4. CARBON AND ITS COMPOUNDS
C + O2 → CO2 + heat & light
CH4 + 2O2 → CO2 + 2H2O + heat & light
CH3CH2OH + 3O2 → 2CO2 + 3H2O + heat & light
Why do substances burn with or without a flame?Carbon compounds are easily oxidised on combustion.
A flame is produced only when gaseous substances burn. So a candle or LPG burns with a flame.
Wood, coal or charcoal burn with a flame at first due to the volatile substances in them. After that they just glow red and gives out heat.
Atoms of gas substance are heated and glow to produce flame. Each element produces characteristic colour. E.g. heating a copper wire in flame gives bluish green flame.
Yellow colour of a candle flame is due to the incomplete combustion of carbon particles. When light falls on them, they scatter yellow colour.
Formation of coal and petroleum (fossil fuels)
Fossil fuels were formed from biomass by biological and geological processes.
Millions of years ago, trees, ferns and other plants were crushed into the earth due to earthquakes or volcanic eruptions. They were pressed down by layers of earth and rock. They slowly decayed into coal.
Dead marine tiny plants and animals sank to the sea bed and were covered by silt. Due to bacterial action, they turned into oil & gas under high pressure. The silt was compressed into rock. The oil & gas seeped into porous rock parts, and got trapped like water in a sponge.
Unsaturated hydrocarbons add hydrogen in the presence of catalysts such as palladium or nickel to give saturated hydrocarbons. (Catalysts: The
substances that influence the rate of a reaction without changing itself).
Saturated hydrocarbons are unreactive and inert in the presence of most reagents.
However, in the presence of sunlight, hydrocarbons undergo a substitution reaction very fast. E.g.
CH4 + Cl2 → CH3Cl + HCl (in the presence of sunlight)
Here, chlorine replaces the hydrogen atoms one by one.
Higher homologues of alkanes can form many products.