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Question
Discuss the preparation of the hydrides of group 15 elements.
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Solution
Ammonium chloride and slaked lime can be heated in a lab to produce ammonia.
\[\ce{2NH4Cl_{(s)} + Ca(OH)2 ->[Heat] 2NH3_{(g)} + CaCl2 + 2H2O}\]
Commercial ammonia production involves passing a nitrogen and hydrogen mixture over a catalyst that contains finely divided iron mixed with molybdenum at 720-770 K and 200 atmospheres of pressure (Haber’s process).
\[\ce{N2 + 3H2 <=>[720-770 K, 200 atm][Fe Mo] \underset{Ammonia}{2NH3}}\]
White phosphorus and concentrated NaOH can be boiled in an inert carbon dioxide atmosphere to produce phosphorus, or calcium phosphide can be treated with water.
\[\ce{\underset{phosphorus}{\underset{White}{P4}} + 3NaOH + 3H2O -> \underset{Sod. hypophosphite}{3NaH2PO2} + \underset{Phosphine}{PH3}}\]
\[\ce{Ca3P2 + 6H2O -> 2PH3 + 3Ca(OH)2}\]
Arsine and stibine can be obtained as follows.
\[\ce{\underset{Zinc arsenide}{Zn3As2_{(s)}} + 6HCl_{(aq)} -> \underset{Arsine}{2AsH3_{(g)}} + 3ZnCl2_{(aq)}}\]
\[\ce{\underset{Zinc antimonide}{Zn3Sb2_{(s)}} + 6HCl_{(aq)} -> \underset{Arsine}{2SbH3_{(g)}} + 3ZnCl2_{(aq)}}\]
Bismuthine is very unstable. It can only be made in small amounts by reacting the bismuth-magnesium alloy with dilute sulphuric or hydrochloric acid.
