Comprehensive Study Guide on Ammonia: Preparation, Properties, and Reactions

Laboratory Preparation of Ammonia: Method 1 (Alkali and Ammonium Salt)

• Reagents and Reaction: Ammonia is prepared by heating a solid mixture of calcium hydroxide ($\text{slaked lime}$) and ammonium chloride ($\text{sal ammoniac}$ or rock salt).

  • Equation: $\text{Ca(OH)}_2(s) + 2\text{NH}_4\text{Cl}(s) \rightarrow \text{CaCl}_2(s) + 2\text{H}_2\text{O}(l) + 2\text{NH}_3(g)$.

• Reagent Selection and Ratios:

  • Higher Ratio of Alkali: A higher ratio by weight of the alkali (usually $1:2$ ratio of $\text{NH}_4\text{Cl}$ to $\text{Ca(OH)}_2$) is used to counteract any loss of ammonium chloride that may occur due to sublimation.
  • Slaked Lime over Pellets: Slaked lime is preferred because it is not deliquescent, unlike sodium hydroxide ($\text{NaOH}$) or potassium hydroxide ($\text{KOH}$).
  • Excluded Salts: Ammonium nitrate ($\text{NH}_4\text{NO}_3$) and ammonium nitrite ($\text{NH}_4\text{NO}_2$) are not used because they are explosive in nature and do not yield ammonia gas upon decomposition; instead, they decompose into nitrogen or nitrogen oxides.

• Procedural Precautions:

  • Inclined Flask: During heating, the round-bottom flask is kept in an inclined position. This ensures that any water formed during the reaction does not trickle back into the hot bottom of the flask, which could cause the glass to crack.

• Drying of Ammonia:

  • Drying Agent: Calcium oxide ($\text{CaO}$, quicklime) is used.
  • Compatibility: Quicklime is used because it is basic, and since ammonia is also basic, they do not react with each other.
  • Incompatible Drying Agents: Acidic drying agents cannot be used because they react with ammonia to form salts:
    • Reaction with Sulphuric Acid: $2\text{NH}_3 + \text{H}_2\text{SO}_4 \rightarrow (\text{NH}_4)_2\text{SO}_4$
    • Reaction with Phosphorus Pentoxide: $6\text{NH}_3 + \text{P}_2\text{O}_5 + 3\text{H}_2\text{O} \rightarrow 2(\text{NH}_4)_3\text{PO}_4$
  • Reaction with Calcium Chloride: Ammonia cannot be dried using fused calcium chloride ($\text{CaCl}_2$) because it forms an addition compound: $\text{CaCl}_2 + 8\text{NH}_3 \rightarrow \text{CaCl}_2 \times 8\text{NH}_3$.

• Collection of Ammonia:

  • Method: Collected in an inverted dry gas jar by the downward displacement of air.
  • Reasoning: Ammonia is highly soluble in water (precluding downward displacement of water) and is lighter than air.

Laboratory Preparation of Ammonia: Method 2 (Metal Nitrides)

• Principle: Ammonia is produced by the action of hot water on metal nitrides (binary compounds of a metal and nitrogen) via hydrolysis.
• Example (Magnesium Nitride): $\text{Mg}_3\text{N}_2 + 6\text{H}_2\text{O} \rightarrow 3\text{Mg(OH)}_2 + 2\text{NH}_3$
• Procedure:
  • Magnesium nitride is placed in a round-bottom flask with a thistle funnel and delivery tube.
  • Hot water is added dropwise through the funnel.
  • The liberated gas is dried using a quicklime tower and collected by downward displacement of air.

Preparation and Forms of Aqueous Ammonia

• Aqueous Solution Preparation:

  • Because ammonia is extremely soluble in water, it dissolves faster than it is produced, which can create a partial vacuum and lead to "back suction" (water rushing into the reaction flask).
  • Funnel Arrangement: To prevent back suction, an inverted funnel is used at the end of the delivery tube. This arrangement also provides a larger surface area for gas absorption.
  • Mechanism: As water rises in the funnel due to low pressure, the rim of the funnel loses contact with the water surface. Air then enters to equalise the pressure, causing the water to drop back down.

• Classifications of Ammonia:

  • Liquid Ammonia: Ammonia gas condensed into a liquid state ($\text{NH}_3$).
  • Liquor Ammonia: Ammonia gas dissolved in water ($\text{NH}_4\text{OH}$).
  • Liquor Ammonia Fortis: A saturated solution of ammonia in water with a specific gravity of $0.88$.

Industrial Preparation: The Haber Process

• Reaction: A reversible, exothermic reaction between nitrogen and hydrogen.

  • Equation: $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3 + 22.4\text{ kcal}$ (or $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3 + \text{heat}$).

• Equilibrium Conditions (Le Chatelier's Principle):

  • Pressure: High pressure favours the forward reaction because 4 volumes of reactants yield 2 volumes of product. Optimum range: $200 \times 500\text{ atm}$.
  • Temperature: The reaction is exothermic, so low temperatures favor the forward reaction. However, at very low temperatures, the reaction rate is too slow. Therefore, an optimum temperature of $450^{\circ}\text{C}$ (not exceeding $500^{\circ}\text{C}$ to prevent decomposition) is used.
  • Catalyst: Finely divided iron ($\text{Fe}$) acts as the catalyst to speed up the reaction. Molybdenum ($\text{Mo}$) is used as a promoter to increase catalyst efficiency.
  • Alternate Catalyst: A mixture of ferric oxide ($\text{Fe}_2\text{O}_3$), potassium oxide ($\text{K}_2\text{O}$), and aluminium oxide ($\text{Al}_2\text{O}_3$) can also be utilized.

• Process Steps:

  1. Compressor: Reactants ($\text{N}_2$ from fractional distillation of air, $\text{H}_2$ from Bosch process/Natural gas) are dried, purified, and compressed to $200 \times 500\text{ atm}$.
  2. Catalyst Chamber: The mixture is heated to $450^{\circ}\text{C}$ in the presence of the catalyst and promoter. Heat exchangers use the heat generated by the reaction to warm incoming gases.
  3. Condenser: Ammonia is recovered by liquefaction or by dissolving it in water. Unreacted $\text{N}_2$ and $\text{H}_2$ (roughly $2/3$ of the volume) are recycled.

Physical Properties and The Fountain Experiment

• Density: Ammonia has a density of $0.769\text{ kg/m}^3$ at STP, making it lighter than air.

• Solubility: Highly soluble; $1\text{ litre}$ of water can dissolve $460\text{ litres}$ of ammonia at room temperature ($25^{\circ}\text{C}$).

• Fountain Experiment:

  • Setup: A flask of dry $\text{NH}_3$ gas with a jet tube and a water dropper is inverted over red litmus solution.
  • Effect: Squeezing the dropper dissolves the gas, creating a vacuum. Atmospheric pressure forces the red litmus up the tube, where it turns into a blue fountain due to the basic nature of ammonia.

• States of Matter:

  • Boiling Point: $-33.5^{\circ}\text{C}$.
  • Freezing Point: $-77.7^{\circ}\text{C}$.

• Physiological Impact: Ammonia has a strong, pungent, irritating odour. It affects the tear glands and respiratory muscles (causing choking) and can be fatal in large quantities.

Chemical Properties of Ammonia

• Combustibility:

  • Ammonia is non-combustible and does not support combustion.
  • In Oxygen (without catalyst): Burns with a yellowish-green (greenish-yellow) flame when mixed with $15\% \times 25\%$ air.
  • Equation: $4\text{NH}_3 + 3\text{O}_2 \rightarrow 2\text{N}_2 + 6\text{H}_2\text{O}$.

• Catalytic Oxidation (Ostwald Process Step):

  • Ammonia mixed with dry oxygen is passed over heated platinum ($800^{\circ}\text{C}$).
  • Reaction: $4\text{NH}_3 + 5\text{O}_2 \rightarrow 4\text{NO} + 6\text{H}_2\text{O}$.
  • Observation: Colourless nitric oxide ($\text{NO}$) forms, which then reacts with air to form reddish-brown nitrogen dioxide ($\text{NO}_2$) fumes: $2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2$.
  • Indicator Change: Moist red litmus first turns blue (basic $\text{NH}_3$), then the resulting $\text{NO}_2$ gas turns blue litmus red (acidic nitrogen dioxide dissolves to form nitric acid).

• Basic Nature:

  • Ammonia molecules are covalently bonded. Dry/liquid ammonia doesn't conduct electricity or affect litmus.
  • Ionization: When dissolved in water, it ionizes: $\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-$. The lone pair of electrons on nitrogen facilitates the formation of the ammonium ion ($\text{NH}_4^+$).

• Reaction with Acids (Forming Ammonium Salts):

  • $\text{NH}_3 + \text{HCl} \rightarrow \text{NH}_4\text{Cl}$ (Ammonium chloride - dense white fumes)
  • $2\text{NH}_3 + \text{H}_2\text{SO}_4 \rightarrow (\text{NH}_4)_2\text{SO}_4$ (Ammonium sulphate)
  • $\text{NH}_3 + \text{HNO}_3 \rightarrow \text{NH}_4\text{NO}_3$ (Ammonium nitrate)

Precipitation and Identification of Metallic Radicals

• Aqueous ammonia reacts with metallic salt solutions to precipitate metallic hydroxides. The colour of the precipitate helps identify the radical.
  • Ferrous Sulphate ($\text{Fe}^{2+}$): $\text{FeSO}_4 + 2\text{NH}_4\text{OH} \rightarrow \text{Fe(OH)}_2 \downarrow (\text{Dirty green}) + (\text{NH}_4)_2\text{SO}_4$
  • Ferric Chloride ($\text{Fe}^{3+}$): $\text{FeCl}_3 + 3\text{NH}_4\text{OH} \rightarrow \text{Fe(OH)}_3 \downarrow (\text{Reddish brown}) + 3\text{NH}_4\text{Cl}$
  • Zinc Nitrate ($\text{Zn}^{2+}$): \text{Zn(NO}_3)_2 + 2\text{NH}_4\text{OH} \rightarrow \text{Zn(OH)}_2 \downarrow (\text{Gelatinous white}) + 2\text{NH}_4\text{NO}_3$\n * **Excess NH4OH**: Precipitate dissolves to form complex salt: \text{Zn(OH)}_2 + 4 ext{NH}_4 ext{OH} ightarrow [ ext{Zn(NH}_3)_4]( ext{OH})_2 + 4 ext{H}_2 ext{O} (Tetra-amine zinc hydroxide).\n * **Lead(II) Nitrate ( ext{Pb}^{2+}$)**:$\text{Pb(NO}_3)_2 + 2 ext{NH}_4 ext{OH} ightarrow ext{Pb(OH)}_2 \downarrow ( ext{Chalky white}) + 2 ext{NH}_4 ext{NO}_3$. Note: Insoluble in excess ammonia.
  • Copper(II) Sulphate ($\text{Cu}^{2+}$): \text{CuSO}_4 + 2\text{NH}_4\text{OH} \rightarrow \text{Cu(OH)}_2 \downarrow (\text{Pale blue}) + (\text{NH}_4)_2\text{SO}_4$\n * **Excess NH4OH**: Precipitate dissolves to form a deep blue/violet solution: \text{Cu(OH)}_2 + 2 ext{NH}_4 ext{OH} + ( ext{NH}_4)_2 ext{SO}_4 ightarrow [ ext{Cu(NH}_3)_4] ext{SO}_4 + 4 ext{H}_2 ext{O} (Tetra-amine copper(II) sulphate).\n\n# Reducing Properties of Ammonia\n\n* **Reduction of Chlorine**:\n * **Ammonia in Excess**: \text{8NH}_3 + ext{3Cl}_2 ightarrow ext{N}_2 + ext{6NH}_4 ext{Cl}$. Unreacted$\text{NH}_3$reacts with$\text{HCl} to form dense white fumes.\n * **Chlorine in Excess**: \text{NH}_3 + ext{3Cl}_2 ightarrow ext{NCl}_3 ( ext{Explosive yellow liquid}) + ext{3HCl}.\n\n* **Reduction of Metal Oxides**:\n * **Copper(II) Oxide**: Dry \text{NH}_3$passed over heated black$\text{CuO}$. Ammonia reduces it to brownish-pink metallic copper:$2 ext{NH}_3 + 3 ext{CuO} ightarrow 3 ext{Cu} + 3 ext{H}_2 ext{O} + ext{N}_2 \uparrow.\n * **Water Test**: Condensate turns cobalt chloride paper pink.\n * **Nitrogen Test**: Burning magnesium in the collected gas produces \text{Mg}_3 ext{N}_2.\n * **Lead(II) Oxide**: Ammonia reduces yellow \text{PbO}$(which is reddish-brown when hot) to silvery-white Lead:$2 ext{NH}_3 + 3 ext{PbO} ightarrow 3 ext{Pb} + 3 ext{H}_2 ext{O} + ext{N}_2 \uparrow.\n\n# Analytical Tests for Ammonia gas\n\n1. **Odour**: Strong, pungent, and irritating.\n2. **Indicators**: \n * Moist red litmus turns blue.\n * Turmeric paper turns brown.\n * Phenolphthalein solution turns pink.\n * Methyl orange turns from orange to yellow.\n3. **Reaction with HCl**: A glass rod dipped in concentrated \text{HCl}$held near the gas produces dense white fumes ($ ext{NH}_4 ext{Cl}).\n4. **Copper Sulphate Test**: Passing the gas into \text{CuSO}_4$solution first gives a pale blue precipitate ($ ext{Cu(OH)}_2$$), which dissolves in excess ammonia to form a deep blue solution (tetra-amine copper sulphate).