Page 154 - ICSE Chemistry 8

Pb(s) + 2NaOH(aq) Na PbO (aq) + H (g) 
2
2
2
Lead Sodium hydroxide Sodium plumbate Hydrogen
Preparation of Hydrogen by Electrolysis of Water
When electric current is passed through acidulated water, it dissociates into hydrogen and oxygen. This
process of bringing about a chemical change in a substance in its dissolved or molten state under the
infl uence of electric current is called electrolysis.

Acidulated water is made by adding a few drops of dilute sulphuric acid to pure water to make it a be er
conductor of electricity.
2H O(l) Electric current 2H (g)  + O (g) 
2
2
2
(Acidulated Hydrogen Oxygen
water) gas gas
Hydrogen produced is collected at the cathode (nega ve electrode) and oxygen is collected at the posi ve
electrode (anode) in the ra o 2:1 by volume. This whole process is called electrolysis of water.

hydrogen oxygen

Thinking Fountain
Why are a few drops of dilute
sulphuric acid added to pure
platinum
electrodes water during electrolysis to
acidulated obtain hydrogen and water?
water
(electrolyte)

Fig. 7.2: Hydrogen preparation by electrolysis

There are some compounds that conduct electricity due to the presence of ions. Such compounds are
called electrolytes. Here, acidulated water is an electrolyte.

The process is carried out in a device which converts electrical energy into chemical energy or vice versa.
The device is called an electroly c cell.
Electrodes are the conduc ng rods that allow electric current to enter and leave an electrolyte. They are
made of inert on unreac ve substances so that they do not react with the electrolyte.

Laboratory Preparation of Hydrogen

In the laboratory, hydrogen is prepared by the ac on of dilute sulphuric acid (H SO ) or dilute hydrochloric
2
4
acid (HCl) on zinc granules.
Zn(s) + H SO (aq) ZnSO (aq) + H (g) 
2
2
4
4
Zinc Sulphuric acid Zinc sulphate Hydrogen
Zn(s) + 2HCl(aq) ZnCl (aq) + H (g) 
2
2
Zinc Hydrochloric acid Zinc chloride Hydrogen
140