Question

give reason....

1. methane doesn't undergo addition reaction but Ethene does

2. ethyne is more reactive than ethane

Answer 1

2.
In organic compounds triple covalent bonds are, in general, less stable than double bonds. Double bonds are less stable than single bonds. 

It's supposed to work like this: 

In a triple bond and in the double bond there will be six, respectively four electrons trying to occupy a small space, in between the two carbon atoms of ethyne and ethene, while they will repel one another (same charge particles). So double and triple bonds will practically burst. 

The single covalent bond is the most stable, thus the least reactive, because the 2 electrons are more comfy in their orbits.