1. Molar Enthalpy - DH
It is impossible to measure all the energy associated with each chemical reaction.
Every molecule has kinetic energy (due to movement: translational, vibrational, rotational, and even centripetal of electrons around nucleus) and potential energy (stored within bonds arising from electrostatic and nuclear attractions of electrons & protons).

2. Therefore it is easier to measure the change in heat content of a reaction, known as enthalpy (DH). DH = Hproducts – Hreactants (where H is really q) The amount of heat generated by a reaction will depend on the amount of substance being reacted (m). So it is more convenient to express the energy from a reaction on a mole basis. Molar enthalpy = amount of energy from a chemical reaction in terms of number of moles of chemical reacted or produced.

3. Types of Molar Enthalpies
Many reactions have specific meanings:
DHofus – heat of fusion (melting solid to liquid)
DHovap - heat of vapourization (boiling liquid to gas)
DHocomb- heat of combustion (burning substance)
DHosol- heat of solution (dissolving substance)
DHoneut- heat of neutralization (neutralizing acid/base)
DHof-heat of formation (making substances from its elements)
DHor-heat of reaction (energy from that specific reaction)

4. Representing Enthalpy Changes
Four different ways:
Heat term in the chemical equation
Eg: 2H2(g) + O2(g)  2H2O(g) kJ
DH value
Eg: 2H2(g) + O2(g)  2H2O(g) DHo = kJ
Negative values indicate exothermic reactions
Positive values indicate endothermic reactions

5. Standard molar values of enthalpy
Eg: H2(g) + 1/2O2(g)  H2O(g) DHo = kJ/mol
Reaction is carried out under standard conditions SATP for 1 mole of specific product made or reactant used
Energy Diagrams
Eg: