How do you know if a reaction is endothermic?

So if the sum of the enthalpies of the reactants is greater than the products, the reaction will be exothermic. If the products side has a larger enthalpy, the reaction is endothermic. You may wonder why endothermic reactions, which soak up energy or enthalpy from the environment, even happen.

Only one reactant yields 2 or more products. meaning you will only have 1 product and it will break down into 2 or more products. eg. h2o -> h2 + o2

In reversible reaction chemistry equilibrium is attained when the forward and reverse reactions are taking place at the same rate, thus, the concentrations of reactants and products remain the same. a reversible reaction is a chemical reaction where the reactants form products that, in turn, react together to give the reactants back. in irreversible reactions on the other hand, the reactants react to form the products, which cannot revert back into reactants.