Energy cannot be created or destroyed.  It can be changed from one form to another as follows:

Electrical energy to light and heat energy in a light bulb.

Electrical energy to heat energy in an electric stove.

Chemical bond energy to heat energy in a chemical reaction.

Molar heat is the amount of heat involved when one mole of a substance reacts.  The symbol is DH.

An exothermic reaction is one in which heat is given off.  It is part of the products.  (Think of EXIT  =  OUT).

The DH (enthalpy change) is negative.

   H₂(g)  +  1/2 O₂(g) -->  H₂O(l)  +  68 kcal        DH  =  -68 kcal/mol H₂

An endothermic reaction is one in which heat is absorbed.  It is part of the reactants.  (Think of IN). 

The DH (enthalpy change) is positive.

   H₂O(l)  +  68 kcal  -->  H₂(g)  +  1/2 O₂(g)        DH  =  +68 kcal/mol H₂O

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