Oxygen (O): Chemical Properties and Reactions

Chemical Properties Overview

Oxygen (O, atomic number 8) is a highly reactive non-metal, a member of Group 16 (chalcogens) of the periodic table.

Reactivity: Oxygen is the second most electronegative element (after fluorine) and is a powerful oxidizing agent. It readily forms compounds with almost all other elements.
Electronegativity: High electronegativity (Pauling scale: 3.44) leads to the formation of polar bonds and a strong tendency to gain electrons.
Oxidation States: Typically exhibits -2 oxidation state in most compounds, but can show -1 (peroxides), -1/2 (superoxides), and +2 (oxygen difluoride, OF₂).
Allotropes: Dioxygen (O₂) and Ozone (O₃) are the two main allotropes.

Action of Air and Oxygen (Combustion/Oxidation)

Oxygen supports combustion and reacts vigorously with many elements and compounds, especially when heated.

With Non-metals

Carbon (C):
- Complete combustion:
```
C(s) + O₂(g) → CO₂(g)
```
- In limited oxygen (incomplete combustion):
```
2C(s) + O₂(g) → 2CO(g)
```
Sulfur (S):
```
S(s) + O₂(g) → SO₂(g)
```
Phosphorus (P₄):
- In excess oxygen (to form phosphorus pentoxide):
```
P₄(s) + 5O₂(g) → P₄O₁₀(s)
```
- In limited oxygen (to form phosphorus trioxide):
```
P₄(s) + 3O₂(g) → P₄O₆(s)
```

With Metals

Magnesium (Mg):
```
2Mg(s) + O₂(g) → 2MgO(s)
```
Iron (Fe):
- Burning iron (in pure oxygen, or with finely divided iron):
```
3Fe(s) + 2O₂(g) → Fe₃O₄(s)
```
- Rusting (slow oxidation in presence of moisture):
```
4Fe(s) + 3O₂(g) + nH₂O(l) → 2Fe₂O₃·nH₂O(s)
```
Sodium (Na):
- Forms sodium peroxide (major product with excess oxygen):
```
2Na(s) + O₂(g) → Na₂O₂(s)
```
- Forms sodium oxide (minor product, or with limited oxygen):
```
4Na(s) + O₂(g) → 2Na₂O(s)
```
Potassium (K):
- Forms potassium superoxide:
```
K(s) + O₂(g) → KO₂(s)
```

With Compounds (Hydrocarbons, Ammonia)

Methane (CH₄):

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

Ethanol (C₂H₅OH):

C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g)

Ammonia (NH₃):
- Catalytic oxidation (Ostwald process for nitric acid production, with Pt/Rh catalyst at high temp):
```
4NH₃(g) + 5O₂(g) \xrightarrow{Pt/Rh, 800^\circ C} 4NO(g) + 6H₂O(g)
```
- Complete oxidation (non-catalytic, in excess oxygen):
```
4NH₃(g) + 3O₂(g) → 2N₂(g) + 6H₂O(g)
```

Action of Water and Steam

Elemental oxygen gas (O₂) does not react chemically with water or steam. It is sparingly soluble in water, a property vital for aquatic life. The solubility of oxygen in water decreases with increasing temperature.

Action of Acids and Bases

Elemental oxygen gas (O₂) is a neutral gas and does not react with common acids or bases. Its oxides, however, can be acidic (e.g., CO₂, SO₂), basic (e.g., Na₂O, MgO), amphoteric (e.g., Al₂O₃, ZnO), or neutral (e.g., CO, NO).

Key Laboratory Test/Identification Reactions

Oxygen gas can be identified by the following test:

Glowing Splint Test:
- When a glowing (not flaming) wooden splint is inserted into a gas jar containing oxygen, the splint immediately re-ignites and bursts into flame. This is due to oxygen’s ability to support combustion.
- Observation: Glowing splint re-ignites.