A brief note on Phosphorus, Boron, Silicon, Noble gases and Environmental Pollution for class 11.

                         PHOSPHORUS :

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Allotropes of phosphorus :

Phosphorus exists on following allotropic forms :

1. White or yellow phosphorus
2. Red phosphorus
3. Black phosphorus
4. Violet phosphorus
5. Scarlet phosphorus

Among them white and red phosphorus are more common.

        1. White Phosphorus :

      Chemical properties of (white) phosphorus:

• Action with air (O₂) : Phosphorescence : Phosphorus is highly reactive and it burns(oxidize) in air giving yellow-green flame forming phosphorus pentoxide and trioxide. This phenomenon is called phosphorescence.

           

Q) why is phosphorus stored in water?

→ Phosphorus is highly reactive and it easily oxidized in the presence of oxygen at room temperature and burns with yellow- green flame. This phenomenon is called phosphorescence. Due to this reason phosphorus is stored in water.

• Action with chlorine : Phosphorus forms trichloride and pentachloride with chlorine

           

Note: similar reaction for bromine and iodine too.

• Action with metal : phosphorus combines with metals like Na, K, Mg, Ca, etc to form metal phosphide.

           

• Reducing property : Phosphorus is reducing agent. It reduces conc. HNO₃ to NO₂ and conc. H₂SO₄ to SO₂.

           

• Action with caustic alkali (i.e. Caustic soda or caustic potash) :

When phosphorus is boiled with aqueous solution of caustic soda(NaOH) or caustic potash(KOH), phosphine gas is formed.

       

Structure of white phosphorus :

         

    2. Red Phosphorus :

When white phosphorus is heated at about 250⁰C in an inert atmosphere( of nitrogen or carbondioxide or coal gas) for several hours then it is converted into red phosphorus.

               

Structure of red phosphorus:

         

Uses of phosphorus :

• It is to prepare poison for killing rats.
• It is used in match industries.
• It is used in manufacture of phosphate fertilizer.
• It is used to prepare phosphine gas, phosphoric acid, etc.

Phosphorus hydride (Phosphine) (PH₃) :

Laboratory Preparation of phosphine :

Phosphine gas is prepared in laboratory by heating white phosphorus with caustic soda or caustic potash in an inert atmosphere.

         

During the preparation of phosphine, some quantity of diphosphine(P₂H₄) is formed. Due to presence of highly inflammable diphosphine, PH₃ gas burns with air as it comes out of water and forms vertex rings of phosphorus pentoxide.

           

Hence, during the preparation of phosphine gas all air inside the reaction flask is replaced by inert gas like CO₂ to minimize the risk of explosion. The phosphine gas evolved should be first passed through freezing mixture where diphosphine is condenced but phosphine is not condensed. Then the pure phosphine gas is collected in a gas jar by downward displacement of water.

           

Chemical Properties of Phosphine :

• Decomposition : Phosphine gas dissociates at 4400C to give red phosphorus and hydrogen.

               

• Combustion ( action of air) : Phosphine is non-supporter of combustion. But when phosphine is heated with air phosphorus pentoxide or phosphoric acid is formed.

               

• Reducing properties :
• Reduction of AgNO₃ solution : When PH3 gas is passed through AgNO3 solution then metallic silver along with phosphorus acid and nitric acid is formed.

           

Q) What happens when the gas produced by the action of white phosphorus with sodium hydroxide is passed through silver nitrate solution?

• With CuSO₄ solution : Phosphine gives black ppt. of copper phosphide with CuSO₄ solution. This is test reaction of phosphine.

         

Uses of phosphine :

• It is used to the formation of smoke screen during wars.
• It can be used as reducing agent.

Structure of Phosphine gas :

Oxides of phosphorus:

Phosphorus trioxide (P₂O₃) :

Uses : It is used to prepare Phosphorous acid and Phosphoric acid.

Structure of P₂O₃ : It exists in dimer form i.e. P₄O₆.

Phosphorus Pentoxide (P₂O₅) :

Uses :

• It is used as dehydrating agent in organic reactions.
• It is used for preparing phosphoric acid.

Structure of P₂O₅ :

 

Oxyacids of phosphorus :

S.No.	Oxyacid	Formula
1	Hypophosphorus acid	H3PO2
2	Orthophosphorus acid	H3PO3
3	Orthophosphoric acid	H3PO4
4	Pyrophosphoric acid	H4P2O7
5	Hypophosphoric acid	H4P2O6
6	Metaphosphoric acid	HPO3

Orthophosphoric acid (H₃PO₄) :

Preparation :

In laboratory, phosphoric acid is prepared by heating red phosphorus with concentrated HNO3 in presence of iodine as catalyst.

       P₄ + conc. 20 HNO₃      →    4H₃PO₄ + 4H₂O + 20 NO₂

Chemical Properties :

• Action of heat :   When orthophosphoric acid is heated, it produces pyrophosphoric acid at 250⁰-260⁰C and metaphosphoric acid at 300⁰C with removal of water molecule.

                   

• Acidic nature :It is tribasic( triprotic )acid. It ionizes in three steps and forms three series of salts with alkali.

        

Uses of phosphoric acid :

• It is used in the manufacture of phosphate fertilizers.
• It is used as rust removal agent, household cleaning agent, sanitizing agent, etc.

Structure of orthophosphoric acid and phosphate ion :

     

                     

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                           BORON :

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Occurance : it is not found in free state in nature. It is found in combined states as boric acid, borax, etc.

Uses of boron :

• It is used in the steel industry for increasing hardness of steel.
• Small amount of boron compound is essential for plant growth.
• It is used as semiconductor for making electronic devices.

Boric acid (Ortho boric acid) :

Molecular formula : H₃BO₃ or B(OH)₃

Uses :

• It is used as an antiseptic. Its aqueous solution is used in washing eyes, nose, ear,etc.
• It is used as a preservative for milk and food stuffs.

Borax :

Molecular formula : Na₂B₄O₇. 10H₂O

Uses :

• It is used to detect basic radicals in salt analysis (borax bead test).
• is used as a preservative for food stuffs.
• It is used in manufacture of soaps, washing powder, etc.

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                                  SILICON :

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Uses of silicon :

• It is used to make chips used in electric devices.
• It is used to make carborundum which is used as an abrasive.
• Silica (SiO2) is used as building material in making cement.

Carborunbum : Silicon carbide (SiC).

Use : It is used as abrasive.

Silica : SiO₂

Uses :

• Used as building material in making cement.
• Used for preparing glass.
• Used for making silica gel which is used for absorbing moisture.

Silica gel : i.e hydrated silica : SiO₂. xH₂O

Use : It is used for absorbing moisture and as an adsorbent in chromatography.

Sodium silicate : Na₂SiO₃

Uses :

• It is used for making glass.
• It is used for making silica gel.

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                         Noble Gases

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The elements which are present in zero group or 18 group of Modern periodic table are known as noble gases. Helium, neon, argon, krypton, xenon and radon constitute the noble gas family. They are very less reactive at normal temperature and pressure.

Uses of noble gases :

• Helium is used for filling ballons.
• Argon is used in electric bulb.
• Radon is used for the treatment of cancer i.e. in radiography.

Q. what are noble gases? Why are they chemically inert?

→ The elements which are present in zero group or 18 group of Modern periodic table are known as noble gases. They are chemically inert because of stable valence shell with octet electrons except in helium( duplet).

Eg. He : 1s² Ne : 1s²2s²2p⁶

They have high ionization energy so transfer and share of electrons is very difficult. Hence, noble gases are chemically inert.

Q) The first ionization energy of noble gases is higher than that of halogens, explain.

→ The electronic configuration of noble gases is very stable due to presence of fulfilled electrons in the outermost orbital. So, very high amount of energy is required to remove electrons from the outermost orbital and the value of ionization energy is very high. But in case of holegens, there are seven electrons in outermost orbital, i.e. not fulfilled . Hence, I.E. of noble gases is higher than that of halogens.

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          Environmental Pollution :

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Air Pollution : The contamination of air with harmful gases, dust, smoke , etc. is known as air. The pollutants like CO, NO, NO₂, SO₂, O₃, smoke, dust, etc. which pollute the air are called air pollutants.

Smog : It is the combination of smoke and fog.

• Chemical smog : Sulphur dioxide (SO₂) and particular matter present in air form this type of smog during cold and humid climate. When SO₂ combines with water vapour, it forms sulphurous acid which condenses on smoke, dust particles etc. and form a mixture called chemical smog.

         

→ It causes respiratory problems such as bronchitis, pneumonia, etc.

• Photochemical smog : The smog which is formed by the reaction of automobile exhaust in the presence of sunlight is called photochemical smog.

The major components of photochemical smog are NO, CO, NO₂, hydrocarbons, etc.

→ The photochemical smog causes headache, eye problem, skin problem, etc.

→ it reduces visibility and affects the air and road traffic.

Acid Rain :

Generally rain water is slightly acidic having pH about 5.6 due to dissolved CO₂ in water. However, pH of the rain water is decreased below pH 5.6 if the acidic oxides like oxides of sulphur and nitrogen get dissolved in it. Such rain water is called acid rain.

               

→ It can destroy the life of aquatic animals.

→ It causes damage of buildings and rocks.

→ It increases the acidity of soil and decreases the fertility of soil.

Green house effect :

The green house effect is a natural process that warms the earth. When the energy from sun reaches the earth’s atmosphere, some of it is reflected back to space and the rest is absorbed by greenhouse gases.

Greenhouse gases include water vapour, carbondioxide, methane, nitrous oxide, ozone and some artificial chemicals such as chlorofluorocarbon (CFCs).

The absorbed energy warms the atmosphere and the surface of the earth. This process maintains the temperature of our earth due to which the life is possible in earth.