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Revision Guide • Class 10-12 / JEE / NEET

Hydrogen (H): Comprehensive Study Guide for CBSE, JEE, NEET

By Periodic Table India
CBSE / JEE Prep Notes
Hydrogen H Chemistry Elements JEE NEET CBSE Inorganic Chemistry Periodic Table

Introduction

Hydrogen (H) is the first element in the periodic table, possessing the simplest atomic structure. It is the most abundant chemical substance in the universe, comprising approximately 75% of all normal matter. On Earth, hydrogen is primarily found in combined forms, notably in water, hydrocarbons, and various organic compounds, playing a fundamental role in both natural processes and industrial applications. Its unique position and properties make it crucial for understanding chemistry, biochemistry, and astrophysics.

CBSE/JEE Quick Revision Notes

  • Atomic Number (Z): 1
  • Atomic Mass (A): 1.008 u (or amu)
  • Electronic Configuration: $1s^1$
  • Valency: 1 (can exhibit +1 or -1 oxidation state)
  • Group: 1 (occasionally placed separately due to its non-metallic character)
  • Period: 1
  • Nature: Non-metal
  • State at STP: Gaseous (colorless, odorless, tasteless diatomic gas, $H_2$)
  • Isotopes:
    • Protium ($^1_1$H): No neutrons (most common, ~99.98%)
    • Deuterium ($^2_1$H or D): One neutron (heavy hydrogen)
    • Tritium ($^3_1$H or T): Two neutrons (radioactive)
  • Ionization Enthalpy: High (1312 kJ/mol), comparable to halogens.
  • Electronegativity: 2.20 on the Pauling scale.

Electron Configuration & Bonding Behavior

Hydrogen’s electronic configuration is $1s^1$. This unique configuration leads to its dual nature in chemical bonding:

  1. Electropositive Character (Formation of $H^+$):

    • Hydrogen can lose its single valence electron to form a bare proton ($H^+$).
    • This behavior is analogous to alkali metals (Group 1).
    • Due to its extremely small size and high charge density, $H^+$ is highly unstable and never exists freely. It always associates with other molecules, primarily water, to form hydronium ion ($H_3O^+$) or other protonated species.
    • Example: In acids like $HCl \rightarrow H^+ + Cl^-$.

  2. Electronegative Character (Formation of $H^-$):

    • Hydrogen can gain an electron to achieve a stable $1s^2$ (duet) configuration, forming the hydride ion ($H^-$).
    • This behavior is analogous to halogens (Group 17), which gain an electron to complete their octet.
    • $H^-$ ions are typically formed when hydrogen reacts with highly electropositive metals (Group 1 and 2), forming ionic hydrides.
    • Example: $Na + H_2 \rightarrow NaH$ (sodium hydride).

  3. Covalent Character (Formation of Covalent Bonds):

    • Hydrogen readily shares its single electron with other elements to form a single covalent bond, completing its duet.
    • This is the most common bonding mode for hydrogen, especially with non-metals.
    • Examples:
      • $H_2$ (hydrogen molecule)
      • $CH_4$ (methane)
      • $H_2O$ (water)
      • $HCl$ (hydrogen chloride)

Crucial Chemical Reactions

1. Reaction with Oxygen (Combustion)

Hydrogen is highly flammable and burns with a pale blue flame to form water, releasing significant heat.

$2H_2(g) + O_2(g) \rightarrow 2H_2O(l) \quad (\Delta H = -285.8 , \text{kJ/mol})$

2. Reaction with Halogens ($X_2$)

Hydrogen reacts with halogens to form hydrogen halides ($HX$). The reactivity decreases from fluorine to iodine.

  • Fluorine: Explosive reaction, even in the dark and at low temperatures. $H_2(g) + F_2(g) \rightarrow 2HF(g)$
  • Chlorine: Requires light or heat. $H_2(g) + Cl_2(g) \xrightarrow{\text{light}} 2HCl(g)$
  • Bromine: Requires heat. $H_2(g) + Br_2(g) \xrightarrow{\text{heat}} 2HBr(g)$
  • Iodine: Reversible reaction, requires higher temperature and is endothermic. $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$

3. Reaction with Nitrogen (Haber-Bosch Process)

Hydrogen reacts with nitrogen to produce ammonia, a vital industrial process.

$N_2(g) + 3H_2(g) \xrightarrow{\text{Fe catalyst, high T, P}} 2NH_3(g)$

4. Reaction with Metals (Formation of Hydrides)

Hydrogen forms ionic (saline) hydrides with Group 1 and 2 metals (except Be).

  • $2Na(s) + H_2(g) \xrightarrow{300-400^\circ C} 2NaH(s)$
  • $Ca(s) + H_2(g) \xrightarrow{400^\circ C} CaH_2(s)$

5. Reducing Agent Properties

Hydrogen can reduce oxides of less reactive metals to the respective metals.

  • $CuO(s) + H_2(g) \xrightarrow{\text{heat}} Cu(s) + H_2O(l)$
  • $WO_3(s) + 3H_2(g) \xrightarrow{\text{heat}} W(s) + 3H_2O(l)$
  • $Fe_3O_4(s) + 4H_2(g) \xrightarrow{\text{heat}} 3Fe(s) + 4H_2O(l)$

6. Hydrogenation of Unsaturated Hydrocarbons

Hydrogen adds across double or triple bonds in the presence of catalysts (Ni, Pt, Pd) to form saturated compounds. This process is important in the food industry.

$R-CH=CH-R’ + H_2 \xrightarrow{\text{Ni, Pt or Pd catalyst}} R-CH_2-CH_2-R’$ (e.g., conversion of vegetable oil to vegetable ghee)

7. Water Gas Shift Reaction (for H$_2$ production)

This reaction is used to increase the yield of hydrogen from water gas (a mixture of CO and H$_2$).

$CO(g) + H_2O(g) \xrightarrow{\text{FeCrO}_4 \text{ catalyst, 400}^\circ C} CO_2(g) + H_2(g)$

Industrial and Biological Importance

Industrial Importance

  1. Ammonia Synthesis: Most significant use, primarily for fertilizers via the Haber-Bosch process.
  2. Methanol Synthesis: Used in the production of methanol ($CH_3OH$), a solvent and fuel. $CO(g) + 2H_2(g) \xrightarrow{\text{ZnO/Cr}_2O_3} CH_3OH(l)$
  3. Hydrogenation of Oils: Conversion of unsaturated vegetable oils into solid fats (vanaspati ghee/margarine).
  4. Hydrochloric Acid Production: Used in the chemical industry.
  5. Rocket Fuel: Liquid hydrogen is a powerful rocket propellant.
  6. Metallurgy: Used as a reducing agent in the extraction of certain metals from their oxides.
  7. Fuel Cells: Emerging as a clean energy carrier in fuel cells, producing electricity with water as the only byproduct.
  8. Welding: Atomic hydrogen torch provides extremely high temperatures for welding.

Biological Importance

  1. Water: Hydrogen is a fundamental component of water ($H_2O$), which is essential for all known forms of life. It acts as a solvent and participates in countless biochemical reactions.
  2. Organic Molecules: It is a ubiquitous element in all organic compounds, including carbohydrates, proteins, lipids, and nucleic acids (DNA, RNA), forming the structural backbone of life.
  3. pH Regulation: The concentration of hydrogen ions ($H^+$) dictates the pH of biological fluids, a critical factor for enzyme activity and cellular processes.
  4. Energy Transfer: Proton gradients (differences in $H^+$ concentration) across membranes are central to cellular respiration and photosynthesis, driving the synthesis of ATP (adenosine triphosphate), the primary energy currency of cells.
  5. Hydrogen Bonds: Hydrogen bonding, involving hydrogen atoms, plays a crucial role in maintaining the structure and function of macromolecules like proteins and DNA.
H

Hydrogen (H)

Atomic Number 1

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