Wednesday, March 28, 2007

NON-METALS - III : LONG ANSWERS

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  1. Mention physical properties of nonmetals.
    ANS :

    • Nonmetals occur in all three states : solid, liquid and gas.
    • They are neither malleable nor ductile.
    • They do not conduct heat and electricity because they do not have free electrons (graphite is an exception).
    • They are lustreless.
    • They are hard and brittle.
    • They generally have low melting points and boiling points.
    • They are light in weight.
    • Solid nonmetals do not produce ringing sound.


  2. Explain chemical properties of nonmetals.
    ANS : Nonmetals are electronegative elements because they accept electrons and form negatively charged ions. For example, oxygen forms negative oxide ion O-2 by accepting two electrons.

    1/2 O2 + 2 e- → O-2
    1/2 Cl2 + e- → Cl-



    REACTION WITH OXYGEN : Nonmetals form either acidic oxide or neutral oxide with oxygen.

    • When carbon burns in air, it forms carbon dioxide which is an acidic oxide.

      C(s) + O2(g) → CO2(g)



      Carbon dioxide dissolves in water to form carbonic acid [H2CO3].

      CO2(g) + H2O(l) → H2CO3(aq)



    • When sulphur burns in air, it reacts with oxygen to form sulphur dioxide which is an acidic oxide.

      S(s) + O2(g) → SO2(g)



      Sulphur dioxide dissolves in water to form sulphurous acid [H2SO3]

      SO2(g) + H2O(l) → H2SO3(aq)



    • Phosphorus forms phosphurus pentoxide (2P2O5) with oxygen which dissolves in water to form phosphoric acid [H3PO4(aq)].

      P4(s) + 5O2(g) → 2P2O5(g)

      2P2O5(g) + 6H2O → 4H3PO4(aq)


    • Oxides like CO, N2O, H2O are neutral oxides as their aqueous solutions are neither acidic nor basic or they do not have any effect on any litmus paper.



    REACTION WITH ACIDS : Nonmetals do not react with dilute acids as they are not able to displace hydrogen from acids.

    REACTION WITH CHLORINE : Nonmetals form chlorides with chlorine. These chlorides are either liquid or gas.



    REACTION WITH HYDROGEN : Nonmetals form covalent hydrides with hydrogen.



    Methane (CH4), hydrochloric acid (HCl), etc. are hydrides of nonmetals.

  3. Describe the Importance of nonmetals.
    ANS :

    • Hydrogen gas reacts with nitrogen to form ammonia gas.
    • Carbon is used in the preparation of useful substances like gunpowder, sugar, cold drinks, dry ice, urea, etc.
    • Oxygen supports life on earth.
    • Nitrogen is used in the preparation of nitric acid, nitroglycerin(explosives) and nitrogenous fertilisers.
    • Sulphur is used in the preparation of paper pulp, fungicides, etc.
    • Phosphorus is used in making matchsticks, insecticides, etc.


  4. Write a few sentences on Hydrogen.
    ANS :

    • Hydrogen was discovered in 1766 AD by English chemist Cavendish.
    • Its chemical symbol is H.
    • It is the lightest of all elements.
    • It is the first element of the periodic table.
    • Hydrogen does not occur in free state (monoatomic molecule) in the atmosphere.
    • It is a diatomic molecule.
    • it is a colourless, odourless gas.
    • It is present in outer space.
    • It is the main element present in sun.


  5. Describe preparation of hydrogen in laboratory.
    ANS :


    AIM : To prepare hydrogen gas in laboratory.

    APPARATUS & MATERIAL : Conical flask, Thistle funnel, glass container, bee-hive shelf, glass jar, glass tube, zinc granules, hydrochloric acid, water, etc.

    PROCEDURE :

    • Zinc granules are taken in conical flask and the equipment is set up as shown in figure.
    • Dilute hydrochloric acid is added slowly to the flask through thistle funnel.


    OBSERVATION : The reaction between zinc granules and dilute hydrochloric acid produces hydrogen gas which is collected by downward displacement of water in an inverted jar.

    CHEMICAL REACTION :

    Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

    NOTE : In place of dilute HCl we can also use dilute H2SO4.

    Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)

  6. Describe manufacture of hydrogen from natural gas.
    ANS :

    • Hydrogen gas is manufactured on large scale from natural gas by steam reforming process.
    • Methane (the main constituent of natural gas) is mixed with steam and passed over nickel catalyst at 800°C and 30 atm pressure.
    • The reaction produces carbon dioxide and hydrogen.


    • Hydrogen gas can also be obtained by the electrolysis of acidified water.


  7. Describe chemical properties of hydrogen.
    ANS :

    1. Hydrogen gas burns in air (oxygen) with a blue flame forming watervapour.

      2H2(g) + O2(g) → 2H2O(g)


    2. It burns in chlorine to form white fumes of hydrogen chloride gas.

      H2(g) + Cl2(g) → 2HCl(g)

      Similarly it forms hydrogen fluoride gas with fluorine.

      H2(g) + F2(g) → 2HF(g)


    3. It forms hydrides with metals like lithium, sodium and calcium.

      H2(g) + 2Li(s) → 2LiH(s)

      H2(g) + 2Na(s) → 2NaH(s)

      H2(g) + Ca(s) → CaH2(s)


    4. Hydrogen reduces oxides of those metals, which are less reactive, to free metals. Thus, when hydrogen is passed over hot copper oxide copper metal and water are produced.

      CuO(s) + H2(g) → Cu(s) + H2O(l)



  8. Write a detailed note on ammonia, its manufacture, properties and uses.
    ANS :
    AMMONIA : Ammonia is a very important industrial chemical as it is used for the manufacture of a wide range of chemicals. It is manufactured commercially by Haber's process.

    MANUFACTURE : A mixture of nitrogen and hydrogen in the ratio of 1 : 3 is heated under 200 - 300 atm pressure in presence of catalyst (finely divided iron + molybdenum) at about 450 °C. The reaction is reversible.



    PHYSICAL PROPERTIES :

    • It is a colourless gas.
    • It has characteristic pungent smell which irritates eyes and nose.
    • It is poisonous.
    • It is highly soluble in water.


    CHEMICAL PROPERTIES :

    • Ammonia reacts with hydrochloric acid to form ammonium chloride.

      NH3(g) + HCl(l) → NH4Cl(s)

    • Ammonia decomposes in presence of electric discharge.



    • Ammonia reacts with oxygen in presence of catalyst platinum at 1073 K to form nitric oxide.



    • Aqueous solution of ammonia acts as a base. [NOTE : This property is given as physical property in the text book]


    USES :

    • In the manufacture of fertilisers such as urea, ammonium nitrate and ammonium sulphate.
    • In the manufacture of nitric acid.
    • In the manufacture of baking soda and washing soda.
    • In the manufacture of some medicines such as para-aminobenzoic acid (PABA), folic acid, etc.
    • In the preparation of dyes and explosives.


  9. Describe Extraction of Sulphur.
    ANS : Pure sulphur is obtained by (i)Frasch Process or (ii) from natural gas and oil by chemical process.

    FRASCH PROCESS : The process is used to obtain pure sulphur from the sulphur bed in the ground. It is based on the fact that sulphur has relatively low melting point.


    • A hole of about 30 cm diameter is bored upto the sulphur bed.
    • Three concentric pipes of different diameters are descended upto sulphur deposits.
    • Pressurised superheated water at about 170°C is forced down through the outermost pipe. The heat of water melts the underground sulphur.
    • Hot compressed air is passed through the innermost pipe.
    • A mixture of molten sulphur and water comes out through central pipe.
    • The sulphur-water emulsion is allowed to settle in settling tanks. Yellow solid sulphur separates from water after cooling. This sulphur is 99.5% pure.


    FROM NATURAL GAS AND OIL :

    • Sulphur compounds in natural gas and oil are first converted into hydrogen sulphide. (H2S).
    • Hydrogen sulphide is heated in presence of oxygen to obtain sulphur dioxide.
    • This sulphur dioxide is then heated with additional hydrogen sulphide in the presence of catalyst Fe2O3 to obtain sulphur in free state.




  10. Write a note on Allotropes of sulphur.
    ANS :

    • Different arrangement of molecules in the crystals give rise to allotropes of sulphur.
    • There are two allotropes of crystalline sulphur. (i)Rhombic sulphur and (ii) Monoclinic sulphur.
    • Rhombic sulphur is stable below 96°C and its crystals have octahedral shape.
    • Monoclinic sulphur is stable above 96°C and its crystals are needle-shaped.
    • As one allotrope changes into another at 96°C, it is called TRANSITION TEMPERATURE for sulphur.
    • Both allotropes have similar chemical properties but their physical properties differ due to their different physical structures.
    • When solid sulphur is heated its molecular chain breaks which on further heating yields viscous liquid sulphur. This on heating starts boiling.


  11. Mention Chemical properties of sulphur.
    ANS :

    • Sulphur burns with blue flame and produces sulphur dioxide.

      S(s) + O2(g) → SO2(g)



    • REACTION WITH ACIDS : Sulphur is oxidised to sulphur dioxide when it reacts with hot and concentrated sulphuric acid.

      S(s) + 2 H2SO4(aq) → 2H2O(l) 3SO2(g)



      Sulphur is oxidised to sulphuric acid when it reacts with hot and concentrated nitric acid.

      S(s) + 6HNO3(aq) → H2SO4(aq) + 6NO2(g) + 2H2O(g)



    • REACTION WITH CARBON : Sulphur reacts with carbon at high temperature to give carbon disulphide.

      C(s) + 2S(s) → CS2(l)



    • REACTION WITH HYDROGEN : When hydrogen gas is passed through boiling sulphur, foul smelling hydrogen sulphide gas is formed.

      S(s) + H2(g) → H2S(g)




  12. Mention uses of sulphur.
    ANS :

    • In the manufacture of sulphuric acid.
    • As an antiseptic in skin ointments for skin treatment and in cosmetics.
    • For making dyes, matches, gunpowder, pulp of paper and fireworks.
    • In vulcanisation of rubber.
    • For manufacturing carbon disulphide which is a solvent.
    • As a disinfectant, fungicide, germicide, etc. for destroying bacteria, fungi, insects, etc.
    • In printing (dyeing) of clothes.


  13. Write a detailed note on sulphur dioxide.
    ANS : The reaction of sulphur with oxygen gives sulphur dioxide.

    S(s) + O2(g) → SO2(g)



    PHYSICAL PROPERTIES :

    • It is a colourless gas.
    • It has a pungent smell.
    • It is a pollutant gas which causes acid rain due to dissolution in rain water.


    CHEMICAL PROPERTIES :

    • Sulphur dioxide dissolves in water to form sulphurous acid (H2SO3).

      SO2(g) + H2O(l) ↔ H2SO3(aq)



    • Sulphur dioxide reacts with aqueous sodium hydroxide to form sodium sulphite (Na2SO3).

      SO2(g) + 2NaOH(aq) → Na2SO3(aq) + H2O(l)



    • When sulphur dioxide gas is passed through lime-water, the solution turns cloudy due to the formation of insoluble calcium sulphite (CaSO3).

      SO2(g) + Ca(OH)2(aq) → CaSO3(s) + H2O(l)



    • When sulphur dioxide gas is passed through an aqueous solution of hydrogen sulphide, it turns milky (turbid) due to formation of colloidal sulphur.

      SO2(g) + 2H2S(aq) → 3S(s) + 2H2O(l)



    • Sulphur dioxide combines with oxygen in presence of catalyst vanadium pentoxide [V2O5] at 450°C to form sulphur trioxide.




    USES :

    • For the manufacture of sulphuric acid.
    • As a preservative in orange and lemon squashes and jams.
    • For bleaching of wood pulp in paper industry.


  14. Describe manufacture of sulphuric acid by Contact Process.
    ANS : The contact process is divided into different stages and uses various units such as sulphur burner, precipitator, steam scrubber, drying tower, arsenic purifier, contact chamber and absorption unit.

    PREPARATION OF SO2 : SO2 is obtained by burning sulphur or pyrites.

    Δ
    S(s) + O2(g) → SO2(g)


    CONVERSION OF SO2 into SO3 IN CONTACT CHAMBER : The mixture of purified SO2 and air is passed over catalyst vanadium pentoxide [V2O5] to produce SO3.



    ABSORPTION OF SO3 IN CONC. H2SO4 : SO3 obtained from the contact chamber is dissolved in concentrated H2SO4 acid giving oleum, H2S2O7. [Oleum is also known as 'fuming sulphuric acid'.]



    Sulphuric acid of any concentration can be obtained by diluting oleum with water.

    H2S2O7(l) + H2O(l) → 2H2SO4(aq)



    Contact process yields sulphuric acid which is pure and of about 100% strength.

  15. Write physical properties, chemical properties and uses of sulphuric acid.
    ANS :
    PHYSICAL PROPERTIES

    • Concentrated sulphuric acid is colourless viscous liquid containing 98% H2SO4 and 2% water.
    • Concentrated sulphuric acid is a powerful water-absorbing agent.
    • Dilute sulphuric acid contains 10% H2SO4 and 90% water.

    CHEMICAL PROPERTIES

    • Concentrated sulphuric acid dehydrates sugar to carbon. The black mass of carbon is called sugar charcoal and the reaction is called charring of sugar.



    • Dilute sulphuric acid reacts with metals like zinc and iron to form metal sulphates and hydrogen gas.

      Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)

      Fe(s) + H2SO4(aq) → FeSO4(aq) + H2(g)

    • Sulphuric acid is strong dibasic acid from which two hydrogen atoms can be displaced in solution.

      H2SO4(l) + 2H2O(l) → 2H3O+(aq) + SO4(aq)-2

    USES

    • In the preparation of fertilisers.
    • In the manufacture of pigments, dyes and paints.
    • In the preparation of detergents.
    • In storage batteries.



  16. ANS :

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