I was thinking about the reaction of $C$ with $O_2$ . The reaction is $C+O_2 \to CO_2+$HEAT. In this reaction the mass of carbon is $12$ and the mass of $2$ oxygens is $32$ in the reactant side total mass is $44$ and in product side total mass is $44$ then where is this heat generated. First rule of thermodynamics states that energy can neither be created, nor be destroyed, it can only be converted from one form to another, then in this reaction where does the energy come from?
Look at the bond energy data of the. Chemical substances involved . When you calculate the change in enthalpy using the data you would realise that the change is negative . This means that some energy previously stored as bond energy is released in form of heat from the system .
And , First law of thermodynamics is nothing but energy conservation , which holds to be true for this macroscopic world