Unit 5: Thermochemistry

Section 2: Enthalpy

Enthalpy is a property of a system and is equal to E + PV, where E is the
internal energy of the system, P is the pressure, and V is the volume. In a
chemical reaction, the enthalpy change is equal to the total enthalpy of the
products minus the enthalpy of the reactants. The following equation is used
to determine the change in enthalpy in a chemical reaction:

H = Hproducts - Hreactants

When the products have a greater enthalpy than the reactants, H ends
up positive. In these situations, the higher enthalpy in the products
results in the absorbtion of heat by the system, thus an endothermic
reaction. Also, if H is negative, that means that the reactants have
more enthalpy, and must "release" it on its way to becoming products. How do
the reactants "release" their enthalpy? They generate heat, and the reaction
is exothermic. So, when H is positive, the reaction is endothermic,
and when H is negative, the reaction is exothermic.