1. Change in Enthalpy: H H = Esystem + PV H = Esystem + PV
Change in Enthalpy  Change in system energy
...if the volume change or work done by the system is nearly zero
In chemistry we measure H for reactions knowing that in many cases it is the same as the change in internal energy when V0.

2. Changes in Chemical Enthalpy
H = Hproducts - Hreactants
H = -
251 kJ/mol
(Experimentally Measured)
Reactants have high enthalpy
Enthalpy (H)
(Internal Energy)
Hreactants > Hproducts
H= kJ/mol
....Negative H
...Exothermic Rxn.
Products have lower enthalpy
... Releases energy to surroundings
Reactants
Products 
Heat is a product
Xe(g) F2(g)
XeF4(g)
kJ/mol

3. Changes in Chemical Enthalpy
H = Hproducts - Hreactants
H = +
68 kJ/mol
(Experimentally Measured)
Products have high enthalpy
Enthalpy (H)
(Internal Energy)
Hreactants < Hproducts
H= kJ/mol
....Positive H
Reactants have low enthalpy
...Endothermic Rxn.
... Absorbs energy from surroundings.
Reactants
Products 
Heat is a reactant
N2(g) O2(g)
2NO2(g)
68 kJ/mol +