I realize that if the reactants give off heat when they undergo reactions it is exothermic and when they absorb heat it is endothermic, but how can I tell by looking at an equation whether or not it is going to be endothermic or exothermic.

Do the states of the reactants have anything to do with it?

ex/ MgO(s) + 2HCl(aq) -> MgCl2(aq) + H2O (l)

the oxygen atoms are more tightly packed with the Magnesium, so when they join with the hydrogen ions to form liquid water they must have more space to move, meaning the thermal energy of the products are higher than of the reactants, so why is it still exothermic?!

A lot of people have told me its simply because MgO will react with HCl and the heat will rise therefore making it exothermic, but my concern is why it will rise.

Thanks!

I guess nobody knows