(i) Ethene and ethyne contain double and triple carbon-carbon bonds respectively. These bonds are very strong and even upon combustion, carbon atoms remain together to form soot rather instead of reducing to water and carbon dioxide.
(ii) Ethane does not have any double or triple bonded carbon atoms. It is completely saturated and hence, cannot undergo any addition reactions.
(iii) Ethyne burns in oxygen to generate very high temperatures (of about 3000oC). The equation is:
As a result, it is used in welding and cutting metals, since most metals melt at such high temperatures.