Some observations related to an unknown inorganic substance A are presented below. • A is a yellowis — Organic Chemistry Chemistry Question
Identification of an inorganic compound
Some observations related to an unknown inorganic substance A are presented below.
• A is a yellowish – white deliquescent solid and it sublimes on heating. It has a molecular weight of 266.
• A reacts violently with water, forming solution B.
• When a solution of NH4Cl and NH3 is added to solution B, a white gelatinous precipitate is obtained.
• A sample of B also gives a curdy white precipitate C on addition of dilute nitric acid and silver nitrate solution. This white precipitate C readily dissolves when dilute aqueous NH3 solution is added, though a gelatinous white precipitate D is formed in its place with excess of NH3 .
• Precipitate D is filtered off and is dissolved in excess NaOH to give a clear solution E.
• When CO2 is passed through solution E, compound D is reprecipitated.
• Substance A dissolves unchanged in dry ether. When this solution is reacted with LiH, a product F is formed. If LiH is used in excess, F transforms to G.
Identify the unknown compound A.
Model Answer
The white gelatinous precipitate in group (III) obtained by qualitative analysis of solution B indicates the presence of Al3+ ions. The white precipitate with AgNO3 indicates the presence of Cl– ions.
From the above data the compound A must be a dimer of aluminium chloride Al2Cl6.
Write down the appropriate chemical equations for the given reactions and identify the different products from B to G.
Model Answer
The equations of the reactions in ionic form are as follows:
• Reaction of A with water to form B:
Al2Cl6 + 12 H2O → 2 [Al(H2O)6]3+ + 6 Cl-
(where A = Al2Cl6, B = [Al(H2O)6]3+)
• Formation of C:
Cl- + Ag+ → AgCl(s)
(where C = AgCl)
• Dissolution of C in ammonia:
AgCl(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Cl-
• Formation of D:
Al3+(aq) + 3 NH3 + 3 H2O → Al(OH)3(s) + 3 NH4+
(where D = Al(OH)3)
• Dissolution of D in excess NaOH to form E:
Al(OH)3(s) + OH-(aq) → [Al(OH)4]-(aq)
(where E = [Al(OH)4]-)
• Reprecipitation of D:
[Al(OH)4]- + CO2 → Al(OH)3(s) + HCO3-
• Reaction with LiH to form F and G:
Al2Cl6 + 8 LiH → 2 Li[AlH4] + 6 LiCl
(where F = AlH3, G = Li[AlH4])