LECTURE PHYSICAL CHEMISTRY 3rd EDITION ROBERT G. MORTIMER

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For 1 mol of reaction,
The molar enthalpy of any substance depends on its state.
The standard state of a liquid or solid substance is specified to
be the pure substance at a fixed pressure of exactly 1 bar
(100,000 Pa), which we denote by P ◦ . The standard state for a
gas is defined to be the corresponding ideal gas at pressure
P ◦ . The difference between the molar enthalpy of a real gas at
1 bar pressure and the corresponding ideal gas at 1 bar is
numerically very small.
If substance number i is in its standard state, its molar
enthalpy is denoted by H ◦ m (i).
A standard-state reaction is one in which all substances are in
their standard states before and after the reaction. The
enthalpy change for a standard-state reaction is denoted by
∆H ◦ . The standard-state pressure was at one time defined to
equal 1 atm (101,325 Pa).
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