(i) Hydrogen economy: The basic principle of hydrogen economy is the storage and transportation of energy in the form of liquid or gaseous dihydrogen.
(ii) Hydrogenation: Hydrogenation means addition of hydrogen across double and triple bonds in presence of catalyst to form saturated compounds.
Vegetable oil + H₂ \(\overset { Ni,437K }{ \longrightarrow } \) Vegetable Ghee
(iii) Syngas: The mixture of CO and H₂ are called synthesis or syngas. It can be produced by the reaction of steam on hydrocarbon or coke at high temperature in the presence of nickel catalyst
CH₄(g) + H₂O(g) \(\overset { 1270K }{ \underset { Ni }{ \longrightarrow } } \) CO(g) + 3H₂(g)
The process of producing syngas from coal is called 'Coal gasification'.
C(g) + H₂O(g) \(\overset { 1270K }{ \underset { Ni }{ \longrightarrow } } \) CO(g) + H₂(g)
(iv) Water-gas shift reaction: The amount of hydrogen in the syngas can be increased by the action of CO of syngas mixture with steam in the presence of iron chromate as catalyst.
C(s) + H₂O(g) \(\overset { 673K }{ \underset { { FeCrO }_{ 4 } }{ \longrightarrow } } \) CO(g) + H₂(g)
This is called water-gas shift reaction.
(v) Fuel-Cell: It is a cell which converts chemical energy of fuel directly into electrical energy.

\(CaO(s)+H_{ 2 }O(g)\rightarrow Ca(OH)_{ 2 }(aq)\) (hydrolysis reaction)

\(2MnO_{ 4 }^{ - }(aq)+{ 5H }_{ 2 }{ O }_{ 2 }(l)+6{ H }^{ + }(aq)\rightarrow 2Mn^{ 2+ }(aq)+8{ H }_{ 2 }O(l)+{ 5O }_{ 2 }(g)\)
(Redox reduction)

\(PbS(s)+{ 4 }H_{ 2 }{ O }_{ 2 }(aq)\rightarrow PbSO_{ 4 }(s)+{ 4H }_{ 2 }O(l)\) (redox reaction)

0.2 L(or 200 mL) of 5M solution will contain 
\(\frac { 340\times0.2 }{ 2 } =34g{ H }_{ 2 }{ O }_{ 2 }\)
\(\underset { 2\times 34=68g }{ { 2H }_{ 2 }O_{ 2 } } \rightarrow { 2H }_{ 2 }O+\underset { 2\times 16=32g }{ { O }_{ 2 } } \)
\(\because \ 68g \ { H }_{ 2 }O_{ 2 }\) on decomposition will give 32g\(O_{ 2 }\)
\(\therefore \ 34g \ { H }_{ 2 }O_{ 2 }\) on decomposition will give
\(\frac { 32\times 34 }{ 68 } =16g{ O }_{ 2 }\)

Molar mass of \({ H }_{ 2 }{ O }_{ 2 }=34 \ gmol^{ -1 }\)
1L of 5M solution of \({ H }_{ 2 }O_{ 2 }\)will contain 34 x 5g \({ H }_{ 2 }O_{ 2 }\)
2L of 5M solution of \({ H }_{ 2 }O_{ 2 }\)will contain 34 x 5 x 2 = 340g \({ H }_{ 2 }O_{ 2 }\)
Mass of \({ H }_{ 2 }O_{ 2 }\)present in 2L of 5 molar solution = 340g

Dihydrogen can be obtained by treating zinc with dilute HCl
\(Zn(s)+2HCl(aq)\rightarrow Zn{ Cl }_{ 2 }(aq)+{ H }_{ 2 }(g)\)

(a) Element with Z = 15, belongs to p-block. It forms covalent hydride, PH₃
(b) Element with Z = 19 belongs to s-block. It forms ionic or saline hydride, KH
(c) Element5 with Z = 23 belongs to d-block and Vth group elements. It forms interstitial hydride, VH_1.6. It is non-stoichiometric hydride.
(d) Element with Z = 44 belongsto d-block and VIIth group elements. It is ruthinium. It does not form any hydride (hydride gap). Only ionic hydride, KH reacts violently with water producing dihydrogen gas
\(KH(s)+H_{ 2 }O(aq)\rightarrow KOH(aq)+H_{ 2 }(g)\)

\({ C }_{ 3 }{ H }_{ 8 }(g)+3{ H }_{ 2 }O(g)\overset { Ni,1270K }{ \rightarrow } 3CO(g)+7{ H }_{ 2 }(g)\)

\({ H }_{ 2 }(g)+{ M }_{ m }{ O }_{ 0 }(s)\underrightarrow { \Delta } \)