Page 153 - ICSE Chemistry 8

• From hot water: Magnesium reacts with hot water, libera ng hydrogen and forming magnesium
hydroxide.
Mg + 2H O Mg(OH) 2 + H 
2
2
Magnesium Hot water Magnesium hydroxide Hydrogen
• From steam: Metals like aluminium, zinc and iron react with steam to liberate hydrogen gas and
form corresponding oxides.
2Al + 3H O Al O 3 + 3H 
2
2
2
Aluminium Water Aluminium oxide Hydrogen
(steam)
Zn + H O ZnO + H 
2
2
Zinc Water Zinc oxide Hydrogen
(steam)
3Fe + 4H O Fe O 4 + 4H 
3
2
2
Iron Water Iron oxide Hydrogen
(steam)
Thirst for Knowledge
Preparation of Hydrogen by Action of Dilute Acids
on Metals The reac on between iron and
steam is a reversible reac on.
Dilute sulphuric acid and dilute hydrochloric acid liberate hydrogen
when they are treated with metals placed above hydrogen in the ac vity series of metals.
Mg + H SO MgSO 4 + H 
2
4
2
Magnesium Sulphuric acid Magnesium sulphate Hydrogen
Zn + H SO ZnSO 4 + H 
4
2
2
Zinc Sulphuric acid Zinc sulphate Hydrogen
Zn + 2HCl ZnCl 2 + H 
2
Zinc Hydrochloric acid Zinc chloride Hydrogen
Fe + 2HCl FeCl 2 + H 
2
Iron Hydrochloric acid Iron chloride Hydrogen
• The reac on with potassium, sodium and calcium is explosive and dangerous. Such a reac on
must never be a empted.

• The reac on with lead stops because the product (PbCl or PbSO ) formed is insoluble and forms
4
2
a layer over the metal preven ng further reac on.
• Dilute nitric acid is not used to prepare hydrogen because it is a strong oxidising agent and liberates
nitric oxide or nitrogen dioxide.

Preparation of Hydrogen by Action of Alkalis on Metals

Metals like aluminium, zinc and lead react with hot alkali solu on and liberate hydrogen.
2Al + 2NaOH + 2H O 2NaAlO 2 + 3H 
2
2
Aluminium Sodium hydroxide Water Sodium aluminate Hydrogen
Zn + 2NaOH Na ZnO 2 + H 
2
2
Zinc Sodium hydroxide Sodium zincate Hydrogen

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