Oxygen

Occurrence

1.      In air, 21% by volume.

2.      In water, 89%, and dissolved in it.

3.      In metal and non-metal oxides.

Preparation

Lab method

1.      Heating of metal oxides

2Ag₂O Δ→ 4Ag + O_2↑

2HgO Δ → 2Hg + O_2↑

2Pb₃O₄ Δ → 6PbO + O_2↑

2PbO₂ Δ → 2PbO + O_2↑

2.      Heating hydrogen peroxide
2H₂O₂ Δ (MnO₂) → 2H₂O + O_2↑

3.      Heating of potassium chlorate
2KClO₃ Δ → 2KCl + 3O₂

4.      Reaction of water and sodium peroxide.
2Na₂O₂ + H₂O → 4NaOH + O_2↑

Industrial

1.      From air – dust is removed by filters, CO₂ is removed by potassium hydroxide and water is removed by a drying agent. Air is liquefied by compression and cooling. When warmed, liquid nitrogen boils away first (-196°C) and liquid oxygen next (-183°C). It is stored in cylinders.

2.      From water – the electrolysis of water yields oxygen at the anode.

Properties

Physical

1.      It is a colorless, odorless, tasteless, neutral gas.

2.      It is slightly soluble in water, and slightly heavier than air.

3.      It boils at -183°C.

Chemical

1. It does not burn, but supports combustion.
2. It forms oxides.
1. Acidic oxides – or acid anhydrides, combine with water to form acids.

S + O₂ → SO₂ then SO₂ + H₂O → H₂SO₃

C + O₂ → CO₂ then CO₂ + H₂O → H₂CO₃

4P + 5O₂ → 2P₂O₅ then P₂O₅ + 3H₂O → 2H₃PO₄

These react with bases:

SO₂ + NaOH → Na₂SO₃ + H₂O

CO₂ + NaOH → Na₂CO₃ + H₂O

P₂O₅ + 6NaOH → 2Na₃PO₄ + 3H₂O

2. Basic oxides – are oxides of metals – they react with acids

MgO + H₂SO₄ → MgSO₄ + H₂O

Fe₂O₃ + 6HCl → 2FeCl₃ + 3H₂O

CaO + H₂SO₄ → CaSO₄ + H₂O

Some basic oxides react with water to form alkalis

K₂O + H₂O → 2KOH

Na₂O + H₂O → 2NaOH

CaO + H₂O → Ca(OH)₂

3. Neutral oxides – for example, H₂O, CO, NO, etc.
4. Amphoteric oxides – act as both acids and bases.

ZnO + H₂SO₄ → ZnSO₄ + H₂O

ZnO + 2NaOH → Na₂ZnO₂ + H₂O

3.  Oxygen oxidizes lower oxides                    

2NO + O_{2 (Pt)}→ 2NO₂
2SO₂ +  O₂ → 2SO₃
2CO + O₂ → 2CO₂

4.  Oxidation

2H₂S + 3O₂ → 2H₂O + 2SO₂

CH₄ + 2O₂ → CO₂ + 2H₂O

2ZnS + 3O₂ → 2ZnO + 2SO₂

4NH₃ + 5O_{2 (Pt, 800°C)} → 4NO + 6H₂O

Confirmatory Tests

1. It rekindles a glowing splint
2. In contact with colorless nitric oxide, it produces reddish-brown fumes of nitrogen dioxide
3. It is absorbed by a solution of pyrogallol, which turns brown

Uses

1. For respiration
2. In medicine
3. In oxy-acetylene torch
4. In iron and steel production, to remove impurities
5. In explosives
6. In rockets, liquid oxygen is used for combustion