Hydrogen can be prepared by the following methods,
(i) By action of water with metals
(a) Active metals like Na, K react at room temperature
2M + 2H2O → 2MOH + H2 [M = Na, K etc.]
(b) Less active metals like Ca, Zn, Mg, Al liberate hydrogen only on heating.
2Al + 3H2O → Al2O3 + 3H2
(c) Metals like Fe, Ni, Co, Sn can react only when steam is passed over red hot metals.
3Fe + 4H2O (steam) → Fe3O4 + 4H2
Ferrosoferric oxide
(ii) By the action of water on alkali and alkaline earth metals hydrides
NaH + H2O → NaOH + H2
CaH2 + 2H2O → Ca(OH)2 + 2H2
(iii)By reaction of metals like Zn, Sn, Al with alkalies (NaOH or KOH)
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Zn + 2NaOH ————→ Na2ZnO2 + H2
Sod. zincate
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Al + 2NaOH + H2O ————→ 2NaAlO2 + 2H2
Sod. meta-aluminate
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Si + 2NaOH + 2H2O ————→ Na2SiO2 + 3H2
Silicon
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Sn + 2NaOH ————→ Na2SnO2 + H2↑
Tin Sod. stannite
(iv) By action of metal with acids :All active metals which lie above hydrogen in electrochemical series, can displace hydrogen gas from dilute mineral acids like HCl, H2SO4.
Fe + 2HCl → FeCl2 + H2
(v) By the electrolysis of acidified water
H+/Electrolysis
2H2O ———————→ 2H2O ↑ + O2↑
At cathode At anode
(vi) Laboratory method :In laboratory, it is obtained by action of granulated zinc with dilute .
Zn + dil. H2SO4 → ZnSO4 + H2
It must be noted that
(a) Pure zinc is not used for the preparation of H2 as rate of reaction of pure Zn with dil. H2SO4 is quite slow.
(b) Conc. H2SO4 is not used because then SO2 gas is evolved instead of H2.
(vii) Preparation of pure hydrogen:It can be obtained by
(a) The action of pure dil. H2SO4 on pure magnesium ribbon.
Mg + H2SO4 → MgSO4 + H2
(b) Hydrogen of high purity (> 99.95%) is obtained by electrolysing warm aqueous barium hydroxide between nickel electrodes.
(c) By the action of water on sodium hydride.
NaH + H2O → NaOH + H2↑
(d) By the action of KOH (aq.) on aluminium.
2Al + 2KOH + 2H2O → 2KAlO2 + 3H2↑
(viii) Commercial production of hydrogen
(a) Bosch process : In this method, water gas is mixed with twice its volume of steam and passed over heated catalyst Fe2O3 in the presence of a promoter Cr2O3 or ThO2 at 773 K when CO2 and H2 are obtained. CO2 is removed by dissolving it in water under pressure (20-25 atm) and H2 left undissolved is collected.
About 18% of the world’s production of H2 is obtained from coal.
(b) Lane’s process : By passing steam over spongy iron at 773-1050 K.
3Fe + 4H2O → Fe3O4 + 4H2
The ferrosoferric oxide (Fe3O4) so produced is reduced back to iron with water. this reaction is known as Vivification reactions
Fe3O4 + 4H2O → 3Fe + 4H2O
Fe3O4 + 4CO → 3Fe + 4CO2
(c) By electrolysis of water :Electrolysis of acidified water using platinum electrodes is used for the bulk preparation of hydrogen.
(d) From hydrocarbons :Hydrocarbons (alkanes) react with steam at high temperature to produce carbon monoxide and hydrogen, e.g.,
CH4(g) + H2O(g) —————→ CO(g) + 3H2(g)
The mixture of CO and H2 so obtained can be converted into hydrogen as in Bosch process. About 77% of the world’s production of H2 is obtained from hydrocarbons.
(e) It is also produced as a by-product of the brine electrolysis process for the manufacture of Cl2 and NaOH.
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