Hydrogen Gas

Symbol: H

Molecular formula: H₂

Atomic weight: 1.008

At. No.: 1

Discovery
        Hydrogen gas was discovered by Henry Cavendish in 1776. He gave the name "inflammable gas". Later on, Lavoisier gave the name "Hydrogen". In Greek, Hydros = water & genuous = I produce.
    Hydrogen gas is found in volcanic eruptions and in petroleum substances in combination with it. All hydrocarbons have hydrogen in combination. Hydrocarbons are organic substances.

Preparation of Hydrogen Gas

         Hydrogen gas can be prepared from water, acid and alkalis.

                        Zn + 2HCL = ZnCl₂ + H₂

                        Mg + 2HCL = MgCl₂ + H₂

                        Zn + H₂SO₄ = ZnSO₄ + H₂

1.From water: Hydrogen gas can be prepared from water by electrolysis. Electrolysis is the decomposition of an electrolyte by the passage of electricity. Electrolytes are substances which in solution conduct electricity and at the same time undergo chemical decomposition. 

Fig: Electrolysis of water

     Chemical reaction is this experiment is summarized as follows:

4H₂O = 4H⁺ + 4OH^-

4H^- + 4e^- = 4H = 2H₂ (cathode)

4OH^- - 4e^- = 4OH

4OH = 2H₂O + O₂ (at anode)

During electrolysis of water, we take a voltameter as given in the figure. Water in the volameter is made electrolytic medium by adding few drops of sulphuric acid. An acidic or alkaline medium behaves as electrolytic medium. Two platinum electrodes are dipped in the water. Each electrode is connected to respective terminal of battery (cathode to negative terminal and anode to positive terminal). Two gas jars filled with water are put in as shown in the figure. When switch is on, the mechanism commences. The battery may be of 12V (DC) or 6V (DC). The overall chemical reaction is given above. Hydrogen gas is collected in cathode and oxygen gas in anode. Thus, by electrolysis, hydrogen as well as oxygen can be prepared.