For example, $MnCO_3 → MnO + CO_2$ is endotermic, and occurs at 473 K
the polymerization of ethylene $H_2C=CH_2$ to $n(H_2C=CH_2)$ is exotermic
I suspect it has something to do with the standard enthalpy of formation of the different terms of the equation (right side - left side = $\Delta H$ if negative, the reaction is exotermic and releases $\Delta H$ to the environment as heat, if positive, it is endotermic and needs $\Delta H$ to happen.
Is this correct?
If not, What am I missing?