Metals react with oxygen

Metals react differently with oxygen, water, acids, solutions of other metal salts, and non-metals.

Reaction of metals with oxygen

Almost all metals react with oxygen to form metal oxide, but the reactivity differs for different metals.

metal + oxygen → metal oxide

Sodium reacts with oxygen in air at room temperature to form sodium oxide (Na2O).

4Na + O2 → 2Na2O

Magnesium does not react with oxygen at room temperature but on heating, magnesium burns in air with intense light and heat to form magnesium oxide.

2Mg + O2 heat → 2MgO

Zinc burns in air only on strong heating to form zinc oxide.

2Zn + O2 heat → 2ZnO

Iron does not burn even on strong heating, but iron filings burn vigorously when sprinkled in the flame of the burner.

3Fe + 202 heat → Fe304

Copper is least reactive. It does not burn, but on heating, the hot metal is coated with a black coloured layer of copper oxide.

2Cu + O2 heat → 2CuO

Aluminium develops a thin oxide layer when exposed to air forming aluminium oxide.

4Al + 302 heat → 2Al2O3

Reaction of metal oxides with water: Most metal oxides are insoluble in water but some metal oxides dissolve in water to form alkalis, and they are basic in nature.

Sodium oxide (Na2O) and potassium oxide (K2O) dissolve in water to produce alkalis sodium hydroxide and potassium hydroxide, respectively.

Na2O + H20 → 2NaOH K20 + H20 → 2KOH

Anodising: Anodising is a process of forming a thick oxide layer of aluminium. Aluminium develops a thin oxide layer when exposed to air. This aluminium oxide coat makes it resistant to further corrosion. The resistance can be improved by making the oxide layer thicker.

In this technique aluminium article is used as an anode. Dilute sulphuric acid is used as electrolyte. The anode reaction results in formation of a black coloured thin film of aluminium oxide on the surface of anode.

By putting appropriate dyes in the electrolytic bath, coloured surface with decorative finish can be achieved.

Anodising is done to kitchen articles like anodised pressure cookers, anodised pans and also frames of sliding windows.

Amphoteric oxides: Metal oxides are usually basic in nature but some metal oxides react both with acids as well as bases to produce salt and water e.g. aluminium oxide Al2O3 and zinc oxide ZnO.

Al2O3 + 6HCl (acid) → 2AlCl3 (aluminium chloride salt) + 3H2O (water)

Al2O3 + 2NaOH (base) → 2NaAlO2 (sodium aluminate salt) + H20 (water)