Hydrogen

Hydrogen is the first element in the periodic table. It has a single electron and proton. It exists as a diatomic gas (H₂) where two hydrogen atoms are covalently bonded. Its atomic number is 1 and the electronic configuration is 1s¹. It can show oxidation states of +1 and -1.

Isotopes of Hydrogen

Hydrogen has three isotopes:

• Protium – ¹H (Hydrogen)
• Deuterium – ²H or D (Heavy hydrogen)
• Tritium – ³H or T (Radioactive)

These isotopes have similar chemical properties but differ in their physical properties.

Unique Position of Hydrogen

Hydrogen can lose its one electron to form H⁺, like alkali metals (Group 1). However, unlike alkali metals, H⁺ is not stable. It can also gain an electron to form H⁻ (hydride ion), similar to halogens (Group 7).

Hydrogen forms compounds like:

• LiH, NaH, CaH₂ (like halides: LiCl, NaCl, CaCl₂)

It is similar to halogens in that it's a non-metal, forms covalent compounds, and exists as diatomic molecules. Hydrogen has comparable ionization energy to halogens but much higher than alkali metals.

Laboratory Preparation of Hydrogen

Hydrogen can be prepared by the reaction of active metals with dilute mineral acids or steam:

Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)

Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g)

Hydrogen is dried using anhydrous calcium chloride.

Other Methods of Laboratory Preparation

1. Reaction of zinc or aluminium with sodium hydroxide:

   Zn(s) + 2NaOH(aq) → Na2ZnO2(s) + H2(g)

   2Al(s) + 2NaOH(aq) + 2H2O(l) → 2NaAlO2(s) + 3H2(g)

2. Action of steam on heated metals:

   3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)

Industrial Preparation of Hydrogen

1. From Water Gas (Bosch Process)

CO(g) + H2O(g) ⇌ CO2(g) + H2(g)

CO₂ is removed using water or caustic soda to isolate hydrogen.

2. From Natural Gas (Methane)

CH4(g) → 2H2(g) + C(s)

3. From Hydrocarbons

CH4(g) + H2O(g) → CO(g) + 3H2(g)

Then:

CO(g) + H2O(g) ⇌ CO2(g) + H2(g)

4. From Coal

C(s) + H2O(g) → CO(g) + H2(g)

5. Electrolysis of Brine

Hydrogen is produced as a by-product in the electrolysis of brine or acidified water.

Physical Properties of Hydrogen

• Colourless, odourless, and tasteless gas
• Highly flammable
• Combustible but does not support combustion
• Diatomic (H₂)
• Low density (14.4 times lighter than air)
• Liquefies at high pressure and -235°C
• Insoluble in water

Chemical Properties of Hydrogen

Hydride Formation

Metallic Hydrides

2Li(s) + H2(g) → 2LiH(s)

Ca(s) + H2(g) → CaH2(s)

CaH2(s) + 2H2O(l) → Ca(OH)2(aq) + 2H2(g)

Non-metallic Hydrides

F2(g) + H2(g) → 2HF(g)

Cl2(g) + H2(g) → 2HCl(g)

H2(g) + S(s) → H2S(g)

N2(g) + 3H2(g) → 2NH3(g)

Reducing Properties

PbO(s) + H2(g) → Pb(s) + H2O(g)

Fe3O4(s) + 4H2(g) ⇌ 3Fe(s) + 4H2O(g)

CuO(s) + H2(g) → Cu(s) + H2O(g)

Fe2O3(s) + 3H2(g) → 2Fe(s) + 3H2O(g)

ZnO(s) + H2(g) → Zn(s) + H2O(g)

Combustion

2H2(g) + O2(g) → 2H2O(g)

Uses of Hydrogen

1. Production of ammonia:

   N2(g) + 3H2(g) ⇌ 2NH3(g)

2. Hydrogenation of fats and oils for making margarine and soap
3. Inflating balloons and airships (due to its low density)
4. Used in oxy-hydrogen flames to produce high temperatures
5. Used in the production of methanol and hydrochloric acid
6. Used in the production of synthetic petroleum
7. Component of coal gas and water gas; liquid hydrogen is used in rocket fuel

Compounds of Hydrogen

• Alkali and alkaline earth metal hydrides are crystalline solids with high melting points that conduct electricity when molten and react with water:

  CaH2 + 2H2O → Ca(OH)2 + 2H2

• Boron and aluminium form complex covalent hydrides like lithium tetrahydridoaluminate (III) and sodium tetrahydridoborate (III) used in organic chemistry as reducing agents.

Test for Hydrogen

When a lighted splint is inserted into a jar of hydrogen gas, it produces a pop sound.