How can you determine whether an equation is endothermic or exothermic?

There are a couple of ways to determine if a reaction is
exothermic or endothermic. Endothermic meaning that heat is added to the reaction to
make the reactants interact and exothermic meaning heat is released during the reaction
between the two reactants.

In a chemical equation the word
heat or the triange symbol are used to indicate if the reaction is exothermic or
endothermic.  Thus, Na + HOH --> NaOH + heat would indicate the reaction is
exothermic.    On the other hand, an equation such as 2H2 + O2 +heat --> 2 HOH
would indicate that the reaction is endothermic and no reaction will take place unless
heat is added.

Another way is to look at the energy diagram
for the reaction.  In an energy diagram the energy is on the y-axis and time is on the
x-axis.  The reactants are shown first at their energy level, and the products at their
respective energy levels. In the middle between the two is the activation energy
required to transform the reactants into products. If the energy level of the reactants
is higher than the energy level of the products the reaction is exothermic (energy has
been released during the reaction). If the energy level of the products is higher than
the energy level of the reactants it is an endothermic
reaction.

If the energy is not shown in the chemical
reaction and there is no energy diagram to look at, a last alternative is to calculate
the total bond energy of all the chemical bonds in the reactants and in the products.
There are standard bond energies between almost all elements and you just have to look
them up and do the math.