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It is based on the bonds formed and the bonds that are broken. I believe it is (but can someone confirm): the more bonds that are formed - the more exothermic; the more bonds broken - the more endothermic (generally, not always).
Based on Lewis structures, we know that it takes energy to break a bond, and energy is released to form bonds, so if bonds are being made then the reaction is endothermic, since it is taking in energy, and it bonds are being broken then it is exothermic, since energy is being released.
You would calculate the enthalpies of formation of the substances, and then subtract the enthalpy of the reactants from the enthalpy of the products to see if you get a positive or negative value. Negative, it'll be exothermic, positive endothermic.
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