Precipitation Reactions: Metathetical or precipitation reactions occur when two ionic solutions are mixed, leading to the formation of an insoluble compound that appears as a solid precipitate. The occurrence of these reactions depends on the solubility of the resulting products in the solvent. For instance, AgCl is insoluble in water, and when solutions of BaCl₂ and AgNO₃ are combined, a solid precipitate is formed.
Precipitation Reactions in Liquid SO₂
BaI2 + Zn(CNS)2
liq. SO2
Ba(CNS)2 ↓ + ZnI2
SbCl3 + 3LiI
liq. SO2
SbI3 ↓ + 3LiCl
AlCl3 + 3NaI
liq. SO2
AlI3 + 3NaCl↓
2NH4SCN + SOCl2
liq. SO2
2NH4Cl↓ + SO(SCN)2 ↓
Precipitation Reactions in Water
BaCl2 + Na2SO4
H2O
BaSO4 ↓ + 2NaCl
AgNO3 + NaCl
H2O
AgCl ↓ + NaNO3
Pb(NO3)2 + 2KI
H2O
PbI2 ↓ + 2KNO3
Precipitation Reactions in Liquid NH₃
Precipitation Reactions in Liquid NH<sub>3</sub> (Ammonia)
AgNO3 + KNH2
liq. NH3
AgNH2 ↓ + KNO3
HgI2 + 2KNH2
liq. NH3
Hg(NH2)2 ↓ + 2KI
BiI3 + 3KNH2
liq. NH3
Bi(NH2)3 ↓ + 3KI
Precipitation Reactions in Liquid HF
Precipitation Reactions in Liquid HF
KIO4 + AgF
liq. HF
AgIO4 ↓ + KF
KClO4 + TlF
liq. HF
Tl(ClO4) ↓ + KF
Na2SO4 + 2AgF
liq. HF
Ag2SO4 ↓ + 2NaF
Amphoteric reactions: An amphoteric reaction is a chemical reaction in which a substance may function as both an acid and a base, depending on the nature of the other reactant. This characteristic is exhibited by amphoteric compounds, which can:
- Donate protons (function as acids) in reactions with bases.
- Accept protons (as bases) in reactions with acids.
Amphoteric Reactions of Zinc
Amphoteric Reactions of Zinc
Zn(OH)
2 (s) + 2HCl (aq)
aqueous
⟶
ZnCl
2 (aq) + 2H
2O (l)
Zn(OH)
2 (s) + 2NaOH (aq)
aqueous
⟶
Na
2ZnO
2 (aq) + 2H
2O (l)
Zn(NH
2)
2 (s) + 2NH
4Cl
liq. NH3
⟶
ZnCl
2 (s) + 4NH
3 (g)
Zn(NH
2)
2 (s) + 2NaNH
2
liq. NH3
⟶
Na
2Zn(NH
2)
4 (soln.)
Zn(NH
2)
2 (s) + 2NaNH
2
liq. NH3
⟶
Na
2Zn(NH)
2 (soln.) + 2NH
3 (g)
Amphoterism in Liquid Sulphur Dioxide (SO₂)
Some metal chlorides exhibit amphoteric behavior in liquid SO₂, similar to amphoteric hydroxides in water. Below are three representative reactions.
1. Precipitation of Aluminium Sulphite
2AlCl3 + 3(Me4N)2SO3 → Al2(SO3)3 ↓ + 6Me4NCl
A gelatinous precipitate of aluminium sulphite forms in liquid SO₂.
2. Complexation of Aluminium Sulphite in Excess Sulphite
Al2(SO3)3 + 3(Me4N)2SO3 → 2[Me4N]3[Al(SO3)3]
In excess sulphite, the precipitate dissolves due to formation of a soluble complex.
3. Precipitation of Gallium Sulphite
2GaCl3 + 3(Me4N)2SO3 → Ga2(SO3)3 ↓ + 6Me4NCl
Gallium trichloride reacts similarly in liquid SO₂ to form a precipitate of gallium sulphite.
Amphoteric Reactions in Liquid HF
Amphoteric Reactions in Liquid HF
AlF₃ dissolves in presence of NaF due to complex formation:
AlF₃ + NaF → Na⁺ + [AlF₄]⁻
On adding BF₃, aluminium fluoride is reprecipitated:
Na[AlF₄] + BF₃ → AlF₃ ↓ + NaBF₄
K₃[CrF₆] dissolves in HF with precipitation of CrF₃:
K₃[CrF₆] → 3KF + CrF₃ ↓
On adding excess NaF, CrF₃ redissolves as a complex:
CrF₃ + 3NaF → Na₃[CrF₆]
On addition of BF₃, the complex breaks down and CrF₃ is reprecipitated:
Na₃[CrF₆] + 3BF₃ → CrF₃ ↓ + 3NaBF₄
HClO₄ shows amphoteric behavior in HF:
As a base (accepts proton):
HClO₄ + HF → ClO₄⁻ + H₂F⁺
As an acid (donates proton):
HClO₄ + HF → ClO₃⁺ + H₂F⁻
