Carbon and Its Compounds

Allotropy The phenomenon in which one element exists in two or more forms having same chemical properties but different physical properties is called allotropy. The substances that show allotropy are carbon, sulphur, etc.

Allotropes The different forms of an element having same chemical properties but different physical properties are called allotropes.

Isomerism The phenomenon in which a compound can be represented by different structures having different properties is called isomerism.

Isomers The compounds which can be represented by different structures having different properties are called isomers.
IUPAC – International Union of Pure and Applied Chemistry.

Combustion The process of heating a substance strongly in the presence of excess of oxygen or air is called combustion.

• Carbon and carbon compounds release carbon dioxide, water and a large amount of heat and light on burning.
  

  C + O₂ CO₂ + Heat and Light
  CH₄ + O2 CO₂ + H₂O + Heat and Light
  C₂H₅OH + O₂ CO₂ + H₂O + Heat and Light
  

• Saturated hydrocarbons generally give a clean blue flame because complete combustion of these substances takes place.
  
• Saturated hydrocarbons burn with a yellow flame with lots of black smoke due to the incomplete combustion of carbon contents.
  
• Condition in which saturated carbon compounds burn with sooty flame – If a saturated carbon compound is burnt in limited (insufficient) supply of air, it burns with sooty flame due to incomplete combustion.
  
• The gas/kerosene stoves used at homes have inlets for air so that a sufficient oxygen rich mixture is burnt to give a clean blue flame with a lot of heat.
  
• Sometimes bottoms of cooking vessels get blackened due to incomplete combustion of gas/kerosene. It means the holes are blocked. This problem can be removed by cleaning the burner by opening holes.
• The pollutants released in the environment by burning coal and petroleum are oxides of sulphur and nitrogen e.g. SO₂ , NO, NO₂ , etc.
  
• Combustion of hydrocarbons is regarded as oxidation reaction because during combustion, carbon is oxidised to CO₂ and hydrogen is oxidised to water by addition of oxygen.
  
• Only the gaseous substances produce flame on burning.
  
• Wood and charcoal are solid but they produce flame when ignited because when they are ignited the volatile substances present vaporize and burn with a flame in the beginning only.
  
• Fossil fuels The fuels formed by the dead animals and plants buried under the earth millions of years ago due to intense heat and pressure are called fossil fuels. E.g. coal and petroleum.
  
• When an alcohol is heated in the presence of alkaline KMnO₄ (or acidified K₂ Cr₂ O₇ ) then corresponding carboxylic acid is formed.
  

• Addition reaction The reaction which involves addition of two reactants to form a single product is called an addition reaction. Only unsaturated hydrocarbons perform addition reactions.

CH₂ = CH₂ + Cl₂ CH₂Cl – CH₂Cl

• Industrial application of addition reaction Addition reaction is commonly used in the hydrogenation of vegetable oils using a nickel catalyst to form vegetable ghee.
  

Vegetable oil (liquid) + H₂ Ni (473K) Vegetable ghee (solid)

Saturated fatty acids are harmful for health so oils containing unsaturated fatty acids should be used for cooking because Saturated fats increase the level of bad cholesterol (low density lipoprotein LDL) in blood which sticks to the walls of arteries and causes heart diseases.