🧪 Ammonia (NH₃) – Full Breakdown
1) Preparation of Ammonia
a) Laboratory Method
Reactants: Ammonium chloride (NH₄Cl) + Calcium hydroxide (Ca(OH)₂ — slaked lime)
Equation:
| Lab Preparation of NH₃ Gas |
|---|
| 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₃↑ + 2H₂O |
| NOTES: Heat the mixture gently. NH₃ is collected by downward delivery (upward displacement of air) because it's lighter than air. Drying agent: quicklime (CaO). NH₃ is not passed through conc. H₂SO₄ or CaCl₂ — it reacts with both. |
Why Ca(OH)₂ and not NaOH or KOH? NaOH and KOH are deliquescent (absorb moisture from air) and also react with NH₄Cl to give NH₃ but they're more expensive and harder to handle. Ca(OH)₂ is cheap, easily available, and works perfectly.
b) Industrial Method — Haber's Process
| Haber's Process for NH₃ Production |
|---|
| N₂(g) + 3H₂(g) ⇌ 2NH₃(g) + heat |
| NOTES: Optimum conditions: Pressure = 200-900 atm. Temperature = 450-550°C. Catalyst = Finely divided iron with Mo or Al₂O₃ promoter. N₂ from air, H₂ from water gas / natural gas. Unreacted N₂ and H₂ are recycled. |
Why these conditions?
- High pressure — forward reaction has 4 molecules → 2 molecules (volume decreases). Le Chatelier: high pressure favours forward reaction.
- Moderate temperature (not too high) — reaction is exothermic. Low temp favours forward reaction, but too slow. 450°C is a compromise between rate and yield.
- Iron catalyst — speeds up the reaction without affecting equilibrium.
2) Physical Properties of NH₃
- Colour: Colourless
- Odour: Pungent, characteristic choking smell — irritates eyes and nose
- Density: Lighter than air (vapour density = 8.5, molecular mass = 17)
- Solubility: Highly soluble in water — one volume of water dissolves ~700 volumes of NH₃ at room temp. Highest solubility among common gases.
- Liquefaction: NH₃ is easily liquefied under pressure (boiling point = -33°C). Used as a refrigerant.
- Basic nature: Turns damp red litmus blue (test for NH₃).
3) Fountain Experiment (Similar to HCl)
Same setup as HCl, but the observations differ slightly:
| NH₃ Fountain Experiment |
|---|
| Water rushes into the flask as a fountain (proves high solubility of NH₃) |
| The water turns blue (if colourless or red litmus used — NH₃ is alkaline) |
| NOTES: Proves: (1) NH₃ is highly soluble in water. (2) NH₃ solution is alkaline/basic. |
4) Chemical Properties of NH₃
a) Reaction with Water (Formation of Ammonium Hydroxide)
| NH₃ + H₂O → NH₄OH |
|---|
| NH₃(g) + H₂O ⇌ NH₄OH(aq) |
| NH₄OH ⇌ NH₄⁺ + OH⁻ (partial dissociation — weak base) |
| NOTES: NH₄OH is a weak base (partially dissociates). Turns red litmus blue. Used in analytical chemistry for identifying metal ions. |
b) Reaction with Acids (Neutralisation)
| NH₃ + Acid → Ammonium Salt |
|---|
| NH₃ + HNO₃ → NH₄NO₃ (ammonium nitrate — fertiliser) |
| NH₃ + HCl → NH₄Cl (ammonium chloride — dense white fumes) |
| 2NH₃ + H₂SO₄ → (NH₄)₂SO₄ (ammonium sulphate — fertiliser) |
| NOTES: Reaction with HCl gives dense white fumes of NH₄Cl — this is the test for NH₃ (and for HCl). |
c) Reaction with Oxygen (Combustion)
| NH₃ + O₂ — Two Possible Reactions |
|---|
| In air (burns): 4NH₃ + 3O₂ → 2N₂ + 6H₂O (NH₃ burns with a greenish-yellow flame) |
| With catalyst (Pt/Rh): 4NH₃ + 5O₂ → 4NO + 6H₂O (catalytic oxidation — Ostwald process for HNO₃) |
| NOTES: Without catalyst → N₂ + H₂O. With Pt catalyst at 800°C → NO + H₂O (first step in making HNO₃ industrially). |
d) Reaction with Copper Oxide (Reducing Property)
| NH₃ acts as a reducing agent |
|---|
| 2NH₃ + 3CuO → N₂ + 3Cu + 3H₂O (black CuO → reddish-brown Cu) |
| NOTES: NH₃ reduces CuO to Cu metal. NH₃ itself gets oxidised to N₂. This proves NH₃ is a reducing agent. Black CuO turns reddish-brown. |
e) Reaction with Chlorine (Excess vs Limited)
| NH₃ + Cl₂ — Depends on amount of Cl₂ |
|---|
| Excess NH₃: 8NH₃ + 3Cl₂ → 6NH₄Cl + N₂ (dense white fumes of NH₄Cl) |
| Excess Cl₂: 2NH₃ + 3Cl₂ → N₂ + 6HCl (explosive — NCl₃ can form!) |
| NOTES: With excess NH₃, dense white fumes of NH₄Cl are seen — this is also a test for NH₃ and Cl₂. |
5) NH₃ as a Reducing Agent
NH₃ can reduce metal oxides to metals. The NH₃ gets oxidised to N₂.
| NH₃ Reducing Metal Oxides |
|---|
| 2NH₃ + 3CuO → N₂ + 3Cu + 3H₂O (black → reddish-brown) |
| 2NH₃ + 3PbO → N₂ + 3Pb + 3H₂O (yellow/red → grey) |
| 2NH₃ + 3Fe₂O₃ → N₂ + 6FeO + 3H₂O |
| NOTES: The colour change of CuO (black → brown) is a visual confirmation. N₂ gas is released and can be tested (doesn't support combustion). |
6) Uses of Ammonia
- Fertilisers: NH₃ is used to make (NH₄)₂SO₄, NH₄NO₃, NH₄Cl, and urea
- Nitric acid: Ostwald process — NH₃ → NO → NO₂ → HNO₃
- Refrigerant: In ice plants and cold storage (liquid NH₃)
- Cleaning agent: NH₄OH is used in household cleaners
- Lab reagent: In analytical chemistry (identifying metal ions)
- Explosives: NH₄NO₃ is used in explosives
7) Comparison: NH₃ vs HCl Fountain Experiment
| Feature | NH₃ Fountain | HCl Fountain |
|---|---|---|
| Solution formed | NH₄OH (alkaline) | HCl (acidic) |
| Litmus colour change | Colourless → Blue (basic) | Blue → Red (acidic) |
| Solubility | ~700 vol/1 vol water | ~450 vol/1 vol water |
| Both prove | High solubility in water + nature (acidic/basic) of the solution | |
8) Top ICSE Reasoning Questions
Q1: Why is NH₃ collected by downward delivery?
Answer: NH₃ is lighter than air (vapour density = 8.5, air = 14.4). Downward delivery means the gas is collected in an inverted gas jar (upward displacement of air). Not collected over water — extremely soluble (~700 vol in 1 vol water).
Q2: Why is NH₃ not dried using conc. H₂SO₄ or anhydrous CaCl₂?
Answer: NH₃ is a basic gas. Conc. H₂SO₄ is an acid — they react to form (NH₄)₂SO₄. Anhydrous CaCl₂ reacts with NH₃ to form CaCl₂·8NH₃ (an addition compound). The only suitable drying agent is quicklime (CaO).
Q3: In Haber's process, why is a moderate temperature (450-550°C) used instead of a lower temperature?
Answer: The forward reaction is exothermic. Lower temperature favours forward reaction (more yield), but the reaction rate becomes too slow. 450°C is a compromise — gives reasonable yield at a practical rate. The iron catalyst helps speed up the reaction at this temperature.
Q4: In Haber's process, why is high pressure used?
Answer: The forward reaction (N₂ + 3H₂ → 2NH₃) involves a decrease in volume — 4 volumes → 2 volumes. According to Le Chatelier's principle, high pressure favours the reaction that produces fewer molecules. So high pressure pushes the equilibrium towards NH₃ formation.
Q5: How would you test for NH₃ gas?
Answer: (1) Damp red litmus → turns blue (basic). (2) Glass rod dipped in conc. HCl → dense white fumes of NH₄Cl. (3) Pungent, characteristic smell (irritates nose and eyes).
Q6: What happens when NH₃ is passed over heated CuO?
Answer: Black CuO is reduced to reddish-brown Cu. NH₃ is oxidised to N₂. 2NH₃ + 3CuO → N₂ + 3Cu + 3H₂O. NH₃ acts as a reducing agent, CuO acts as an oxidising agent.
Q7: Distinguish between NH₃ and HCl using a chemical test.
Answer: (1) Damp red litmus — NH₃ turns it blue (basic), HCl keeps it red (acidic). (2) Damp blue litmus — NH₃ keeps it blue, HCl turns it red. (3) Glass rod dipped in NH₄OH — no reaction with NH₃, gives white fumes with HCl. (4) Glass rod dipped in HCl — gives white fumes with NH₃, no reaction with HCl.
Q8: How does NH₃ act as a reducing agent? Give an example.
Answer: NH₃ supplies hydrogen or removes oxygen from other compounds. Example: 2NH₃ + 3CuO → N₂ + 3Cu + 3H₂O. NH₃ removes oxygen from CuO (reduces it) and gets oxidised to N₂ itself. The +3 oxidation state of N in NH₃ goes to 0 in N₂ (oxidation).
1. The drying agent for NH₃ is CaO (quicklime) — NOT CaCl₂, NOT conc. H₂SO₄. This MCQ comes every year.
2. Haber's process conditions: 200-900 atm, 450-550°C, Fe catalyst — memorise the numbers.
3. NH₃ + CuO is the classic reducing agent example — colour change (black → brown) is important.
4. Dense white fumes with HCl is THE test for NH₃ (and vice versa).
5. NH₃ fountain → water turns blue (alkaline). HCl fountain → water turns red (acidic). Easy comparison question.