Properties and Uses of Nitrogen and Phosphorus and Allotrophic Forms of Phosphorus

Chemical Properties of Nitrogen

Reactivity of nitrogen

Nitrogen gas (dinitrogen) is chemically non-reactive at ordinary temperature due to:

• Its small atomic size.
• Its high bond order. The N-N bond in the N₂ molecule has a formal triple bond character.

Reaction electropositive metals

Nitrogen reacts with metals to form corresponding nitrides.

The nitrides of alkaline Earth metals (group 2) are ionic, while those of groups 13 and 14 are covalent in nature.

Reaction with transition metals

The nitrides of the transition metals are interstitial compounds in which nitrogen atoms occupy the interstices (due to its small size).

Reactions with non-metals

• Nitrogen reacts with many non-metals under suitable conditions. For example with hydrogen:

• With oxygen, it forms nitric oxide, which being unstable reacts with more oxygen to form nitrogen dioxide.

This reaction between N₂ and O₂ also takes place during lightening.

Combination with compounds

Nitrogen combines with certain compounds on strong heating. For example,

With calcium carbide:

With alumina in presence of carbon:

Calcium cyanamide, aluminium nitride and magnesium nitride hydrolyze on boiling with water to give ammonia.

Hydrogen bonding

Nitrogen in its compounds with hydrogen shows hydrogen bonding though to a lesser extent than oxygen. For example, ammonia (NH₃) shows intermolecular hydrogen bonding.

Uses

• Nitrogen gas is mainly used for the manufacture of compounds such as, ammonia, nitric acid, calcium cyanamide etc.
• For providing inert atmosphere in many metallurgical operations.
• For producing low temperatures required in cryosurgery, preservation of biological materials etc.

Nitrogen Fixation

The process of conversion of free atmospheric nitrogen into useful nitrogen compounds is called fixation of nitrogen or nitrogen fixation. This is meant to facilitate assimilation of nitrogen.

Types of fixation

Natural fixation of nitrogen

Atmospheric nitrogen can be fixed into usable form by natural processes. Two ways of natural fixation of free nitrogen are:

Fixation of atmospheric nitrogen by electric discharge

Electric discharge by lightning

Combining with rain water

Fixation of atmospheric nitrogen by symbiotic bacteria

Certain plants such as peas, beans, etc., have some special kind of nitrogen fixing bacteria (symbiotic bacteria) in the nodules of their roots. These bacteria fix free nitrogen of the air as nitrogen compounds. These compounds spread into the soil after the decay of such plants.

Artificial fixation of nitrogen

Large amounts of free atmospheric nitrogen is however, fixed artificially by a number of methods. Three important methods are:

The cyanamide process

Nitrogen is also fixed as calcium cyanamide on heating it with calcium carbide at 1000°C in an electric furnace.

The mixture of calcium cyanamide and carbon (trade name nitrolim) is an important fertilizer. Calcium cyanamide is decomposed by water to give ammonia.

Oxides of Phosphorus

P₂O₃: Its structure can be obtained by joining P atoms as in P₄ molecule of white phosphorus via O atoms.

Phosphorus (III) oxide reacts with cold water to give a solution of the weak acid, H₃PO_3.

The pure un-ionised acid has the structure:

Phosphorous acid is a weak acid. Its basicity is 2.

P₂O₅: Phosphorus pentoxide is obtained by burning white phosphorus in air.

Phosphorus (V) oxide is also a white solid, subliming (turning straight from solid to vapour) at 300°C. In this case, the phosphorus uses all five of its outer electrons in the bonding.

Solid phosphorus (V) oxide exists in several different forms - some of them polymeric.

Phosphorus (V) oxide reacts violently with water to give a solution containing a mixture of acids, We usually just consider one of these, phosphoric (V) acid, H₃PO₄ This time the pure un-ionised acid has the structure:

It is a strong acid with basicity of 3.

You can get a reaction with sodium hydroxide in three stages, with one after another of these hydrogens reacting with the hydroxide ions.

Chlorides of Phosphorus

Phosphorus forms two types of halides, PX₃(X = F, Cl, Br, I) and PX₅ (X = F, Cl, Br).

Phosphorus Tricloride

Preparation

It is obtained by passing dry chlorine over heated white phosphorus.

P₄ + 6Cl₂ → 4PCl₃

It can also be prepared by the action of SOCl₂ on phosphorus

Properties

It is a colourless oily liquid and hydrolyses in the presence of moisture.

PCl₃ + 3H₂O → H₃ PO₃ + 3HCl