The Heat of Reaction is the change in the enthalpy of a chemical reaction. In endothermic reactions, ( ΔH>0 ) thermal energy is absorbed via the reaction. Anther way to view endothermic reactions is that more (thermal) energy is NEEDED to overcome the forces of attraction between molecules and to separate them from one another (the activation energy) than (thermal) released when new bonds are formed. heat+6CO2(g)+6H2O(l)↽−−⇀C6H12O6(aq)+6O2(g)  In EXOTHERMIC reactions, ( ΔH<0 ) thermal energy is general with reaction. When new bonds are generated, more thermal energy is released that needed to break bonds in the reactants. In this chemical reaction CaO(s)+H2O(l)↽−−⇀Ca(OH)2(s)+heat(1) the FORWARD reaction is exothermic because energy is released when  CaO(s)  and  H2O(l)  combine to form  Ca(OH)2(s) . The energy to break the bonds in  CaO(s)  and  H2O(l)  on the left side of the equation is less than the energy released from forming the  Ca(OH)2(s)  on the right side of the equation; the net difference is observed as heat on the right side of the equation.HOPE THIS HELPS YOU!!!