Nitrogen, Phosphorus, and Their Compounds

20min Part 3 / Ch5 / Lesson 4

Prerequisites: 3-5-1

Objectives

Ammonia NH₃:

Detection: white fumes with conc. HCl → $\mathrm{NH_4Cl}$

$\mathrm{NH_3 + HCl \rightarrow NH_4Cl}$

Lab preparation:

$\mathrm{2NH_4Cl + Ca(OH)_2 \xrightarrow{heat} CaCl_2 + 2NH_3 + 2H_2O}$

Industrial ammonia synthesis:

$\mathrm{N_2 + 3H_2 \rightleftharpoons 2NH_3} \quad \Delta H < 0$

Conditions: 400–500°C, 200–300 atm, Fe₃O₄ catalyst

Le Chatelier analysis: high pressure shifts equilibrium right. High temperature shifts it left but is needed for reaction rate.

Compound	Color	Reaction with water	Notes
NO	Colorless	Sparingly soluble	Oxidized instantly in air → NO₂
NO₂	Red-brown	HNO₃ + HNO₂	Pungent, toxic
N₂O₄	Colorless	—	Dimer of NO₂

Comparison of nitrogen oxides

$\mathrm{2NO + O_2 \rightarrow 2NO_2}$

$\mathrm{3NO_2 + H_2O \rightarrow 2HNO_3 + NO}$

Properties of nitric acid:

Strong acid
Powerful oxidizer: dissolves Cu, Ag, and other metals
Decomposes in light: $\mathrm{4HNO_3 \rightarrow 4NO_2 + 2H_2O + O_2}$ → stored in brown bottles

Dilute HNO₃ + Cu: $\mathrm{3Cu + 8HNO_3(dilute) \rightarrow 3Cu(NO_3)_2 + 2NO + 4H_2O}$

Conc. HNO₃ + Cu: $\mathrm{Cu + 4HNO_3(conc.) \rightarrow Cu(NO_3)_2 + 2NO_2 + 2H_2O}$

Passivation: Fe, Al, Ni form a protective oxide layer in conc. HNO₃ and stop dissolving.

Ostwald Process

White phosphorus P₄: Very toxic, ignites spontaneously in air → stored under water
Red phosphorus: Stable, used in match striking surfaces
Phosphorus pentoxide P₄O₁₀: Powerful dehydrating agent

Q1 What catalyst is used in the Haber process?

Q2 Why doesn't Fe dissolve in concentrated nitric acid?

Q3 What color is NO gas?

Q1. Write the three steps of the Ostwald Process and show that the overall conversion is NH₃ → HNO₃.

Solution

The NO produced in step 3 is recycled into step 2. The overall equation simplifies to:

$\mathrm{NH_3 + 2O_2 \rightarrow HNO_3 + H_2O}$