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Which is a more favorable reaction: an endothermic or an exothermic reaction?
Exothermic is a more favorable reaction.
First, a "more favorable reaction" is a reaction which has products that are of a lower energy level than its reactants. Heat is a form of energy.
An endothermic reaction uses heat while an exothermic reaction gives off heat.
Here is a diagram of an exothermic reaction. As you can see, the products are at a lower potential energy from giving off heat.
In contrast, this is a diagram of an endothermic reaction.
Since endothermic reactions absorb heat, the products have a higher energy level than the reactants.
If you were wondering which reaction (exo or endo) is spontaneous (meaning that it happens on its own without the input of energy) then there are more factors than just heat. Gibb's Free Energy determines spontaneity.