Why does the temperature change during a reaction

One of the simplest definitions of a chemical reaction is changes in the bonds . All reactions proceed with some bonds cleaving concurrently with others forming. Each 

Like internal energy, we can never know the enthalpy of a reagent only the change in enthalpy during a reaction or process is knowable. Nevertheless, we can think of changes in H. Consider the preparation of ammonia  

We obtain the standard enthalpy change on reaction A H as a sum of the molar enthalpies of each chemical participating in the reaction  

In summary, the temperature of a reaction mixture changes because energy is released or liberated. The temperature of the reaction mixture is only ever constant in the unlikely event of A H being zero. (This argument requires an adiabatic reaction vessel see p. 89.) 

Some standard enthalpies have special names. We consider below some of the more important cases. 

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