forms unipositive ion (H⁺), forms oxides and halides.
Resemblance with Halogens: Ionization enthalpy is high, requires one electron to complete octet (He config), forms diatomic molecule (H₂).

Isotopes of Hydrogen

Isotope	Symbol	Neutrons	Nature
Protium	₁H¹	0	Common (99.9%)
Deuterium	₁H² (D)	1	Stable (Heavy)
Tritium	₁H³ (T)	2	Radioactive (β^-)

2. Preparation & Hydrides

Commercial Preparation:

Electrolysis of Water: With traces of acid/base using Pt electrodes. High purity H₂ (>99.95%) obtained from Ba(OH)₂.
Bosch Process: From Water Gas (CO + H₂).
C + H₂O → CO + H₂ (Water Gas Shift Reaction uses FeCrO₄ catalyst).

Types of Hydrides

1. Ionic (Saline) Hydrides:

Formed by s-block elements (NaH, CaH₂). Crystalline, non-volatile, conduct electricity in molten state.

2. Covalent (Molecular) Hydrides:

Formed by p-block elements. Classified into:

Electron Deficient: Group 13 (e.g., B₂H₆). Lewis Acids.
Electron Precise: Group 14 (e.g., CH₄). Tetrahedral.
Electron Rich: Group 15-17 (e.g., NH₃, H₂O). Have lone pairs (Lewis Bases).

3. Metallic (Interstitial) Hydrides:

Formed by d & f block. H occupies interstitial sites. Non-stoichiometric (e.g., LaH_2.87). Good for H-storage.

3. Water & Hardness

Hardness: Due to presence of Ca²⁺ and Mg²⁺ salts. It prevents lather formation with soap.

Type	Caused By	Removal Methods
Temporary	Bicarbonates (HCO₃^-) of Ca, Mg	1. Boiling 2. Clark's Method (Ca(OH)₂)
Permanent	Chlorides & Sulphates of Ca, Mg	1. Washing Soda (Na₂CO₃) 2. Calgon Method 3. Ion Exchange (Zeolite) 4. Synthetic Resin

4. Hydrogen Peroxide (H₂O₂)

Structure:
Non-planar structure. Often called "Open Book" structure. Dihedral angle is 111.5° in gas phase and 90.2° in solid phase.

Chemical Properties

Acts as both Oxidizing and Reducing agent in acidic and basic media.

Oxidizing: PbS(s) + 4H₂O₂ → PbSO₄ + 4H₂O (Bleaching action).
Reducing: HOCl + H₂O₂ → H₃O⁺ + Cl^- + O₂ (Antichlor).

5. Uses & Hydrogen Economy

Hydrogen Economy:

The transportation and storage of energy in the form of liquid or gaseous dihydrogen. Advantages:

High calorific value.
Pollution free (Product is water).
Used in fuel cells for electric power.

Specific Uses:

Liquid H₂: Rocket fuel.
Atomic Hydrogen Torch: For cutting and welding (temp ~4000 K).
Vegetable Ghee: Hydrogenation of vegetable oils.

Numericals & HOTS

Q1. H₂O₂ Molarity

Calculate the molarity of a "20 volume" hydrogen peroxide solution.

Solution:
Relationship: Volume Strength = 11.2 × Molarity (M)
20 = 11.2 × M
M = 20 / 11.2
M = 1.79 M

Q2. Percent Strength (w/V)

Calculate the percentage strength (w/V) of a "10 volume" H₂O₂ solution.

Solution:
1. Find Molarity: M = 10 / 11.2 = 0.89 M.
2. Strength (g/L) = Molarity × Molar Mass (34 g/mol)
Strength = 0.89 × 34 = 30.26 g/L.
3. % w/V (g per 100 mL) = 30.26 / 10
Ans: 3.03%

Q3. Hardness Calculation

A sample of hard water contains 120 mg of MgSO₄ per kg of water. Calculate the degree of hardness in ppm. (Molar mass MgSO₄ = 120).

Solution:
1. Convert MgSO₄ to CaCO₃ equivalent.
120 g MgSO₄ ≡ 100 g CaCO₃.
So, 120 mg MgSO₄ ≡ 100 mg CaCO₃.

2. Hardness in ppm = (Mass CaCO₃ eq / Mass Water) × 10⁶
Mass Water = 1 kg = 1000 g = 10⁶ mg.
ppm = (100 / 10⁶) × 10⁶
Hardness = 100 ppm

Q4. H₂ from Hydrolith

Hydrolith is Calcium Hydride (CaH₂). Calculate the volume of H₂ produced at STP when 42 g of CaH₂ reacts with excess water. (Ca=40, H=1).

Solution:
Reaction: CaH₂ + 2H₂O → Ca(OH)₂ + 2H₂.
Molar Mass CaH₂ = 40 + 2 = 42 g/mol.

1 mole (42 g) CaH₂ produces 2 moles of H₂.
Volume of 2 moles H₂ at STP = 2 × 22.4 L
Ans: 44.8 L

Q5. Neutrons in D₂O

Calculate the total number of neutrons present in 1 mole of Heavy Water (D₂O).

Solution:
Deuterium (₁H²): 1 Proton, 1 Neutron.
Oxygen (₈O¹⁶): 8 Protons, 8 Neutrons.

In D₂O molecule: 2(1) + 8 = 10 Neutrons.
In 1 Mole D₂O: 10 × N_A neutrons.
Ans: 6.022 × 10²⁴ neutrons

Q6. Lime Requirement (HOTS)

1000 Litres of water contains 16.2 g of Calcium Bicarbonate. Calculate the mass of Lime (Ca(OH)₂) required to soften this water. (MW Ca(HCO₃)₂ = 162).

Solution:
Reaction: Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃ + 2H₂O.
Stoichiometry: 1 mole Bicarbonate reacts with 1 mole Lime.

Moles of Bicarbonate = 16.2 g / 162 g/mol = 0.1 mol.
Moles of Lime required = 0.1 mol.
Mass of Lime = 0.1 × 74 g/mol
Ans: 7.4 g

Q7. Normality Calculation

Find the Normality of a "5.6 volume" H₂O₂ solution.

Solution:
Relationship: Volume Strength = 5.6 × Normality (N).
5.6 = 5.6 × N
N = 1.
Ans: 1 N

Q8. Saline Hydride Electrolysis

During the electrolysis of molten saline hydride (e.g., NaH), hydrogen gas is liberated at which electrode?

Solution:
Saline hydrides contain H^- (Hydride ion).
At Anode (Oxidation): 2H^- → H₂(g) + 2e^-.
Ans: Anode (This proves the existence of H^- ion).

Q9. Bleaching Mechanism (HOTS)

H₂O₂ acts as a bleaching agent. Is this bleaching permanent or temporary? Compare with SO₂.

Solution:
H₂O₂: Bleaches by Oxidation (Nascent Oxygen). The effect is Permanent.
SO₂: Bleaches by Reduction (Nascent Hydrogen). The effect is Temporary (colour restores on oxidation by air).

Q10. KMnO₄ Reaction

Why does acidified KMnO₄ solution become colourless when H₂O₂ is added to it?

Solution:
Here H₂O₂ acts as a Reducing Agent.
Reaction: 2MnO₄^- (Purple) + 5H₂O₂ + 6H⁺ → 2Mn²⁺ (Colourless) + 5O₂ + 8H₂O.
Reduction of Mn(+7) to Mn(+2) causes loss of colour.

Important Formulae & Reactions

1. Strength of H₂O₂

Volume Strength (V):

Vol. Strength = 11.2 × Molarity

Vol. Strength = 5.6 × Normality

Percentage Strength (% w/V):

% Strength = (Vol. Strength / 11.2) × (34 / 10)

(34 is Molar Mass of H₂O₂)

2. Water Hardness

Degree of Hardness (ppm):

ppm = (Mass of CaCO₃ eq. / Mass of Water) × 10⁶

Calgon Formula (Sodium Hexametaphosphate):

Na₆P₆O₁₈

(Commercially represented as Na₂[Na₄(PO₃)₆])

3. Key Preparation Reactions

Coal Gasification (Syn Gas):

C(s) + H₂O(g) → CO(g) + H₂(g)

Water Gas Shift Reaction:

CO(g) + H₂O(g) &xrightarrow{FeCrO_4} CO₂(g) + H₂(g)

Lab Prep (Amphoteric Nature):

Zn + 2NaOH → Na₂ZnO₂ + H₂

(Sodium Zincate is formed)

20 Golden Facts (NEET)

1. Tritium: The only radioactive isotope of Hydrogen. It is a low energy β^- emitter and has a half-life of 12.33 years.
2. Hydride Gap: Metals of Group 7, 8, and 9 do NOT form hydrides. This region in the periodic table is called the Hydride Gap. (Exception: CrH only).
3. Ortho vs Para Hydrogen: At room temp, H₂ is 75% Ortho (parallel nuclear spin) and 25% Para (anti-parallel). At very low temps, Para form is more stable (100% Para at 0K).
4. Density of Ice: Ice has a cage-like structure due to H-bonding, leaving vacant spaces. Thus, Density of Ice < Density of Water. Max density of water is at 4°C.
5. 10 Volume H₂O₂: This means 1 Litre of this H₂O₂ solution will produce 10 Litres of O₂ at STP. It corresponds to ~3% strength.
6. H₂O₂ Storage: It decomposes slowly on exposure to light. Stored in wax-lined glass or plastic bottles in dark. Urea is added as a stabilizer.
7. Temporary Hardness: Caused by Bicarbonates (HCO₃^-) of Ca and Mg. Removed simply by Boiling (converts to insoluble carbonates) or Clark's method.
8. Permanent Hardness: Caused by Chlorides and Sulphates. Removed by Washing Soda, Calgon, or Ion-Exchange (Zeolites/Resins). Boiling does NOT remove it.
9. Heavy Water (D₂O): Prepared by exhaustive electrolysis of water. Used as a moderator in nuclear reactors to slow down neutrons.
10. Non-stoichiometric Hydrides: Transition metals (d-block) form hydrides like LaH_2.87, TiH_1.5-1.8. They are good potential hydrogen storage media.
11. Antichlor: H₂O₂ is used as an antichlor to remove excess chlorine from fabrics after bleaching.
12. Structure of H₂O₂: It has a non-planar, Open Book structure. The dihedral angle is 111.5° in Gas phase and 90.2° in Solid phase.
13. Atomic Hydrogen Torch: Recombination of atomic hydrogen generates very high temp (4000 K), used for cutting and welding high-melting metals.
14. Nascent Hydrogen: Hydrogen produced in situ (e.g., Zn + H₂SO₄) is called Nascent Hydrogen. It is a much stronger reducing agent than molecular H₂.
15. Permutit/Zeolite: Hydrated Sodium Aluminium Silicate (NaAlSiO₄). Used for softening water by exchanging Na⁺ with Ca²⁺/Mg²⁺.
16. High Ionization Enthalpy: Hydrogen has a very high IE (1312 kJ/mol) for a single electron species, resembling Halogens more than Alkali metals in this aspect.
17. Oxidizing Action of H₂O₂: In acidic medium, H₂O₂ oxidizes Fe²⁺ to Fe³⁺ (Ferrous to Ferric) and PbS to PbSO₄ (Black to White).
18. Clark's Method: Calculates exact amount of Lime (Ca(OH)₂) needed. If excess lime is added, water becomes hard again due to formation of soluble Calcium Bicarbonate.
19. Synthetic Resin: Cation exchange resins (R-SO₃H) exchange H⁺ for Ca²⁺, and Anion exchange resins (R-NH₂) exchange OH^- for Cl^-. This produces demineralized water.
20. Fuel of Future: Dihydrogen yields more energy per unit mass than petrol (approx 3 times). However, storage (requires heavy cylinders) is the main challenge.

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