Preparation of Hydrogen

Preparation of hydrogen:

The various methods for the preparation of hydrogen are given below:

1. From water: Alkali and certain alkali metals when reacted with cold water liberates hydrogen gas.
$$\ce{2Na + H2O->[\text{Cold water}]NaOH + H2 ^ }$$
$$\ce{2K + H2O->[\text{Cold water}]KOH + H2 ^ }$$
Things to know: The reaction of alkali metal is very vigorous and exothermic so that heat is evolved and may catches fire easily. So, amalgams are used in order to slow down the reaction.

2. From hot water: Less reactive metals like zinc, magnesium, aluminum gets decomposed in boiling water and liberates hydrogen.
$$\ce{Zn + H2O->[\text{Hot water}]ZnO + H2 ^ }$$
$$\ce{Mg + H2O->[\text{Hot water}]MgO + H2 ^ }$$
$$\ce{2Al + 3H2O->[\text{Hot water}]Al2O3 + 3H2 ^ }$$
3. From steam: Some metals like iron, tin, nickel when passed with steam decomposes to give hydrogen gas. This reaction is reversible.
$$\ce{2Cu + $\underset{\text{Steam}}{\ce{H2O}}$->$\underset{\text{Ferrosoferic oxide Oxide}}{\ce{Fe2O3}}$ + H2 ^ }$$
4. From metallic (ionic) hydride: Hydrides of alkali and alkaline earth metal reacts with water to give hydrogen.
$$\ce{LiH + H2O->LiOH + H2 ^ }$$
$$\ce{CaH2 + 2H2O->Ca(OH)2 + 2H2 ^ }$$

5. By the electrolysis of water: When water is electrolyzed in presence of some acid or base, hydrogen gas is evolved.
$$\ce{2H2O->[\text{Electrolysis}]H2 ^ + O2 ^ }$$

Note: When water is electrolyzed without the presence of acid or base. There will be no liberation of hydrogen gas. This is because pure water is a bad conductor of heat and electricity. And when acid and base is added the degree of ionization of water increases thereby decomposes to hydrogen and oxygen.

6. From acids: Acids when treated with metals that lie above the electrochemical series can displace hydrogen from their respective mineral acids.
$$\ce{Fe + 2HCl->FeCl2 + H2 ^ }$$
$$\ce{Zn + H2SO4->ZnSO4 + H2 ^ }$$
$$\ce{Mg + 2HCl->MgCl2 + H2 ^ }$$

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