oxides of nitrogen

Nitrogen forms several oxides by showing various oxidation states. Some of the compounds formed are neutral oxides of nitrogen, while others are acidic. Different oxides have different physical appearances, for example, nitrogen dioxide colour is brown (gas), and nitrogen trioxide is blue (solid). They also differ in their chemical properties and structures. Given below are some important oxides of nitrogen.

Dinitrogen oxide [Nitrogen (I) oxide]

Formula – N2O (molar mass = 44.013 g/mol)

It is also known as nitrous oxide, laughing gas, or nitrous.

The oxidation state of N in dinitrogen oxide is +1.

Methods of preparation

Industrial method

On an industrial scale, nitrous oxide is prepared by heating ammonium nitrate (NH4NO3) at 250OC or 523K. It decomposes to liberate N2O gas and water vapour. Phosphate salts can be used as catalysts.

NH4NO3 →  2 H2O + N2O

Laboratory methods

• Heating sodium nitrate and ammonium sulphate

2 NaNO3 + (NH4)2SO4 → Na2SO4 + 2 N2O + 4 H2O

• By oxidising ammonia

2 NH3 + 2 O2 → N2O + 3 H2O

Structure

Nitrous oxide has a linear geometry. The bond length between two nitrogen atoms is shorter than the bong length of N-O.

Properties

• It is a neutral, colourless gas.
• It is soluble in water.
• It is non-flammable.
• It has a melting point of −90.86 °C.
• Its boiling point is −88.48 °C.
• It is inert at room temperature.

Uses

• It is used as an anesthetic. 
• It is used in cleaning products like dishwasher detergent.
• It helps make propellants.

Nitrogen monoxide [Nitrogen (II) oxide]

Formula – NO (molar mass = 30.006 g·mol−1)

It is also known as nitric oxide.

The oxidation state of N in nitrogen monoxide is +2.

Methods of preparation

Industrial method

It is produced by oxidising ammonia at 750-900°C. Platinum is used as a catalyst.

4 NH3 + 5 O2 → 4 NO + 6 H2O

Laboratory methods

• Using copper to reduce dilute nitric acid

8 HNO3 + 3 Cu → 3 Cu(NO3)2 + 4 H2O + 2 NO

• From potassium nitrate

3 KNO2 + KNO3 + Cr2O3 → 2 K2CrO4 + 4 NO

Structure

Nitric oxide is a linear molecule. The bond length between nitrogen and oxygen is 115pm. It forms two resonating structures.

Properties

• It is colourless and neutral.
• It has a sharp, sweet odour.
• Its melting point is −164 °C.
• It has a boiling point of −152 °C.

Uses

• It is used in the preparation of nitrosyl carbonyls.
• It is an important messenger molecule in the human body. It transmits signals to the cardiovascular and immune systems. Therefore, it is also used in the manufacture of some drugs.

Dinitrogen trioxide [Nitrogen (III) oxide]

Formula – N2O3 (molar mass = 76.01 g/mol)

The oxidation state of N in dinitrogen trioxide is +3.

Method of preparation

It is prepared by mixing nitric oxide and nitrogen dioxide in equal proportions.

NO + NO2 ⇌ N2O3

Structure

Dinitrogen trioxide molecules are planar. The N-N is unusually long (186pm). It shows resonance.

Properties

• It is acidic.
• It is blue in solid and liquid states.
• It has a melting point of −100.7 °C.
• Its boiling point is 3.5 °C.
• It is soluble in water and ether.

Uses

• It is used in fuels because it is highly combustible.
• It also works as an oxidising agent.

Nitrogen dioxide [Nitrogen (IV) oxide]

Formula – NO2 (molar mass = 46 g/mol)

The oxidation state of N in nitrogen dioxide is +4.

Methods of preparation

Oxidation of nitric oxide

2 NO + O2 → 2 NO2

Thermal decomposition of metal nitrates

2 Pb(NO3)2 → 2 PbO + 4 NO2 + O2

Reduction of nitric acid by a metal

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

Structure

Nitrogen dioxide has a bent shape. The N-O is 120pm long. The angle between is 134o.

Properties

• It is acidic in nature.
• Nitrogen dioxide colour is reddish-brown in a gaseous state.
• It has a pungent, chlorine-like odour.
• Its melting point is −9.3 °C.
• It has a boiling point of 21.15 °C.
• It hydrolyses in water.

Uses

• It is used to manufacture nitric acid.
• It plays the role of an oxidiser in rocket fuel.
• It is used as a nitrating agent in the production of chemical explosives.

Dinitrogen tetroxide [Nitrogen (IV) oxide]

Formula – N2O4 (molar mass = 92.011 g/mol)

It is also known as nitrogen tetroxide (NTO) and amyl.

The oxidation state of N in dinitrogen tetroxide is +4.

Method of preparation

Nitrogen tetroxide is made by the oxidation of ammonia in the presence of a catalyst. It involves the following steps:

1. 4 NH3 + 5 O2 → 4 NO + 6 H2O
2. 2 NO + O2 → 2 NO2
3. 2 NO2 ⇌ N2O4

Structure

Nitrogen tetroxide has a planar structure. The N-N distance is longer than N-O.

Properties

• It is acidic.
• It is colourless in solid and liquid states.
• In the gaseous state, it appears orange.
• It is diamagnetic. This is because it has no unpaired electrons.
• Its melting point is −11.2 °C.
• It has a boiling point of 21.69 °C.

Uses

• It is used as an oxidising agent.
• It is an important rocket propellant.
• It is an intermediate in the production of nitric acid.

Dinitrogen pentoxide [Nitrogen (V) oxide]

Formula – N2O5 (molar mass = 108.01 g/mol)

It is commonly called nitrogen pentoxide or nitric anhydride.

The oxidation state of N in dinitrogen tetroxide is +5.

Method of preparation

The dehydration of nitric acid forms dinitrogen pentoxide by phosphorous oxide.

P4O10 + 12 HNO3 → 4 H3PO4 + 6 N2O5

Structure

Dinitrogen pentoxide is salt in solid-state (nitronium nitrate) but forms covalent bonds in the gaseous state. It has a planar structure.

Properties

• It absorbs UV rays in the gaseous state.
• It is a colourless, acidic solid.
• It is highly unstable. It decomposes to give nitrogen dioxide and oxygen.
• It has a melting point of 41 °C.
• Its boiling point is 47 °C.

Uses

• It is an important nitrating agent in organic chemistry.
• It is used as an oxidising agent in high-fuel rockets.

Effects of nitrogen oxides

• Increased levels of nitrogen dioxide in the air causes damage to the human respiratory tract.
• Prolonged exposure to oxides of nitrogen can cause chronic lung diseases.
• Nitrogen dioxide is harmful to flora.

Conclusion

The oxides formed by nitrogen differ from each other in their structures and physical and chemical properties. Aside from a detailed definition, we also covered different methods of preparation and uses of nitrogen oxides.