Exothermic or Endothermic reactions Can we determine whether a reaction is endothermic or exothermic? For example if we are given the following reactions:

$$\ce{2SO2 + O2 -&gt; 2SO3}$$

$$\ce{H2 + I2 -&gt;2HI}$$

$$\ce{N2 + 3H2 -&gt; 2NH3}$$

Can we determine weather these reactions are endothermic or exothermic or do we have to just memorize this? Are there any general trends that could make memorizing this information more manageable?
In general just by looking at these equations there is no surefire way to know which is endo or exothermic. However you can look at the bond strength or the ∆H of formation for each molecule. In short you have to know something. 



The energy stored in the molecules(specifically in the bonds) is lower before the reaction in an endothermic reaction and higher in an exothermic reaction. This means if you know relatively how strong the bonds or how much energy it takes to form each of the molecules then you can know whether the reaction is endo or exothermic.

If you have a few ∆H of formation values you can use Hess's Law