Page 159 - ICSE Chemistry 8

• Reac on with carbon: Hydrogen combines with carbon to form compounds called hydrocarbons.
This reac on occurs at a temperature above 520°C in the presence of nickel or cobalt ac ng as a
catalyst.
2H (g) + C(s) CH (g)
2
4
Hydrogen Carbon Methane
The reac on can be reversed on further raising the temperature.

• Reac on with metals: Hydrogen reacts with certain highly reac ve metals at a high temperature
to form salt-like compounds called metal hydrides.
Thirst for Knowledge
2Na(s) + H (g) 2NaH(s)
2
Sodium Hydrogen Sodium hydride
The mixture of hydrogen and
oxygen when ignited by a spark
Ca(s) + H (g) CaH (s)
2
2
Calcium Hydrogen Calcium hydride causes explosion.
Tests for Hydrogen

1. Pure hydrogen burns with a pale blue fl ame in air. But impure hydrogen burns in air with a
characteris c pop sound.
2. When hydrogen is burnt in air, water is formed. When a few drops of water so formed are sprinkled
on white anhydrous copper sulphate, the la er turns into blue anhydrous copper sulphate.

OXIDATION AND REDUCTION

The process of adding oxygen to an element or a compound is called oxida on.
The process of removing oxygen or adding hydrogen to an element or a compound is called reduc on.

A substance that brings about oxida on is called an oxidising agent. A substance that brings about
reduc on is called a reducing agent.

In a redox reac on, both oxida on and reduc on occur simultaneously. When hydrogen gas is passed
over hot oxides of metals like copper, lead, iron and zinc, removal of oxygen from metal oxides takes
place. In this process, hydrogen itself gets oxidised into water.
reduc on

CuO + H Cu + H O
2
2
Copper oxide Hydrogen Copper Water
oxida on
PbO + H Pb + H O
2
2
Lead(II) oxide Hydrogen Lead Water
Fe O 3 + 3H 2Fe + 3H O
2
2
2
Ferric oxide Hydrogen Iron Water
ZnO + H Zn + H O
2
2
Zinc oxide Hydrogen Zinc Water
In these reac ons, hydrogen acts as a reducing agent.

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