manojsir-science blog: SOME IMPORTANT CHEMICAL COMPOUNDS-III : LONG ANSWERS

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Describe Solvay's Ammonia-soda process (manufacture of sodium carbonate or washing soda).

ANS : AMMONIA-SODA PROCESS

A cold and concentrated solution of sodium chloride (brine) is saturated with ammonia to obtain ammoniacal brine.
Ammoniacal brine is allowed to drop from the top of the reactor called carbonating tower.
Simultaneously, carbon dioxide gas is introduced into the tower from the bottom.
Following reaction takes place :

Sodium hydrogen carbonate is separated from ammonium chloride.
When sodium hydrogen carbonate is heated, it decomposes to give sodium carbonate.

The sodium carbonate obtained in this process is anhydrous and is called 'Soda Ash'.
Soda ash is dissolved in water and crystallized to get washing soda.

RE-USE OF BY-PRODUCTS:

The chief attraction of Solvay's process is that the by-products can be re-used.
The carbon dioxide used in the process is obtained by decomposition of CaCO₃ in a lime-kiln (furnace).

Lime is heated with water to obtain slaked lime.

When this slaked lime is heated with ammonium chloride (also a by-product), it yields ammonia, which is recycled to the process.

Mention the properties of washing soda.

ANS:

It is a colourless, crystalline solid.
When exposed to air, the crystals of washing soda lose 9 molecules of water of crystallization to give sodium carbonate monohydrate. This process is called 'EFFLORESCENCE'.

On strong heating it loses all the water of crystallization and forms anhydrous sodium carbonate (soda ash).
As it is a salt of weak acid and strong base, the aqueous solution of washing soda is alkaline. It turns red litmus to blue.
Its reaction with acid solution produces carbon dioxide gas.

Mention the properties of baking soda.

ANS:

It is a colourless crystalline solid.
It is soluble in water.
It is a salt of weak acid and strong base. Therefore, its aqueous solution is slightly alkaline.
On heating it decomposes to give sodium carbonate.

It reacts with acid to form carbon dioxide.

Write a note on bleaching powder.

ANS:
MANUFACTURE : Bleaching powder is manufactured by Hesenclever process. In this process, chlorine gas is passed over slaked lime, Ca(OH)₂, to obtain bleaching powder.

PROPERTIES OF BLEACHING POWDER :

It is yellowish white powder with strong smell of chlorine.
In moist air it liberates chlorine gas.
When it reacts with carbon dioxide or dilute sulphuric acid or dilute hydrochloric acid, it produces chlorine gas.

USES OF BLEACHING POWDER :

For making drinking water germ-free.
As an oxidising agent in industry.
For bleaching of cotton, linen and wood-pulp.
For bleaching (removing stains from ) washed clothes in laundry.

Write a note on Plaster of Paris.

ANS:

The chemical formula of Plaster of Paris is CaSO₄.1/2 H₂O.

[ONLY FOR INFORMATION: The formula of Plaster of Paris should be written as (CaSO₄)₂.H₂O because a molecule never exists in fraction. Here 1/2 molecule is written only for convenience.]
Its chemical name is calcium sulphate hemihydrate.
Plaster of Paris derived its name from the fact that it was made from gypsum which was found mainly in Paris.
PREPARATION : Plaster of Paris is prepared by heating gypsum[CaSO₄.2H₂O] in a kiln to 100°C.

The gypsum should be heated carefully under controlled condition because if it is heated above 100°C, then anhydrous calcium sulphate (CaSO₄) called 'dead burnt plaster' is formed which does not set like Plaster of Paris on adding water.
PROPERTIES :

It is a white powder.
On addition of 1/3 parts of water, it forms crystals of gypsum and sets into a hard mass.

Its setting time increases if NaCl is added to it.
If gypsum is heated above 200°C, it forms dead burnt plaster which does not set like Plaster of Paris on adding water.[NOTE : This point is given in the text book as a property of Plaster of Paris. Actually, it is not the property of Plaster of Paris].

USES :

In making toys, idols and casts for statues.
In preparation of denture castings.
In bone-setting, i.e. fixing fractured limbs, bone joints, etc.
For sealing air-gaps in laboratory apparatus during experiments.
For making black-board chalks,decorative materials,POP false ceilings, cosmetics, etc.

Write a note on lime.

ANS:

Lime is calcium oxide ( also called slaked lime).
MANUFACTURE :

Lime is manufactured by heating lime stone (CaCO₃) to 1000°C in a lime kiln.
Limestone decomposes to form lime and carbon dioxide gas.

The above reaction is reversible. Therefore to convert all the limestone into lime, carbon dioxide is allowed to escape from the kiln.
The process is known as CALCINATION (calcining) of limestone.

PROPERTIES :

It is a colourless amorphous solid.
Its melting point is 2000°C.
When lime is heated in an oxy-hydrogen flame, it produces bright white light called "limelight".
Lime reacts with water vigorously releasing a lot of heat. Slaked lime is produced.

Lime forms calcium chloride with wet (moist) hydrogen chloride gas but it does not react with dry HCl gas.

When lime is heated at 2000°C in an electric arc furnace with carbon, it forms calcium carbide.

USES :

As a drying agent for drying of gases like ammonia and alcohols.
In the manufacture of glass and cement.
In the manufacture of dyes.
For preparing basic lining in furnaces.
For white washing of walls.

Write a note on cement.

ANS:

Cement is an important building material.
Cement was discovered by Joseph Aspdin of England in 1824.
It is called 'Portland Cement' as the hardness obtained by mixing cement with water resembles Portland rock of England.
MANUFACTURE :

A finely powdered mixture of limestone and clay is heated at 1500°C in a rotary kiln to form clinker (cement).
After cooling, the clinker is mixed with 2 to 5 % gypsum.
The mixture is then ground to obtain finely powdered cement (Portland Cement).
Dicalcium silicate, tricalcium silicate and tricalcium aluminate are the major constituents of Portland cement.

PROPERTIES :

Cement is fine gray powder.
If water is mixed to it , it sets to a very hard mass.
If kept open in moist air, it becomes hard like a rock.

USES :

For making roads, bridges, dams and other building materials.
For the manufacture of concrete/R.C.C.[Reinforced Concrete Cement].

R.C.C. : The concrete having an iron framework inside it is called Reinforced Concrete Cement or Reinforced Cement Concrete (R.C.C.). RCC is used for making structures like pillars, roofs (floors), bridges, tunnels, dams, etc.

Write the chemical formula of ordinary glass and describe its manufacture.

ANS: The approximate composition of ordinary glass is Na₂O.CaO.6SiO₂.

MANUFACTURE :

A mixture of sodium carbonate, calcium carbonate and silica (sand) is heated at 1400°C in a furnace.
The silica decomposes carbonate compounds into silicates and releases carbon dioxide.
A mixture of sodium silicate and calcium silicate along with silica is obtained in molten state.

This mixture is molten glass which on cooling forms solid glass.
The hot molten glass is cooled carefully, i.e. neither very quickly nor very slowly.
If it is cooled very quickly it becomes very brittle and cracks easily.
If it is cooled very slowly it becomes opaque.

Describe different types of glass briefly.

ANS:

Soda Glass (Soft Glass) :

Soda glass is the ordinary glass also called soft glass.
It is made by melting soda ash, sand and quick lime.
It is used for making glass tumblers, dishes, bottles, mirrors, window panes, light bulbs, etc.

Potash Glass (Hard Glass) :

Hard glass is prepared by melting a mixture of potassium carbonate, calcium carbonate and silica.
It is more resistant to acids.
It is used for making hard glass laboratory apparatus.

Lead Crystal Glass :

Lead crystal glass is made by melting potassium carbonate, lead oxide and silica.
It is used for making costly glass apparatus and for producing flashes by reflection of light.

Pyrex Glass (Heat Resistant Glass) :

Pyrex glass is made by melting a mixture of sand, quick lime, borax and alkali carbonate.
It is also known as Borosilicate glass.
It is used for making ampoules and laboratory apparatus.[An ampoule is used to store liquid medicines, usually for injections.]

Optical Glass :It is used for making lenses for spectacles, cameras, microscopes, telescopes and similar optical instruments.
Photochromatic Glass :

Photochromatic glass is the one which darkens temporarily when exposed to bright light.
Silver bromide in the glass gives it its property of automatic darkening.
It is used for making spectacles to protect eyes from sun's heat(sunglass).

Safety Glass :

It is prepared by placing a thin sheet of transparent plastic between two layers of glass.
It is safe as it does not break easily.
It is used for making the windscreens of automobiles, trains and aeroplanes.
It is also used in making bullet-proof glass.

Glass Fibre :

It is heat resistant.
It is used in refrigerators, ovens, etc.
Optical fibres are used in telecommunication because their efficiency is more than copper cables.
Optical fibres are also used in endoscopy to examine the internal organs of human body.

Coloured Glass :

Coloured glass is obtained by adding small amounts of colouring material like certain metal oxides in molten state during preparation of glass.
For example, Ferric oxide for brown glass, chromium oxide for green glass, manganese dioxide for purple(violet) glass and cobalt oxide for blue glass.
Coloured glass is used for making fancy articles, artificial gems and window panes.

What is steel ? Describe its manufacture using Bessemer convertor.

ANS: Steel is an alloy of iron and carbon containing 0.1 to 1.5 % carbon. Different elements like nickel,manganese and silicon are added to it in different proportion to obtain steel of desired characteristics.

MANUFACTURE :

Bessemer convertor is used to produce steel from pig iron.
Bessemer convertor is lined with heat-resistant bricks from inside.
First, the Bessemer convertor is turned sideways and molten pig iron is poured in to it.
It is then brought back to vertical position.
A blast of hot air with pressure is passed into the convertor from the bottom.
This removes oxides of manganese, silicon, phosphorus, etc present as impurities in pig iron in the form of slag or gases.
After the rem,oval of impurities, required amount of carbon is added to pure molten iron to obtain desired steel.

manojsir-science blog

Friday, March 16, 2007

SOME IMPORTANT CHEMICAL COMPOUNDS-III : LONG ANSWERS

GIVE ANSWER IN DETAIL

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