Give reasons for:

(i) Methane does not undergo addition reactions, but ethene does.

(ii) Ethyne is more reactive than ethane.

(iii) Hydrocarbons are excellent fuels.

(i) Methane is a saturated hydrocarbon whereas Ethene is an unsaturated hydrocarbon. In saturated hydrocarbons, all the four valencies of each carbon atom are satisfied by the hydrogen atoms, forming single covalent bond. Thus, due to non-availability of electrons methane is less reactive and hence does not undergo addition reactions.
On the other hand, ethene has two carbon atoms forming a double covalent bond as their valencies are not fully satisfied by hydrogen atoms. The availability of electrons in the double bond makes ethene more reactive and it undergoes addition reactions.

(ii) Ethyne is an unsaturated hydrocarbon having two carbon atoms forming a triple covalent bond as their valencies are not fully satisfied by hydrogen atoms whereas ethane is a saturated hydrocarbon as all the four valencies of its two carbon atoms are satisfied by the hydrogen atoms. The availability of electrons in the triple bond in case of ethyne makes it more reactive than ethane which has does not have electrons available in the single covalent bond.

(iii) Hydrocarbons have high calorific value. They are easily combustible and the reaction is exothermic releasing heat energy. Hence, they are excellent fuels.