1.

i] Give a chemical test to distinguish between saturated and unsaturated hydrocarbon. ii] Name the product formed when ethane burns is air. Write the balanced chemical equation for the reaction showing the types of energies liberated. iii] Why is the reaction between methane and chlorine in the presence of sunlight considered a substitution reaction?

Answer»

Solution :i]a) Saturated HYDROCARBON burns with blue and non smoke flame, due to their complete combustion, Unsaturated hydrocarbons GENERALLY burns with sooty flame due to their INCOMPLETE combustion.
b) Unsaturated hydrocarbons decolourise bromine water.
Eg: `CH_2 = CH_2 + Br_2 (aq) to underset(Br)underset(|)(CH_2)-underset(Br)underset(|)(CH_2)`
c] Saturated hydrocarbon does not decolourises bromine water.
ii) When ethane burns in air Carbon dioxide and water are formed
`CH_3 -CH_2 +7//2O_2 to 2CO_2 + 3H_2O`
OR
`2(CH_3 - CH_3) + 7O_2 to 4CO_2 + 6H_2O`
iii) The reaction in which a reagent substitutes or replaces atom or a group of atoms from the reactant are called substitution reaction.
Generally saturated hydrocarbons undergo substitution reaction. When methane reacts with chlorine in sunlight the hydrogen atoms in methane are successively replaced lay chlorine atoms and forms four substitution products.
`CH_4 + Cl_2 overset("Sunlight")(rarr) underset("CHLOROMETHANE")(CH_3Cl + HCl)`
`CH ""_(3)Cl + Cl_(2) overset("Sunlight")(rarr)underset("Dichlorimethane")(CH_3Cl + HCl)`
`CH ""_(3)Cl_(2) + Cl_(2) overset("Sunlight")(rarr)underset("CHLOROFORM"("Trichloroo methane"))(CHCl_3 + HCl)`
`CHCl_(3) + Cl_(2) overset("Sunlight")(rarr) underset("Carbon tetrachloride" ("Tetrachloromethane"))(C Cl_4 + HCl)`


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