Group 15 Hydrides
Ammonia ($NH_3$): Haber's Process & Chemical Properties
Ammonia is a pungent-smelling gas characterized by its ability to form hydrogen bonds and act as a Lewis base. It is the most critical compound of Nitrogen, serving as a precursor for fertilizers like Urea.
1. Preparation of Ammonia
On a small scale, ammonia is obtained from ammonium salts (e.g., $NH_4Cl$). Commercially, it is manufactured via the Haber's Process.
Haber's Process Equation
$$ N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g); \quad \Delta_f H = -46.1 \text{ kJ/mol} $$
Optimum Conditions (Le Chatelier's Principle)
- High Pressure: $200 \times 10^5 \text{ Pa}$ ($\sim 200 \text{ atm}$) favors forward reaction (volume decreases).
- Optimum Temperature: $\sim 700 \text{ K}$ (Exothermic reaction, but low temp slows rate).
- Catalyst: Iron Oxide with small amounts of $K_2O$ and $Al_2O_3$ (previously Mo was used as promoter).
Figure: Complete Reaction Chart of Ammonia ($NH_3$)
2. Chemical Properties
The properties of ammonia are dominated by the lone pair on the Nitrogen atom, making it a Lewis Base.
Basic Nature
$NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$
Turns red litmus blue.
Precipitation of Metal Hydroxides
$ZnSO_4 + 2NH_4OH \rightarrow \underset{\text{White ppt}}{Zn(OH)_2} + (NH_4)_2SO_4$
Complex Formation (Detection of Ions)
Ammonia acts as a ligand to form colored complexes, useful in qualitative analysis.
- Copper (II): Blue solution with $NH_3$.
$Cu^{2+}(aq) + 4NH_3(aq) \rightleftharpoons [Cu(NH_3)_4]^{2+}(\text{Deep Blue})$ - Silver (I): White ppt of $AgCl$ dissolves in $NH_3$.
$AgCl(s) + 2NH_3(aq) \rightarrow [Ag(NH_3)_2]Cl(aq)$ (Colorless)
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