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I am aware that exothermic reactions release energy and is bond making so the enthalpy change is negative, whereas endothermic reactions absorb energy and is bond breaking and the enthalpy change is positive. According to enthalpy diagrams, for exothermic reaction, the enthalpy of the reactants is higher than that of the products, and I get that its because heat energy is lost to the surroundings, hence the increase in temperature. However, why is it that in an exothermic reaction, the sum of the bond enthalpies of the products is higher that than of the reactants? Does it mean that there are stronger covalent bonds formed in the products of an exothermic reaction?
Yes, think of reaction as the process of achieving a stable state. The reactants are unstable, they got loads of energy so they try to lose energy (exothermic) to form stable products.
bond broken en. is smaller than bond made en. so enthalpy of reaction is negative.