Final answer:
A reaction is endothermic when the bonds in the products are weaker than in the reactants, resulting in energy absorption from the surroundings and a positive enthalpy change.
Step-by-step explanation:
A reaction is generally endothermic if the bonds in the products are weaker than the bonds in the reactants. During an endothermic reaction, energy is absorbed from the surroundings because the products have higher potential energy compared to the reactants, which requires the input of energy. This results in a positive change in enthalpy (ΔH), indicating that the reaction is energetically "uphill".
In contrast, an exothermic reaction occurs when the bonds in the products are stronger than those in the reactants, leading to a release of energy, usually in the form of heat, resulting in a negative change in enthalpy (ΔH).