CBSE Chemistry Subject Notes

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Chemistry

Carbon and Its Compounds

COAL AND PETROLEUM

When a fuel is burned , the energy is released mainly as heat which can be used for various purposes like cooking, heating , etc. Most of the fuels are either free carbon or carbon compounds. They are obtained from coal, petroleum and natural gas which are also called Fossil Fuels because they were formed by the decomposition of the remains of the pre-historic plants and animals buried under the earth long ago.

Formation of coal -  It is formed by the decomposition of large land plants and trees buried under the earth millions of years ago. Due to high temp. and pressure inside the earth, and in absence of air, wood was converted into coal.

Formation of petroleum -  It was formed by the decomposition of  the remains of extremely small plants and animals buried under the sea millions of years ago. It is a dark coloured, viscous and foul smelling crude oil which is a mixture of  several solid, liquid and gaseous hydrocarbons mixed with water , salt an dearth particles.

• The fuels such as coal and petroleum have some nitrogen and sulphur in them . So, when they burn they lead to oxides of nitrogen and sulphur which are major pollutants of air.

  Burning of substances with or without flame ----
  A flame is a region where combustion of gaseous substances takes place. So, a flame is produced only when gaseous substances burn.

• All gaseous substances burn with a flame but only those solid and liquid fuels which vaporize on heating burn with a flame.
• Two types of flames  -  Blue ;  Yellow
• Colour of flame depends upon the proportion of oxygen available for burning of fuel.

• Blue  Flame  - When the oxygen supply is sufficient .  It does not produce much light – Non-luminous .eg. gas stove.
• Yellow  Flame  -  Oxygen supply insufficient . Yellow color is due to the glow of hot, unburnt carbon particles produced due to incomplete combustion of fuel .It produces light – Luminous. Eg. candle
• Solid and liquid fuels which do not vaporise on heating , burn without producing a flame.eg. charcoal and coal.

CHEMICAL PROPERTIES OF CARBON COMPOUNDS

• Combustion --Process of burning of a carbon compound in air to give CO₂, water, heat and light.
  1. Alkanes burn in air to produce lot of heat due to which they are excellent fuels. Eg. methane , Butane (LPG). They burn in air with blue, non-sooty flame  because the % of carbon in the saturated hydrocarbons is comparatively low which gets oxidized completely by the oxygen present in the air.
  2. The stoves have tiny holes for air so that sufficient oxygen is available for complete burning of fuel to produce a smokeless blue flame.
  3. Mixture of acetylene(ethyne) and air is not used for welding because burning of ethyne in air produces a sooty flame , which is not hot enough to melt metals for welding.
Substitution reactions --- The reaction in which one or more hydrogen atoms of hydrocarbons are replaced by some other atoms like chlorine ( chlorination).
1. Saturated hydrocarbons ( alkanes) are unreactive because they contain only carbon-carbon single bonds. They do not react with many substances.
2. They undergo substitution reactions with chlorine in the presence of sunlight.
3. Methane reacts with chlorine in the presence of sunlight to form chloromethane and hydrogen chloride.
   CH₄ + Cl₂ →  CH₃Cl + HCl
   1 H atom of methane is replaced by Cl atom and we get chloromethane .By supplying more chlorine , all H atoms can be replaced by Cl one by one.
   So, we can obtain 3 more compounds ---
   Dichloromethane  /  methylene dichloride --- CH₂Cl₂
   Trichloromethane / Chloroform  ---- CHCl₃

4. Addition  Reactions - The unsaturated hydrocarbon combines with another substance to give a single product ( addition of H, Cl or Br)
   1. These reactions are given by all unsaturated hydrocarbons containing double or triple bonds ( alkenes and alkynes)
   2. Simplest alkene is Ethene which reacts with hydrogen when heated in the presence of nickel catalyst to form ethane  .
   3. Unsaturated hydrocarbons add on hydrogen in the presence of catalysts such as Nickel or Palladium(Pd) to form saturated hydrocarbons  which is called HYDROGENATION . This process is used to prepare vegetable Ghee form vegetable oils.

Submitted By Mrs. Kritika Bhola
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