8.1 Preparation

  Hydrogen

1. Electrolysis of acidified water using platinum electrodes gives hydrogen.

$$2{H_2}O\left( l \right)\mathop \to \limits_{Traces{\text{ }}of{\text{ }}acid/base}^{Electrolysis} 2{H_2}\left( g \right) + {O_2}\left( g \right)$$

2. It is usually prepared by the reaction of granulated zinc with dilute hydrochloric acid.

$$Zn + 2{H^ + } \to Z{n^{2 + }} + {H_2}$$

3. High purity ($>99.95\%$) dihydrogen is obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes.

4. Hydrogen can be obtained by reaction of metals like $Zn$, $Al$, $Be$ etc. with alkalies.

$$\begin{equation} \begin{aligned} Zn + 2NaOH \to N{a_2}Zn{O_2} (\text{Sodium zincate})+ {H_2}\end{aligned} \end{equation} $$$$\begin{equation} \begin{aligned} Be + 2NaOH \to N{a_2}Be{O_2} (\text{Sodium beryliate})+ {H_2} \end{aligned} \end{equation} $$

5. It is obtained as a byproduct in the manufacture of sodium hydroxide and chlorine by the electrolysis of brine solution. During electrolysis, the reactions that take place are:

$$\begin{equation} \begin{aligned} {\text{At Anode: }}2C{l^ - }\left( {aq} \right) \to C{l_2}\left( g \right) + 2{e^ - } \\ {\text{At cathode: }}2{H_2}O\left( l \right) + 2{e^ - } \to {H_2}\left( g \right) + 2O{H^ - } \\ {\text{The overall reaction is:}} \\ 2N{a^ + }\left( {aq} \right) + 2C{l^ - }\left( {aq} \right) + 2{H_2}O\left( l \right) \to C{l_2}\left( g \right) + {H_2}\left( g \right) + 2N{a^ + }\left( {aq} \right) + 2O{H^ - }\left( {aq} \right) \\\end{aligned} \end{equation} $$

6. Lane’s Process:

$$3Fe + 4{H_2}O\mathop \to \limits^\Delta F{e_3}{O_4} + 4{H_2}$$

7. Dihydrogen can be prepared by action of water or dilute mineral acid on metals.

$$2N{a^ + }\left( s \right) + 2{H_2}O\left( l \right) \to 2NaOH\left( {aq} \right) + {H_2} \uparrow $$ This reaction is vigorous. To minimize the rate of reaction, alkali metals are used in the form of amalgams (alloys of metal with $Hg$).

$$2Na\left( {Hg} \right) + 2{H_2}O \to 2NaOH + {H_2} \uparrow + 2Hg$$

8. Bosch Process:

Reaction of steam on hydrocarbons or coke at high temperatures in the presence of catalyst yield hydrogen.

$${C_n}{H_{2n + 2}} + n{H_2}O\mathop \to \limits_{Catalyst}^{1270K} nCO + \left( {2n + 1} \right){H_2}$$

The mixture of $CO$ and $H_{2}$ is called water gas. As this mixture of $CO$ and $H_{2}$ is used for the synthesis of methanol and a number of hydrocarbons, it is also called synthesis gas or 'syngas'. Nowadays 'syngas' is produced from sewage, saw-dust, scrap wood, newspapers etc. The process of producing 'syngas' from coal is called 'coal gasification'.