How do you tell endothermic reaction from an exothermic reaction by looking at the chemical equation?

Answer 1

You have to add up the bond energies of all the bonds on the products side and the reactants side. When bonds are formed energy is released. Conversely energy has to be put into a system to break bonds (like smashing a block of ice with a Baseball bat) If there is more bond energy on the products side bonds were created (energy was released) which means the reaction is exothermic. If there is more bond energy on the reactants side, bonds were broken (energy put in) and so the reaction is endothermic.

Answer 2

In an endothermic reaction, the products will have more energy than the reactants, so the enthalpy change (ΔH) will be positive in the chemical equation. In an exothermic reaction, the products will have less energy than the reactants, so the enthalpy change (ΔH) will be negative in the chemical equation.