CHAPTER 4. THERMODYNAMICS: THE FIRST LAW

F. Thermochemistry
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A very important application of thermodynamics to chemistry is the study of the heat released
or absorbed in a chemical reaction. Chemical reactions are most commonly carried out at constant
pressure (i.e., in an open vessel) and for this specified path, the heat changes are the same as the
enthalpy changes. Since enthalpy is a state function, it is possible to calculate the change in enthalpy
accompanying a chemical reaction simply from a knowledge of the enthalpies of the reactants and
the enthalpies of the products. In actual, practice, enthalpies for chemical reactions may be
o
determined from tabulated standard enthalpy changes, denoted by the symbol H . The term
standard means that the enthalpy change is for a reaction in which the reactants and products are in
their so-called standard states.
Standard state:
the pure substance at a specified
temperature and a pressure of 1 bar.
Although enthalpy changes may be tabulated for any temperature, the most common tabulations are
for 298 K.
1. Enthalpy of formation
The most useful thermochemical tabulations give standard enthalpies of formation denoted
o
by the symbol fH .
Standard enthalpy of formation:
the enthalpy change accompanying the formation of a
substance in its standard state from the most stable forms of
the constituent elements of the substance in their standard
states.
For example, the standard enthalpy of formation for benzene is the enthalpy change for the reaction
6C + 3H ! C H .
(s,gr) 2(g) 6 6(l)