Vardaan Learning Institute
Ammonia
Ammonia is a basic compound of nitrogen and hydrogen. It occurs in nature in small quantities in air and soil, formed by the bacterial decomposition of nitrogenous organic matter (like urea and animal excreta).
1. General Properties & Molecular Profile
Molecular Profile
- Molecular Formula: $NH_3$
- Molecular Mass: $14 + 3(1) = 17\text{ a.m.u.}$
- Structure & Bonding: Pyramidal geometry with a nitrogen atom bonded to three hydrogen atoms by single covalent bonds, leaving one unshared lone pair of electrons on nitrogen.
- Vapour Density ($V.D.$): $\frac{17}{2} = 8.5$. Since the vapour density of air is $14.4$, ammonia gas is roughly 0.589 times lighter than air.
- Solubility: Exceptionally soluble in water ($1\text{ volume of water dissolves } \approx 702\text{ volumes of } NH_3\text{ gas at } 20^\circ\text{C}$ and $1\text{ atm}$).
- Physical Nature: Colourless gas with a characteristic pungent, choking smell. It causes tears in eyes and acts as an alkaline gas in aqueous solution.
2. Laboratory Preparation of Ammonia Gas
Diagram Placeholder
Figure 9.1: Laboratory Preparation of Ammonia Gas
Apparatus diagram showing round-bottom flask tilted downwards with NH₄Cl and Ca(OH)₂, drying tower packed with Quicklime (CaO), and downward displacement of air collection in an inverted gas jar.
Reaction & Setup
Reactants: Ammonium chloride ($NH_4Cl$) and Slaked lime / Calcium hydroxide ($Ca(OH)_2$).
Reactant Ratio: Mixed in a ratio of $2:3$ by mass (excess $Ca(OH)_2$ is used to counteract the loss of $NH_4Cl$ due to sublimation on heating).
Chemical Equation:
$2NH_4Cl + Ca(OH)_2 \xrightarrow{\Delta} CaCl_2 + 2H_2O + 2NH_3\uparrow$
Important Technical Details
- Sloping Flask Position: The round-bottom flask is tilted with its neck sloping downwards. This prevents any water vapor formed during the reaction from condensing on the neck and trickling back onto the hot bottom of the flask, which would cause the glass to crack.
- Why is $Ca(OH)_2$ preferred over $NaOH$ or $KOH$?
Calcium hydroxide is cheap, non-deliquescent (does not absorb moisture to form a solution), and does not attack glass as aggressively as caustic soda ($NaOH$) or caustic potash ($KOH$).
- Why are $NH_4NO_3$ and $NH_4NO_2$ not used?
Ammonium nitrate and ammonium nitrite are explosive salts. On heating, they undergo thermal decomposition to yield nitrous oxide ($N_2O$) and nitrogen ($N_2$) respectively, rather than ammonia:
$NH_4NO_3 \xrightarrow{\Delta} N_2O + 2H_2O$
$NH_4NO_2 \xrightarrow{\Delta} N_2 + 2H_2O$
- Drying of Ammonia Gas: The moist gas is passed through a drying tower containing Quicklime ($CaO$).
Other common drying agents like conc. $H_2SO_4$, anhydrous $CaCl_2$, or phosphorus pentoxide ($P_2O_5$) cannot be used because they react chemically with basic ammonia:
$2NH_3 + H_2SO_4 \to (NH_4)_2SO_4$
$CaCl_2 + 8NH_3 \to CaCl_2 \cdot 8NH_3\text{ (addition compound)}$
$P_2O_5 + 3H_2O + 6NH_3 \to 2(NH_4)_3PO_4$
- Collection Method: Collected by downward displacement of air (upward delivery) in an inverted gas jar because it is lighter than air ($V.D. = 8.5$) and highly soluble in water (cannot be collected over water).
- Identification Tests:
- Dense white fumes of ammonium chloride ($NH_4Cl$) are evolved when a glass rod dipped in concentrated hydrochloric acid is brought near the mouth of the gas jar.
- Turns moist red litmus paper blue.
- Turns moist turmeric paper brown.
- Turns phenolphthalein solution deep pink.
- Turns Nessler's reagent ($K_2[HgI_4]$) from pale yellow to brown (or gives a brown precipitate).
3. General Preparation of Ammonia
A. From Metal Nitrides using Warm Water
Ammonia gas is liberated when metal nitrides react with warm water:
- Magnesium Nitride:
$Mg_3N_2 + 6H_2O\text{ (warm)} \to 3Mg(OH)_2 + 2NH_3\uparrow$
- Aluminium Nitride:
$AlN + 3H_2O\text{ (warm)} \to Al(OH)_3 + NH_3\uparrow$
- Calcium Nitride:
$Ca_3N_2 + 6H_2O\text{ (warm)} \to 3Ca(OH)_2 + 2NH_3\uparrow$
B. From Ammonium Salts using Alkalis
When any ammonium salt is warmed with an alkali (such as sodium hydroxide, potassium hydroxide, or calcium hydroxide), ammonia gas is evolved along with a metallic salt and water:
$\text{Ammonium Salt} + \text{Alkali} \xrightarrow{\Delta} \text{Salt} + \text{Water} + \text{Ammonia}\uparrow$
- With Sodium Hydroxide ($NaOH$):
$NH_4Cl + NaOH \xrightarrow{\Delta} NaCl + H_2O + NH_3\uparrow$
$(NH_4)_2SO_4 + 2NaOH \xrightarrow{\Delta} Na_2SO_4 + 2H_2O + 2NH_3\uparrow$
- With Potassium Hydroxide ($KOH$):
$NH_4Cl + KOH \xrightarrow{\Delta} KCl + H_2O + NH_3\uparrow$
$(NH_4)_2SO_4 + 2KOH \xrightarrow{\Delta} K_2SO_4 + 2H_2O + 2NH_3\uparrow$
- With Calcium Hydroxide ($Ca(OH)_2$):
$(NH_4)_2SO_4 + Ca(OH)_2 \xrightarrow{\Delta} CaSO_4 + 2H_2O + 2NH_3\uparrow$
4. Industrial Manufacture by Haber's Process
Ammonia is manufactured on an industrial scale by the direct catalytic combination of atmospheric nitrogen and hydrogen.
Diagram Placeholder
Figure 9.2: Flowsheet of Haber's Process
Industrial flowsheet diagram showing compression pump (200 atm), catalytic chamber with Fe/Mo catalyst at 450°C-500°C, condensation cooling coil / liquefier, and recirculation pump for unreacted N₂ + H₂.
Haber's Process Equilibrium & Conditions
Fundamental Reaction:
$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) + 92.4\text{ kJ}\text{ (Exothermic, Reversible)}$
Sources of Reactants: Nitrogen is obtained from the fractional distillation of liquid air. Hydrogen is obtained from water gas (Bosch Process) or natural gas.
Optimum Operating Conditions:
- Temperature: $450^\circ\text{C} - 500^\circ\text{C}$ (An optimum temperature compromise between reaction rate and equilibrium yield).
- Pressure: $200 - 900\text{ atm}$ (typically around $200\text{ atm}$, favoring forward reaction where volume decreases from $4\text{ volumes}$ to $2\text{ volumes}$).
- Catalyst: Finely divided iron ($Fe$).
- Promoter: Traces of molybdenum ($Mo$) or aluminium oxide ($Al_2O_3$) / potassium oxide ($K_2O$).
Separation & Recirculation
- Only $15 - 20\%$ of the reactant gases are converted to ammonia in a single pass.
- Separation of Ammonia:
- By Liquefaction: Ammonia liquefies easily under pressure because of its higher critical temperature ($-33^\circ\text{C}$), whereas $N_2$ and $H_2$ remain gaseous.
- By Dissolution in Water: Ammonia readily dissolves in water forming aqueous ammonia, leaving unreacted $N_2$ and $H_2$ insoluble.
- Recirculation: The unreacted nitrogen and hydrogen gases are dried, pumped back, and recirculated over the catalyst to prevent any waste.
5. Physical Properties & The Fountain Experiment
Diagram Placeholder
Figure 9.3: The Fountain Experiment for Ammonia
Experimental setup showing inverted round-bottom flask of dry NH₃ gas with water dropper and jet tube dipping into red litmus solution, demonstrating extreme solubility and creation of a Blue Fountain.
The Ammonia Fountain Protocol
- Setup: A dry round-bottom flask filled with dry ammonia gas is inverted and fitted with a two-holed rubber cork carrying a dropper with water and a jet tube dipping into a beaker containing red litmus solution.
- Procedure: When water from the dropper is injected into the flask, a large volume of ammonia gas dissolves immediately due to its high solubility.
- Observation: The sudden reduction of pressure creates a partial vacuum inside the flask. Outside atmospheric pressure pushes the red litmus solution up the jet tube, which bursts into a splendid BLUE FOUNTAIN.
- Inference: Proves two essential properties:
- Ammonia gas is highly soluble in water ($1\text{ vol } H_2O$ dissolves $702\text{ vol } NH_3$).
- Ammonia gas in aqueous solution is basic/alkaline in nature ($NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-$).
6. Chemical Properties of Ammonia
A. Combustibility & Burning in Oxygen
Ammonia is non-combustible in air, but it burns in an atmosphere of pure oxygen with a yellowish-green flame:
$4NH_3 + 3O_2 \to 2N_2 + 6H_2O$
B. Catalytic Oxidation of Ammonia (Ostwald's Process First Step)
When a mixture of dry ammonia and oxygen (in the ratio $1:2$ by volume) is passed over heated platinum ($Pt$) gauze at $800^\circ\text{C}$, the following takes place:
Diagram Placeholder
Figure 9.4: Catalytic Oxidation of Ammonia
Apparatus diagram showing platinum gauze at 800°C glowing red-hot from exothermic oxidation of NH₃ to NO, followed by cooling to form dense reddish-brown fumes of NO₂ gas.
Catalytic Oxidation Pathway
Primary Reaction:
$4NH_3 + 5O_2 \xrightarrow{Pt,\ 800^\circ\text{C}} 4NO\text{ (Nitric oxide)} + 6H_2O + \text{Heat}$
Key Observations:
- The platinum gauze continues to glow red-hot even after the external heating is removed, because the reaction is strongly exothermic.
- The colourless nitric oxide ($NO$) formed cools down and reacts immediately with excess oxygen to produce dense reddish-brown fumes of nitrogen dioxide ($NO_2$):
$2NO + O_2 \to 2NO_2\uparrow\text{ (Reddish-brown fumes)}$
C. Reducing Action on Metallic Oxides
Ammonia acts as a reducing agent when passed over heated metallic oxides, reducing them to their respective metals while being oxidised to nitrogen gas and water vapour:
- Action on Black Copper(II) Oxide ($CuO$):
$3CuO\text{ (Black)} + 2NH_3 \xrightarrow{\Delta} 3Cu\text{ (Pinkish-red metallic copper)} + 3H_2O + N_2\uparrow$
- Action on Yellow Lead(II) Oxide / Litharge ($PbO$):
$3PbO\text{ (Yellow)} + 2NH_3 \xrightarrow{\Delta} 3Pb\text{ (Greyish metallic lead)} + 3H_2O + N_2\uparrow$
- Action on Dark Brown Lead(IV) Oxide ($PbO_2$):
$3PbO_2 + 4NH_3 \xrightarrow{\Delta} 3Pb + 6H_2O + 2N_2\uparrow$
D. Reaction with Chlorine ($Cl_2$)
Ammonia reacts with chlorine differently depending on which reactant is in excess:
Two Chlorine Reaction Scenarios
Case 1: When Ammonia is in Excess:
$8NH_3\text{ (Excess)} + 3Cl_2 \to 6NH_4Cl\text{ (Dense white fumes)} + N_2\uparrow$
Observation: Formation of dense white fumes of ammonium chloride and evolution of colourless nitrogen gas.
Case 2: When Chlorine is in Excess:
$NH_3 + 3Cl_2\text{ (Excess)} \to \mathbf{NCl_3\text{ (Nitrogen trichloride)}} + 3HCl$
Observation: Formation of a yellow, oily, highly explosive liquid of nitrogen trichloride ($NCl_3$).
7. Aqueous Ammonia & Analytical Precipitation Reactions
Aqueous ammonia ($NH_4OH$) is a weak alkali that partially dissociates in water: $NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-$.
A. Reaction with Acids
- With Sulphuric Acid: $2NH_4OH + H_2SO_4 \to (NH_4)_2SO_4 + 2H_2O$
- With Nitric Acid: $NH_4OH + HNO_3 \to NH_4NO_3 + H_2O$
- With Hydrochloric Acid: $NH_4OH + HCl \to NH_4Cl + H_2O$
B. Analytical Precipitation Tests for Metal Ions
When ammonium hydroxide solution is added dropwise to metal salt solutions, it precipitates insoluble metal hydroxides:
| Salt Solution |
Precipitate Formed |
Observation on Adding Excess $NH_4OH$ |
| Iron(III) Chloride ($FeCl_3$) |
$Fe(OH)_3$ (Reddish-brown ppt) |
Insoluble in excess $NH_4OH$ |
| Iron(II) Sulphate ($FeSO_4$) |
$Fe(OH)_2$ (Dirty green ppt) |
Insoluble in excess $NH_4OH$ |
| Lead Nitrate ($Pb(NO_3)_2$) |
$Pb(OH)_2$ (Chalky white ppt) |
Insoluble in excess $NH_4OH$ |
| Zinc Nitrate / Sulphate ($ZnSO_4$) |
$Zn(OH)_2$ (Gelatinous white ppt) |
Soluble in excess forming a clear solution of tetraamminezinc(II) sulphate: $[Zn(NH_3)_4]SO_4$ |
| Copper(II) Sulphate ($CuSO_4$) |
$Cu(OH)_2$ (Pale blue ppt) |
Soluble in excess forming a striking deep inky blue solution of tetraamminecopper(II) sulphate: $[Cu(NH_3)_4]SO_4$ |
Complex Formation Equations
With Zinc Salts ($ZnSO_4$ or $Zn(NO_3)_2$):
$ZnSO_4 + 2NH_4OH \to Zn(OH)_2\downarrow\text{ (Gelatinous white)} + (NH_4)_2SO_4$
$Zn(NO_3)_2 + 2NH_4OH \to Zn(OH)_2\downarrow\text{ (Gelatinous white)} + 2NH_4NO_3$
$Zn(OH)_2 + (NH_4)_2SO_4 + 2NH_4OH\text{ (excess)} \to [Zn(NH_3)_4]SO_4 + 4H_2O\text{ (Colourless solution)}$
With Copper Salts ($CuSO_4$):
$CuSO_4 + 2NH_4OH \to Cu(OH)_2\downarrow\text{ (Pale blue)} + (NH_4)_2SO_4$
$Cu(OH)_2 + (NH_4)_2SO_4 + 2NH_4OH\text{ (excess)} \to [Cu(NH_3)_4]SO_4 + 4H_2O\text{ (Deep inky blue solution)}$
8. Tests & Uses of Ammonia
A. Identification Tests
- Choking Smell: Characteristic pungent smell that causes tears in eyes.
- Hydrochloric Acid Glass Rod: Gives dense white fumes of ammonium chloride ($NH_4Cl$).
- Litmus Paper: Turns moist red litmus paper blue.
- Nessler's Reagent: Turns Nessler's reagent ($K_2[HgI_4]$) from pale yellow to brown.
- Copper Sulphate Solution: Turns copper sulphate solution pale blue, which deepens into an intense inky blue solution when excess ammonia is added.
B. Major Industrial & Domestic Uses
- Manufacture of Fertilisers: Production of urea ($NH_2CONH_2$), ammonium sulphate ($(NH_4)_2SO_4$), ammonium nitrate ($NH_4NO_3$), and diammonium phosphate ($DAP$).
- Manufacture of Nitric Acid: Used as the starting reactant in Ostwald's Process.
- Manufacture of Explosives: Ammonium nitrate is widely used in making commercial mining explosives (ANFO).
- Refrigeration: Liquid ammonia is used as a refrigerant in large ice plants due to its high latent heat of vaporisation ($1370\text{ J/g}$). It serves as a non-ozone-depleting alternative to CFCs.
- Cleansing Agent: Aqueous ammonia is used to remove grease and oil stains from clothing, glass windows, and porcelain fixtures.
9. Solved ICSE Board Practice Questions
ICSE Board Question 1 (Lab Preparation)
(a) Write a balanced chemical equation for the laboratory preparation of ammonia gas from ammonium chloride.
(b) Why is higher ratio of slaked lime used during this preparation?
(c) Name the drying agent used and state why concentrated sulphuric acid cannot be used.
Solution:
(a) $2NH_4Cl + Ca(OH)_2 \xrightarrow{\Delta} CaCl_2 + 2H_2O + 2NH_3\uparrow$
(b) Slaked lime is used in higher ratio by mass ($2:3$) to counteract the sublimation of ammonium chloride when heated.
(c) Quicklime ($CaO$) is used as the drying agent. Concentrated sulphuric acid cannot be used because it reacts chemically with basic ammonia to form ammonium sulphate ($(NH_4)_2SO_4$).
ICSE Board Question 2 (Catalytic Oxidation & Burning)
(a) Name the catalyst and the temperature required for the catalytic oxidation of ammonia.
(b) Write the balanced chemical equation for this reaction.
(c) What reddish-brown gas is observed when the products cool in air?
Solution:
(a) Catalyst: Platinum ($Pt$) gauze; Temperature: $800^\circ\text{C}$.
(b) $4NH_3 + 5O_2 \xrightarrow{Pt,\ 800^\circ\text{C}} 4NO + 6H_2O + \text{Heat}$
(c) Nitrogen dioxide gas ($NO_2$) is formed when colourless nitric oxide combines with atmospheric oxygen: $2NO + O_2 \to 2NO_2\uparrow$.
ICSE Board Question 3 (Reducing Action & Chlorine Reactions)
State your observations when:
(a) Dry ammonia gas is passed over heated black copper(II) oxide.
(b) Excess ammonia reacts with chlorine gas.
(c) Ammonia reacts with excess chlorine gas.
Solution:
(a) The black copper(II) oxide turns into pinkish-red metallic copper, and colourless droplets of water condense on the cooler parts of the tube along with the evolution of nitrogen gas.
(b) Dense white fumes of ammonium chloride ($NH_4Cl$) are formed along with the evolution of nitrogen gas ($8NH_3 + 3Cl_2 \to 6NH_4Cl + N_2$).
(c) A yellow, oily, highly explosive liquid of nitrogen trichloride ($NCl_3$) is formed ($NH_3 + 3Cl_2 \to NCl_3 + 3HCl$).
ICSE Board Question 4 (Analytical Reactions)
State what is observed when ammonium hydroxide solution is added dropwise and then in excess to:
(a) Zinc sulphate solution
(b) Copper sulphate solution
Solution:
(a) A gelatinous white precipitate of zinc hydroxide ($Zn(OH)_2$) is formed, which completely dissolves in excess ammonium hydroxide to give a clear colourless solution of tetraamminezinc(II) sulphate.
(b) A pale blue precipitate of copper(II) hydroxide ($Cu(OH)_2$) is formed, which dissolves in excess ammonium hydroxide to produce an intense, inky deep blue solution of tetraamminecopper(II) sulphate.
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