Question

Very pure hydrogen (99.9%) can be made by which of the following processes?
(A) Reaction of methane with steam
(B) Mixing natural hydrocarbons with high molecular weight
(C) Electrolysis of water
(D) Reaction of salt like hydrides with water

Answer 1

The method by which we can obtain pure hydrogen uses electric energy to obtain gaseous hydrogen molecules. Alongside hydrogen, oxygen gas is also obtained by this method.

Complete step by step solution:
There are many methods by which we can make dihydrogen gas.
- We know that in the laboratory, hydrogen gas is prepared by allowing the metals to react with the acid. Some metals like Zn can react with alkali to give hydrogen gas as well.
- The pure hydrogen gas has purity around 99.9%. This is prepared by electrolysis of water.
- In electrolysis of water, the dissociation of the water molecule into hydrogen gas and oxygen gas is done by use of electric energy.
- We know that in electrolysis, the reaction can be divided into two parts; oxidation reaction and reduction reaction. Both of them are shown here.
${\text{Reduction: 2}}{{\text{H}}_2}{O_{(l)}} + 2{e^ - } \to {H_{2(g)}} + 2OH_{(aq)}^ -$
${\text{Oxidation : 4OH}}_{(aq)}^ - \to {O_{2(g)}} + 2{H_2}{O_{(l)}} + 4{e^ - }$
We can see that in the oxidation reaction, oxygen atoms are oxidized and in reduction reaction, hydrogen atoms are reduced.
- The oxidation reaction occurs at anode and the reduction reaction occurs at cathode.
- The overall reaction can be given as:
$2{H_2}{O_{(l)}} \to 2{H_{2(g)}} + {O_{2(g)}}$
Thus, we can conclude that the purest form of hydrogen can be obtained by electrolysis of water.

So, the correct answer is (C).

Note: However, this method of production of hydrogen gas is very costly so it is not widely used. Currently, 77% of hydrogen is obtained from petrochemicals, 18% from coal, 5% from electrolysis and 1% from various other sources.