Thermal cracking of alkanes usually

Cracking breaks large alkane molecules into smaller ones using high temperature (thermal cracking). A big molecule decomposes, so thermal cracking involves decomposition.

It is not exothermic. Heat must be supplied to break the C–C bonds, so it is endothermic. It also does not make only small alkanes, because alkenes (such as ethene) and sometimes hydrogen are formed too. No hydrogen is needed. That is hydrocracking, a different process.