Nitrogen (N): Chemical Properties & Reactions Study Guide

Chemical Properties Overview

Nitrogen (N) is a p-block element residing in Group 15 of the periodic table. Its chemical behavior is profoundly influenced by its electronic configuration ([He] 2s²2p³) and the robust triple bond in its diatomic form (N₂).

• Reactivity Series Position: Molecular nitrogen (N₂) is notably inert at room temperature due to the exceptionally high bond dissociation energy (945 kJ/mol) of the N≡N triple bond. This inertness distinguishes it from more reactive elements in standard reactivity series.
• Electronegativity: With a high electronegativity value (3.04 on the Pauling scale), nitrogen is capable of forming strong polar covalent bonds. It exhibits a wide range of oxidation states, from -3 (e.g., in ammonia, NH₃) to +5 (e.g., in nitric acid, HNO₃).
• General Reactivity:
  • Inertness: N₂ is a colorless, odorless, and largely unreactive gas under ambient conditions, making it a common choice for creating inert atmospheres in laboratories and industries.
  • Reactivity at high temperatures/pressures: Significant reactivity is observed only under harsh conditions, such as high temperatures, high pressures, or in the presence of catalysts. Examples include reactions with very reactive metals (e.g., lithium, magnesium) and non-metals (e.g., hydrogen, oxygen).
  • Variable Oxidation States: Nitrogen can display oxidation states of -3, 0, +1, +2, +3, +4, and +5, showcasing its versatility in compound formation.

Action of Air and Oxygen

Molecular nitrogen (N₂) is unreactive with most components of air at room temperature. Its reaction with oxygen is a high-energy process.

• With Oxygen (O₂):
  • At extremely high temperatures (e.g., during lightning discharge, in internal combustion engines, or in an electric arc, typically above 3000 K), nitrogen combines directly with oxygen to produce nitric oxide. N₂(g) + O₂(g) $\xrightarrow{\text{High Temp / Electric Arc}}$ 2NO(g)
  • Nitric oxide (NO) is a colorless gas that readily reacts with atmospheric oxygen to form reddish-brown nitrogen dioxide (NO₂). 2NO(g) + O₂(g) $\rightarrow$ 2NO₂(g)
• With other components of Air: Nitrogen exhibits no direct chemical reaction with carbon dioxide, noble gases, or water vapor present in the atmosphere under normal conditions.

Action of Water and Steam

Elemental nitrogen (N₂) demonstrates no chemical reactivity with water or steam across a range of typical laboratory or industrial conditions. It is only sparingly soluble in water.

• N₂(g) + H₂O(l) $\rightarrow$ No Reaction
• N₂(g) + H₂O(g) (Steam) $\rightarrow$ No Reaction

Note: Ammonia (NH₃), a compound of nitrogen, is highly soluble in water, forming ammonium hydroxide (NH₄OH). However, this is a property of ammonia, not of elemental nitrogen directly reacting with water.

Action of Acids and Bases

Due to the exceptional stability of the N≡N triple bond and its inherent inertness, molecular nitrogen (N₂) does not react with common acids (whether dilute or concentrated) or strong bases (alkalis) under standard conditions.

• N₂(g) + Acids (e.g., HCl, H₂SO₄, HNO₃) $\rightarrow$ No Reaction
• N₂(g) + Bases (e.g., NaOH, KOH) $\rightarrow$ No Reaction

Key Laboratory Test/Identification Reactions

While elemental nitrogen (N₂) itself is primarily identified by its inert gaseous properties, the chemical identification methods in qualitative analysis typically focus on its common ionic forms: ammonium (NH₄⁺), nitrite (NO₂⁻), and nitrate (NO₃⁻).

1. Identification of Ammonium Ion (NH₄⁺)

• Liberation of Ammonia Gas: When an ammonium salt is heated with a strong alkali (e.g., sodium hydroxide solution or calcium hydroxide), ammonia gas (NH₃) is liberated. NH₄⁺(aq) + OH⁻(aq) $\xrightarrow{\text{Heat}}$ NH₃(g) + H₂O(l) The evolved NH₃ gas is identified by:
  • Its characteristic pungent smell.
  • Turning moist red litmus paper blue (ammonia is basic).
  • Forming dense white fumes of ammonium chloride (NH₄Cl) when a glass rod dipped in concentrated hydrochloric acid is brought near the mouth of the test tube. NH₃(g) + HCl(g) $\rightarrow$ NH₄Cl(s) (Dense white fumes)
• Nessler’s Reagent Test: Ammonia gas, upon passing through Nessler’s reagent (alkaline solution of potassium tetraiodomercurate(II), K₂[HgI₄]), produces a characteristic brown precipitate or yellow-brown coloration. 2K₂HgI₄ + NH₃(g) + 3KOH(aq) $\rightarrow$ O(HgI)(HgNH₂)(s) (Brown ppt, Iodide of Millon’s Base) + 7KI(aq) + 2H₂O(l)

2. Identification of Nitrate Ion (NO₃⁻)

• Brown Ring Test: This is the definitive test for nitrate ions. To the solution containing nitrate, freshly prepared ferrous sulfate (FeSO₄) solution is added. Concentrated sulfuric acid (H₂SO₄) is then carefully added down the side of the test tube, forming a distinct brown ring at the interface of the two layers.
  • Step 1: Reduction of nitrate: The nitrate ion is reduced to nitric oxide (NO) by Fe²⁺ in the acidic medium. NO₃⁻(aq) + 3Fe²⁺(aq) + 4H⁺(aq) $\rightarrow$ NO(g) + 3Fe³⁺(aq) + 2H₂O(l)
  • Step 2: Formation of brown ring complex: The nitric oxide gas then combines with excess Fe²⁺ ions to form a brown colored complex, pentaaquanitrosyliron(II). Fe²⁺(aq) + NO(g) $\rightarrow$ [Fe(H₂O)₅NO]²⁺ (Brown ring complex)

3. Identification of Nitrite Ion (NO₂⁻)

• Starch-Iodide Test: Acidifying a solution containing nitrite ions in the presence of potassium iodide (KI) and starch solution results in a blue-black coloration. The nitrite ion oxidizes iodide ions (I⁻) to iodine (I₂), which then forms a blue-black complex with starch. 2NO₂⁻(aq) + 2I⁻(aq) + 4H⁺(aq) $\rightarrow$ 2NO(g) + I₂(aq) + 2H₂O(l) I₂(aq) + Starch solution $\rightarrow$ Blue-black color
• Reaction with Dilute Acid: Upon acidification with a dilute acid (e.g., HCl), nitrite ions produce nitrous acid (HNO₂), which is unstable and decomposes upon warming, liberating nitric oxide (NO). This NO then reacts with atmospheric oxygen to produce reddish-brown fumes of nitrogen dioxide (NO₂). NO₂⁻(aq) + H⁺(aq) $\rightarrow$ HNO₂(aq) 3HNO₂(aq) $\rightarrow$ HNO₃(aq) + 2NO(g) + H₂O(l) 2NO(g) + O₂(g) $\rightarrow$ 2NO₂(g) (Reddish-brown fumes)