chemistry (acid and alkali)

2.
Ammonia is only slightly ionized in water.
NH3(aq) + H2O(l) ⇌ NH4⁺(aq) + OH⁻(aq)

Therefore, ammonia solution contains mainly ammonia and water molecules, and smallamounts of NH4⁺ and OH⁻ ions. Note that NH4OHdoes not exist. If you write NH4OHin your equation in HKDSE Chemistry, you will scores zero mark, although NH4OHis accepted in Mainland China and Taiwan.

When you write full equations involved ammonia solution, you had better write"NH3 + H2O" instead of "NH4OH",or sometimes NH3 only.
When you write ionic equations, "OH⁻ ions", or sometimes "NH3"are involved.


1.
When ammonia solution is added dropwise to ZnSO4 solution, a whiteprecipitate of Zn(OH)2 is formed.
Full equation: 2NH3­(aq) + 2H2O(l) + ZnSO4(aq)→ Zn(OH)2(s) + (NH4)2SO4(aq)
Ionic equation : Zn²⁺(aq) + 2OH⁻(aq) → Zn(OH)2(s)
Refer to the ionic equation above, SO4²⁻(aq) and NH4⁺(aq)ions are spectacular ions or dummy ions.

When an excess of ammonia solution is added, the white precipitate of Zn(OH)2is re-dissolved.
Full equation : Zn(OH)2(s) + 4NH3(aq) → [Zn(NH4)3](OH)2(aq)
Ionic equation : Zn(OH)2(s) + 4NH3(aq) → [Zn(NH4)3]²⁺(aq) + 2OH⁻(aq)

so simple.

Always remind yourself that when ammonia gas dissolves in a solution, it undergoes "ionization" / partly ionizes in water to give only ammonium ion and water since ammonia is a WEAK ALKALI
This is the equation you should remember:
NH3(g) + H2O(I) ----> NH4+(aq) + OH-(aq)
Note that this is a reversible reaction ( vice versa )

1. ZnSO4 + NH3 ---> ?
Write this equation to remind yourself first:
NH3(g) + H2O(I) ----> NH4+(aq) + OH-(aq)
Then, deduce the bond formations:
Positively charged NH4+(aq) combines with negatively charged SO42-(aq) to form (NH4)2SO4(aq) in water.
Negatively charged OH-(aq) combines with positively charged zinc ions to form
hydroxide Zn(OH)2(s)
Refer to your txtbook, alkalis react with NH4+