NO₂ — Nitrogen Dioxide

Nitrogen dioxide (NO2) is a reddish-brown, toxic gas that plays a significant role in atmospheric chemistry. It is one of the most important oxides of nitrogen, known collectively as NOx gases. NO2 is an air pollutant that contributes to the formation of acid rain, photochemical smog, and tropospheric ozone. It is also a by-product of combustion processes such as automobile engines, industrial furnaces, and power plants that burn fossil fuels.

In environmental chemistry, nitrogen dioxide acts as both a primary and secondary pollutant. It forms directly during combustion and also through the oxidation of nitric oxide (NO). In the atmosphere, it participates in complex photochemical reactions that affect air quality and climate.

Nitrogen dioxide has the molecular formula \(NO_2\). It consists of one nitrogen atom covalently bonded to two oxygen atoms. The molecule is bent with an angle of approximately 134°, due to the presence of a lone electron on the nitrogen atom. The electronic structure of NO2 makes it a free radical with one unpaired electron, accounting for its paramagnetic nature.

The bonding can be explained by resonance structures that show delocalization of electrons between the nitrogen and oxygen atoms:

The unpaired electron resides mainly on the nitrogen atom, contributing to its reactivity and tendency to dimerize. NO2 can combine to form dinitrogen tetroxide (N2O4), especially at lower temperatures, in a reversible equilibrium:

This dimerization causes the color of the gas to fade as the temperature decreases since N2O4 is colorless.

Nitrogen dioxide can be prepared in several ways, both in laboratories and industrially:

• By oxidation of nitric oxide:

  \( 2NO + O_2 \rightarrow 2NO_2 \)

  This reaction occurs readily in air and represents the most common natural and industrial pathway.
• By thermal decomposition of metal nitrates:

  \( 2Pb(NO_3)_2 \xrightarrow{heat} 2PbO + 4NO_2 + O_2 \)

  This is a classic laboratory method for producing nitrogen dioxide gas.
• By reacting copper with concentrated nitric acid:

  \( Cu + 4HNO_3(conc.) \rightarrow Cu(NO_3)_2 + 2NO_2 + 2H_2O \)

  This reaction is used in school demonstrations to produce NO2, characterized by its brown fumes.

Industrially, nitrogen dioxide is produced as a by-product in the manufacture of nitric acid via the Ostwald process.

Physical Properties:

• Reddish-brown gas with a sharp, suffocating odor.
• It condenses into a yellowish-brown liquid near 21°C and solidifies below -11°C.
• It is denser than air and moderately soluble in water.
• Highly corrosive and toxic when inhaled.

Chemical Properties:

• Acidic nature: When dissolved in water, NO2 forms a mixture of nitric acid and nitrous acid:

  \( 2NO_2 + H_2O \rightarrow HNO_3 + HNO_2 \)

• Oxidizing agent: Acts as an oxidizer, converting substances like sulfur dioxide and metals to their oxides.
• Reversible dimerization: At lower temperatures, NO2 forms N2O4:

  \( 2NO_2 \leftrightharpoons N_2O_4 \)

• Reaction with alkalis: NO2 reacts with sodium hydroxide to form a mixture of nitrite and nitrate:

  \( 2NO_2 + 2NaOH \rightarrow NaNO_2 + NaNO_3 + H_2O \)

• Reaction with reducing agents: Reducing agents such as hydrogen sulfide or carbon monoxide convert NO2 into nitric oxide (NO).

Nitrogen dioxide is a major atmospheric pollutant and contributes to several environmental and health issues. It is one of the principal components of photochemical smog in urban areas. When exposed to sunlight, NO2 undergoes photodissociation to form nitric oxide (NO) and a reactive oxygen atom (O), which reacts with oxygen to produce ozone (O3):

This leads to the accumulation of ozone and secondary pollutants, which irritate respiratory tissues and damage vegetation.

In the presence of water vapor and oxygen, NO2 also contributes to acid rain formation by forming nitric acid:

Health hazards: Exposure to high concentrations of NO2 can cause respiratory distress, coughing, and pulmonary edema. Long-term exposure may lead to decreased lung function, bronchitis, and increased susceptibility to infections. The World Health Organization (WHO) recommends keeping NO2 levels below 200 µg/m³ for short-term exposure.

Despite its toxicity, nitrogen dioxide has various industrial and laboratory applications:

• Production of nitric acid: NO2 is a key intermediate in the Ostwald process for the large-scale manufacture of nitric acid:

  \( 4NO_2 + 2H_2O + O_2 \rightarrow 4HNO_3 \)

• Oxidizing agent: Used in organic chemistry as an oxidant for converting alkenes, alcohols, and other organic compounds.
• Rocket propellant: In liquid form (N2O4), it acts as an oxidizer in hypergolic fuels for rockets and spacecraft.
• Analytical chemistry: Employed in the analysis of nitrogen oxides and pollution control studies.

Its applications are tightly controlled due to its environmental impact and toxicity.