An Exothermic Reaction Involves a Net Release of Energy

For any chemical reaction, the total amount of energy absorbed in breaking bonds in reactants is always different from the total amount of the energ released as bonds form in the products. Consider the reaction in which hydrogen and oxygen react to form water  

In the reactants, hydrogen atoms are bonded to hydrogen atoms and oxygen atoms are double-bonded to oxygen atoms. The total amount of energy absorbed as these bonds break is 

In the products there are four hydrogen-oxygen bonds. The total amount of energy released as these bonds form is 

For this reaction, the amount of energy released exceeds the amount of energy absorbed. The net energy of the reaction Ls found by adding the two quantities  

The negative sign on the net energy indicates that there is a net refowe of energy, and so the reaction is exothermic. For any exothermic reaction, energy can be considered a product and is thus sometimes included after the arrow of the chemical equation  

Type of bond Number of moles Bond energy Total energy 

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