Preparation of dihydrogen

Dihydrogen can be prepared by several routes, split broadly into laboratory-scale and industrial-scale methods.

A. Laboratory methods: (i) reacting zinc granules with dilute hydrochloric acid, Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g); (ii) reacting zinc with aqueous sodium hydroxide, Zn(s) + 2NaOH(aq) → Na2ZnO2(aq) + H2(g).

B. Industrial methods: (i) electrolysis of water — pure water conducts electricity poorly, so a dilute aqueous acid or alkali solution is electrolysed instead; electrolysing dilute sulphuric acid, for example, yields two volumes of H2 at the cathode for every one volume of O2 at the anode, 2H2O(l) →(electrolysis, trace acid/alkali) 2H2(g) + O2(g); very pure dihydrogen (over 99.5%) is obtained by electrolysing warm barium hydroxide solution between nickel electrodes. (ii) From carbon or a hydrocarbon, in three stages: Stage 1 reacts steam with a hydrocarbon (or with coke/carbon) at 1270 K over a nickel catalyst to give 'water-gas', a mixture of CO and H2, CH4(g) + H2O(g) →(1270 K, Ni) CO(g) + 3H2(g), or with coke, C(s) + H2O(g) →(1270 K) CO(g) + H2(g); water-gas is also called 'syngas' because it is used to synthesise methanol and various hydrocarbons, and producing it is also the first stage of coal gasification (sawdust or scrap wood can substitute for carbon). Stage 2, the water-gas shift reaction, converts the CO in water-gas into CO2 by reacting it with more steam over an iron chromate catalyst, CO(g) + H2O(g) →(673 K, iron chromate …