Nitrogen dioxide

The chemical element nitrogen with serial number 7 is located in group 5 of the periodic table. This diatomic gas under normal conditions is quite inert. In the earth's atmosphere, it accounts for three quarters. The element is characterized by the following oxidation states: −3, −1, 0, 1, 2, 3, 4, 5. It is part of many compounds. One of them is a reddish-brown poisonous gas (characterized by the ability to irritate the respiratory tract, causes pulmonary edema at high concentrations), which has a characteristic pungent, pungent odor - it is nitrogen dioxide. The formula is of the form NO2. Molar mass of 46.01 g / mol. Density 2.62 g / dm³. Boiling point 21 ° C. When dissolved in water, it reacts with it. Refractive index 1,449 (at 20 ° C).

Nitrogen dioxide plays an important role in atmospheric chemistry, including the formation of tropospheric ozone. At the same time, it is a major air pollutant and an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced every year. This is one of several nitrogen oxides (binary inorganic compounds of nitrogen with oxygen) with oxidation states:

• I - nitrous oxide N2O;

• II - nitrogen monoxide NO;

• III - diazotide trioxide N2O3;

• IV - nitrogen dioxide NO2 and tetraoxide diazot N2O4;

• V is the diazo nitrogen pentoxide N2O5;

• trinitramide N (NO2) 3.

Nitrogen dioxide is easily liquefied. It is heavier than air. Under normal conditions, NO2 is mixed (approximately 1: 1) with a colorless substance (its dimer) N2O4. The chemistry of NO2 is well understood.

When interacting with water, it hydrolyzes, as a result, two acids are formed (nitrous and nitric): 2NO2 + H2O → HNO2 + HNO3.

In reactions with alkalis, salts of the same two acids are formed: 2NaOH + 2NO2 → NaNO2 + NaNO3 + H2O.

It is a strong oxidizing agent, capable of oxidizing SO2 to SO3. The nitrous method for producing sulfuric acid is based on this property . In NO2, many substances, including organic compounds, sulfur, coal, and phosphorus, burn.

Nitrogen dioxide is usually formed as a result of oxidation of nitric oxide by atmospheric oxygen: O2 + 2NO → 2NO2

In the laboratory, NO2 is obtained in two stages: by dehydration of nitric acid to diazotent pentoxide, which is then decomposed thermally:

2HNO3 → N2O5 + H2O,

2N2O5 → 4NO2 + O2.

As a result of thermal decomposition of nitrates of some metals, NO2 can also be obtained:

2Pb (NO3) 2 → 4NO2 + 2PbO + O2.

An oxide can form when nitric acid (concentrated) reacts with metals (for example, copper):

4HNO3 + Cu → 2NO2 + Cu (NO3) 2 + 2H2O.

When exposed to nitric acid (concentrated) on tin, in addition to nitrogen dioxide, tin acid is formed as a by-product:

4HNO3 + Sn → H2O + H2SnO3 + 4NO2.

In some sources, the oxide N2O4 (IV) is called none other than nitrogen tetroxide. But this is the wrong name, since the substance is diazotetroxide. NO2 exists in equilibrium with the colorless gas N2O4: 2NO2↔N2O4.

Since this equilibrium is exothermic, it shifts toward NO2 at higher temperatures, and at lower temperatures toward N2O4. The dimer becomes solid at a temperature of minus 11.2 ° C. At a temperature of 150 degrees, it decomposes: N2O4 → 2NO2, then 2NO2 → 2NO + O2.

Nitric acid slowly releases NO2, which gives a characteristic yellow color to most samples of this acid:

4HNO3 → 4NO2 + 2H2O + O2.

Nitrogen dioxide is easily detectable by smell even at low concentrations; inhalation of its vapor is best avoided. One potential source of NO2 is fuming nitric acid, which releases NO2 at temperatures above 0 degrees. Symptoms of poisoning (pulmonary edema) usually appear after inhalation of potentially fatal doses after a few hours. There is some evidence that long-term exposure to NO2 at concentrations above 40–100 µg / m³ can reduce lung function and increase the risk of respiratory symptoms. Some scientists have found a link between NO2 concentration and sudden infant death syndrome.

Nitrogen dioxide is formed in most combustion processes where air is used as an oxidizing agent.

At elevated temperatures, nitrogen combines with oxygen to form nitric oxide: O2 + N2 → 2NO, then NO is oxidized in air to form dioxide O2 + 2NO → 2NO2:

1. At normal atmospheric concentrations, this is a very slow process.

2. The most likely sources of NO2 are internal combustion engines, thermal power plants, and, to a lesser extent, pulp mills.

3. Gas heaters and furnaces are also sources of this oxide. The excess air required for combustion is introduced by nitrogen, which is converted to nitrogen oxides at high temperatures.

4. In households, kerosene heaters and gas heaters are also sources of NO2.

5. Nitrogen dioxide is produced during atmospheric nuclear tests (the reddish color of the mushroom cloud).

6. In some agricultural areas, its surface concentrations can reach 30 μg / m³.

7. NO2 is also naturally produced during thunderstorms and rain.