Question

laboratory preparation of Hydrogen...............

Answer 1

Preparation of Hydrogen

Hydrogen gas is prepared in the laboratory by the action of dilute hydrochloric acid or dilute sulphuric acid on granulated zinc.

Zn + 2HCL → ZnCL₂ + H₂↑
Zn + H2SO₄ → ZnSO₄ + H₂↑

Granulated zinc is commercial zinc obtained from molten zinc. It contains impurities like copper which have a slight catalytic effect on the reaction and speeds up the reaction.

Answer 2

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By the action of water on Active Metals

(1) Cold Water : Very active metals i.e. alkali and certain alkaline earth metals like Na, K , Ca react with water at room temperature evolving dihydrogen.

2Na + 2H2O ———–> 2NaOH + H2

2K + 2H2O ———–> 2KOH + H2

Ca + 2H2O ———–> 2(Ca)2OH + H2

The reaction with alkali metal is so vigorous and exothermic that the hydrogen involved catches fire. To slow down the reaction, amalgams of these metals are generally used.

In amalgams, only a small surface area of the metal comes in contact with water ,and ,therefore the reaction is slowed down.

(2) Boiling water : Less active metals like Zn, Mg , Al decompose boiling water liberating dihydrogen.

Zn + H2O ———–> ZnO + H2

Mg + H2O ———–> MgO + H2

2Al + 3H2O ———–> Al2O3+ H2