`A(g)+A(g) to A_(2)(g)`, these type of reaction are generally exothermic. Explain.

Explanation

In the reaction, one molecule `A_(2)(g)` is formed from two atoms of A(g). Thus, entropy of the system decreases.
For any chemical reaction at fixed temperature and pressure, `DeltaG=DeltaH-TDeltaS`. For the given reaction, `DeltaS gt0`. The reaction will be spontaneous if `DeltaG lt0`.
So, negative quantity=`DeltaH-(Txx"negative quantity")`
`=DeltaH+`positive quantity
so, negative quantity`=DeltaH-(Txx"negative quantity")`
`=DeltaH+`positive quantity
`therefore DeltaH=(-ve)` quantity `-(+ve)` quantity, so, `DeltaH lt0`
`therefore` The given reaction is exothermic.