Question

Very pure hydrogen $\left(99.9\%\right)$ can be made by which of the following processes?

• A. Reaction of methane with steam
• B. Mixing natural hydrocarbons of high molecular weight
• C. Electrolysis of water
• D. Reaction of salt like hydrides with water

Answer 1

Answer: (C)

Electrolysis of water

Pure hydrogen can be obtained from the electrolysis of water.

Cathode (reduction): $2H_2{O}_{\left(l\right)}+2e^{-}\to H_{2\left(g\right)}+2O{H}_{\left(aq\right)}^{-}$

Anode (oxidation): $4O{H}_{\left(aq\right)}^{-}\to O_{2\left(g\right)}+2H_2{O}_{\left(l\right)}+4e^{-}$

Combining either half reaction pair yields the same overall decomposition of water into oxygen and hydrogen:

Overall reaction: $2H_2{O}_{\left(l\right)}\to 2H_{2\left(g\right)}+O_{2\left(g\right)}$

The number of hydrogen molecules produced is thus twice the number of oxygen molecules.