If an element is more reactive, is it more likely to be found as an element or a compound?

It is more likely to be found as a COMPOUND, as it is more reactive, by the time we found them, they're already reacted with other elements or compounds to form new compounds.
Example is oxygen, it is very reactive, therefore we often found oxygen in water, which is H2O, in earth, instead of just pure oxygen.

Yes, endothermic means it turns cold. and some chemical changes include change of tempature.