Why is dinitrogen ($N_2$) gas so unreactive at room temperature, despite being abundant in the atmosphere?

Dinitrogen ($N_2$) gas is remarkably unreactive at room temperature primarily due to the presence of a very strong triple bond ($N equiv N$) between the two nitrogen atoms. This triple bond has an exceptionally high bond dissociation enthalpy of $941.4, ext{kJ/mol}$. Breaking this bond requires a significant amount of energy, which is not readily available under normal ambient conditions. Consequently, $N_2$ does not easily participate in chemical reactions, making it an inert gas that dilutes oxygen in the atmosphere and prevents rapid oxidation processes.

Explain the significance of the Haber process in modern society.

The Haber process is one of the most significant chemical processes ever developed, as it enables the industrial synthesis of ammonia ($NH_3$) directly from atmospheric nitrogen ($N_2$) and hydrogen ($H_2$). Ammonia is the primary precursor for almost all nitrogen-containing fertilizers. Without the Haber process, it would be impossible to produce enough food to sustain the global population, as natural nitrogen fixation processes are insufficient. It is estimated that the Haber process is responsible for feeding billions of people by providing the necessary nitrogen for agricultural productivity.

How does ammonia act as both a Lewis base and a Brønsted-Lowry base?

Ammonia ($NH_3$) acts as a Lewis base because its nitrogen atom possesses a lone pair of electrons, which it can donate to an electron-deficient species (a Lewis acid) to form a coordinate covalent bond. For example, it forms complex ions with transition metal cations. Simultaneously, ammonia acts as a Brønsted-Lowry base because it can accept a proton ($H^+$) from an acid. When dissolved in water, it accepts a proton from water to form ammonium ions ($NH_4^+$) and hydroxide ions ($OH^-$), demonstrating its basic character.

What are the different products formed when copper reacts with dilute versus concentrated nitric acid?

The reaction of copper with nitric acid yields different nitrogen-containing products depending on the acid's concentration. With dilute nitric acid, copper reacts to produce nitric oxide ($NO$) gas, which is colorless. The balanced equation is: $3Cu + 8HNO_3( ext{dilute}) ightarrow 3Cu(NO_3)_2 + 2NO + 4H_2O$. In contrast, with concentrated nitric acid, copper reacts to produce nitrogen dioxide ($NO_2$) gas, which is a reddish-brown gas. The balanced equation is: $Cu + 4HNO_3( ext{conc.}) ightarrow Cu(NO_3)_2 + 2NO_2 + 2H_2O$. This difference highlights nitric acid's varying oxidizing power.

Why does nitric oxide ($NO$) readily turn reddish-brown when exposed to air?

Nitric oxide ($NO$) is a colorless gas, but it contains an odd number of electrons (11 valence electrons), making it a paramagnetic and relatively reactive molecule. When exposed to air, $NO$ readily reacts with oxygen ($O_2$) present in the atmosphere to form nitrogen dioxide ($NO_2$). Nitrogen dioxide is a reddish-brown gas, which is why colorless $NO$ appears reddish-brown upon contact with air. The reaction is: $2NO(g) + O_2(g) ightarrow 2NO_2(g)$. This rapid oxidation is a characteristic property of nitric oxide.

What is the phenomenon of 'passivity' observed with certain metals in concentrated nitric acid?

Passivity is a phenomenon where certain metals, such as iron (Fe), chromium (Cr), and aluminum (Al), become unreactive or 'passive' when treated with concentrated nitric acid. Instead of dissolving, these metals form a very thin, dense, and non-porous protective oxide layer on their surface. This oxide layer acts as a barrier, preventing further contact between the metal and the acid, thereby inhibiting any further chemical reaction. This protective layer makes these metals resistant to corrosion under specific conditions.

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Nitrogen and its Compounds

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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Nitrogen, a non-metallic element with atomic number 7, is the first member of Group 15 (Pnictogens) in the periodic table. It is a vital constituent of the Earth's atmosphere, comprising approximately 78% by volume as dinitrogen ( $N_2$ ) gas. Its unique electronic configuration ( $1s^2 2s^2 2p^3$ ) and the presence of a stable triple bond in its diatomic form contribute to its relative inertness at r…

Quick Summary

Nitrogen, the most abundant gas in the atmosphere, is characterized by its inert diatomic form ( $N_2$ ) due to a strong triple bond. Despite this, it forms a wide array of crucial compounds, exhibiting oxidation states from -3 to +5.

Key compounds include ammonia ( $NH_3$ ), various oxides of nitrogen ( $N_2O, NO, N_2O_3, NO_2, N_2O_4, N_2O_5$ ), and nitric acid ( $HNO_3$ ). Ammonia is industrially produced via the Haber process, a high-pressure, moderate-temperature catalytic reaction, and is vital for fertilizers.

It's a pyramidal molecule, a Lewis base, and forms complexes. Oxides of nitrogen vary in color, acidity, and oxidation state; for instance, $NO$ is colorless and paramagnetic, while $NO_2$ is reddish-brown and also paramagnetic, readily dimerizing to colorless $N_2O_4$ .

Nitric acid is a strong oxidizing acid, industrially made by the Ostwald process. Its reactions with metals depend on concentration, producing different nitrogen oxides. Understanding these compounds' preparation, properties, structures, and uses is fundamental for NEET, with special attention to industrial processes and reaction conditions.

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Key Concepts

Haber Process Conditions and Le Chatelier's Principle

The Haber process, $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ , is an exothermic reaction ($Delta H =…

Oxidizing Nature of Nitric Acid with Metals

Nitric acid ( $HNO_3$ ) is a powerful oxidizing agent, and its reduction products vary significantly based on…

Ammonia as a Ligand and Complex Formation

Ammonia ( $NH_3$ ) acts as a ligand due to the presence of a lone pair of electrons on the nitrogen atom. This…

Dinitrogen ($N_2$) — Inert due to

N equiv N

bond. Lab prep:

NH_4Cl + NaNO_2 xrightarrow{\text{heat}} N_2

. Industrial: Fractional distillation of liquid air.

Ammonia ($NH_3$) — Pyramidal,

sp^3

N. Lewis base. Haber process:

N_2 + 3H_2 \rightleftharpoons 2NH_3

(Fe catalyst,

450-500^circ C

200,\text{atm}

). Forms complexes (e.g.,

[Cu(NH_3)_4]^{2+}

Oxides of Nitrogen

- $N_2O$ : +1, colorless, neutral, diamagnetic. - $NO$ : +2, colorless, neutral, paramagnetic, oxidizes to $NO_2$ in air. - $N_2O_3$ : +3, blue solid, acidic, unstable. - $NO_2$ : +4, reddish-brown, acidic, paramagnetic, dimerizes to $N_2O_4$ . - $N_2O_4$ : +4, colorless, diamagnetic, dimer of $NO_2$ . - $N_2O_5$ : +5, colorless solid, acidic, strong oxidizing agent.

Nitric Acid ($HNO_3$) — Strong acid, powerful oxidizing agent. Ostwald process:

NH_3 xrightarrow{O_2, Pt/Rh} NO xrightarrow{O_2} NO_2 xrightarrow{H_2O, O_2} HNO_3

- Reactions with metals: Dilute $HNO_3 \rightarrow NO$ ; Conc. $HNO_3 \rightarrow NO_2$ . Passivity with Fe, Cr, Al.

For the Oxides of Nitrogen and their oxidation states, remember: Never Never Never Never Never Never Outside Outside Outside Outside Outside

This represents the number of Nitrogen and Oxygen atoms in the common oxides, and helps recall their oxidation states in increasing order:

$N_2O$ (N=+1) - Never Outside (1 N, 1 O for oxidation state calc) $NO$ (N=+2) - Never Outside (1 N, 1 O for oxidation state calc) $N_2O_3$ (N=+3) - Never Outside (2 N, 3 O for oxidation state calc) $NO_2$ (N=+4) - Never Outside (1 N, 2 O for oxidation state calc) $N_2O_4$ (N=+4) - Never Outside (2 N, 4 O for oxidation state calc) $N_2O_5$ (N=+5) - Never Outside (2 N, 5 O for oxidation state calc)

While the mnemonic itself is simple, the key is to associate the increasing number of 'O's (and 'N's for $N_2O_x$ ) with the increasing oxidation state of nitrogen. For example, $N_2O$ is +1, $NO$ is +2, $N_2O_3$ is +3, $NO_2$ is +4, $N_2O_4$ is +4, $N_2O_5$ is +5. It's a quick way to mentally check the order and corresponding oxidation states.

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