(a) Laboratory preparation:
Reaction of metals with acids. Zn + H+→ Zn2++H2
(b) Commercial Preparation:
Uses of Hydrogen:
(a) Water containing carbonate, chloride and sulphate salts of calcium and magnesium.
Temporary hardens is due to the presence of carbonate salts and can be removed by boiling or by adding lime water.
Ca(HCO3)2 + Ca(OH)2 → 2CaCO3↓+2H2O
Ca(HCO3)2 + Δ → 2CaCO3↓+2H2O +CO2↑
Permanent hardness is due to presence of sulphate and chloride salts and can be removed by treatment with washing soda.
MCl2 + Na2CO3 → MCO3 ↓ + 2NaCl ( M= Mg, Ca)
MSO4 + Na2CO3 → MCO3 ↓ + Na2SO4 ( M= Mg, Ca)
(b) Hard water forms scum/precipitate with soap:
Na2O2 (s) + H2SO4(aq) → H2O2(aq) + Na2SO4(s)
BaO2.8H2O + H2SO4(aq) → H2O2 (aq) + BaSO4(s)
Anhydrous barium oxide is not used because the precipitated BaSO4 forms a protective layer on the unreacted barium peroxide and thus prevents its further participation in the reaction. However it can be overcome by using phosphoric acid.
By the auto-oxidation of 2-ethyl anthraquinol. The net reaction is a catalytic union of H2 and O2 to yield hydrogen peroxide.
(i) Unstable liquid, decomposes to give water and dioxygen and the reaction is slow in the absence of catalyst. It is catalysed by certain metal ions, metal powders and metal oxides.
2H2O2 (l) → 2H2O (l) + O2 (g)
(ii) It is a very powerful oxidising agent and poor reducing agent.
As oxidising agent
In acidic medium: H2O2 + 2H+ + 2e- → 2H2O
In basic medium :H2O2 + OH- + 2e- → 3OH-
As reducing agent
In acidic medium: H2O2 → 2H+ + O2 + 2e-
In basic medium : H2O2 + 2OH- → 2H2O + O2 + 2e-
2Fe2+ + H2O2 + 2H+ → 2Fe3+ + 2H2O
2MnO4- + 5H2O2 + 6H+ → 2Mn2+ + 8H2O + 5O2
Mn2+ + H2O2 → Mn+4 + 2OH-
2Fe3+ + H2O2 + 2OH- → 2Fe2+ + 2H2O + O2
The oxidising property of hydrogen peroxide is put to use in the restoration of old paintings, where the original white lead paint has been converted to black PbS by the H2S in the atmosphere. Hydrogen peroxide oxidises the black PbS into white PbSO4.