General characteristics of oxides
Nitrogen is capable of forming a number of oxides in which it exhibits oxidation states ranging from +1 to +5. It is important to understand that in some molecules (for example, in N₂O or N₂O₃), nitrogen atoms can be in completely different electronic states. Because of this, the indicated oxidation states are often just average calculated values.
All nitrogen oxides are thermodynamically unstable. Due to the high electronegativity of oxygen and nitrogen, they exhibit a pronounced oxidizing effect towards metals, hydrocarbons, and a number of non-metals. For example, magnesium can burn in nitrogen(II) oxide to form magnesium oxide and free nitrogen, while carbon burns similarly in an atmosphere of nitrogen(IV) oxide.
A distinctive feature of the electronic structure of NO and NO₂ molecules is the presence of unpaired electrons. Usually, particles with unpaired electrons are highly reactive free radicals, but these oxides are a rare exception among stable molecules (the oxygen molecule O₂ also has a similar structure).
Properties of individual nitrogen oxides
The physical and chemical characteristics of oxides strongly depend on the oxidation state of nitrogen and the spatial structure of the molecule.
| Oxide | Structural features | Reaction with water | Physical properties |
|---|---|---|---|
| Nitrous oxide (N₂O) | Linear structure | Does not react (salt-indifferent) | Colorless gas |
| Nitric oxide (NO) | Radical | Does not react (salt-indifferent) | Colorless gas |
| Dinitrogen trioxide (N₂O₃) | Asymmetric molecule | Forms nitrous acid | Blue liquid (boils at -3.5 °C) |
| Nitrogen dioxide (NO₂) | Bent molecule, radical | Forms a mixture of nitrous and nitric acids | Brown gas ("fox tail") |
| NO₂ dimer (N₂O₄) | N-N bond present | Yields a mixture of acids (like the monomer) | Colorless liquid (boils at 21 °C) |
| Dinitrogen pentoxide (N₂O₅) | Ionic structure in the solid phase | Forms nitric acid | White crystals (melt at 41 °C) |
Interaction with water and alkalis
With respect to water, oxides are divided into salt-indifferent (N₂O, NO) and acid anhydrides. Nitrogen(III) and (V) oxides are classical anhydrides: upon dissolution in water, they form nitrous (HNO₂) and nitric (HNO₃) acids, respectively.
The reaction of nitrogen(IV) oxide with water is much more complex. The process depends on temperature conditions:
- Disproportionation in cold water. The dimeric form (N₂O₄) enters the reaction. As a result, a mixture of two acids is formed simultaneously—nitrous and nitric. If alkali is added at this point, the acids are neutralized to form a mixture of salts: nitrites and nitrates.
- Reaction in hot water. Nitrous acid is thermally unstable. When the temperature rises, it decomposes into water and two oxides (NO and NO₂). Therefore, the overall equation for the reaction of nitrogen(IV) oxide with hot water leads to the formation of nitric acid only, with the simultaneous release of gaseous nitric oxide.
Environmental aspects and toxicity
From a biological and environmental perspective, compounds of oxidized nitrogen require special attention. All of them are potent toxicants. Fortunately, under normal conditions near the Earth's surface, atmospheric nitrogen and oxygen do not react with each other—this is a vital biological factor that protects the biosphere.
The main source of these toxic gases in modern cities is the anthropogenic factor. At high temperatures generated during the combustion of motor fuel, atmospheric nitrogen is oxidized. This is why automobile exhaust always contains nitrogen oxides (predominantly in the form of the brown gas NO₂), which contribute to the formation of smog and acid rain.
Briefly: answers to topical questions
What physiological functions does nitric oxide perform in the human body?
Nitric oxide in the human body functions as an endothelium-derived relaxing factor, regulating blood flow and exerting a vasodilatory effect. It inhibits platelet aggregation, and in macrophages, it exhibits bactericidal and tumoricidal activity. In addition, in certain types of brain cells, nitric oxide acts as a neurotransmitter.
The brown "fox tail" is always nitrogen(IV) oxide (NO₂), while salt-indifferent gases are easier to remember by their lower oxidation states: N₂O and NO.
Class and exam questions
Why are oxidation states in nitrogen oxides called average values?
In complex oxides, such as N₂O or N₂O₃, the nitrogen atoms within a single molecule are in different electronic states and have different environments. Therefore, the oxidation state assigned to them is a mathematical average.
What happens when the brown gas is dissolved in hot water?
A complex reaction occurs, in which nitric acid is formed and nitric oxide is released. This is because the intermediate nitrous acid rapidly decomposes upon heating.
Why do NO and NO₂ molecules not polymerize despite having unpaired electrons?
They are capable of forming dimers (for example, NO₂ turns into N₂O₄ upon cooling), yet they are sufficiently stable as monomers as well, which is a rare property for free radicals.
What else to study on this topic
- Mechanisms of mutual transformations and equilibrium of nitrogen oxides
- Synthesis of nitric acid and production of nitrogen(V) oxide
- Thermal decomposition of ammonium nitrate and production of N₂O
- Specific combustion reactions of metals in an atmosphere of nitrogen oxides
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