Dear student,

Enthalpies of Formation

Using Hess's Law we can calculate reaction enthalpies for a variety of reactions using tables of known enthalpies

Many experimentally determined enthalpies are listed by the type of process

• D
H for converting various liquids to the gas phase are listed in tables of enthalpies of vaporization
• D
H for melting solids to liquids are listed in tables of enthalpies of fusion
• D
H for for combusting a substance in oxygen are listed in tables of enthalpies of combustion

The enthalpy change associated with the formation of a compound from its constituent elements is called the enthalpy of formation (DH_f )

Conditions which influence enthalpy changes include:

• temperature
• pressure
• state of reactants and products (s, g, l, aq)

The standard state of a substance is the form most stable at 298 ?K (25 ?C, or standard "room temperature") and 1 atmosphere (1 atm) of pressure

When a reaction occurs with all reactants and products in their standard states, the enthalpy change is the standard enthalpy of reaction (DH?)

Thus, the standard enthalpy of formation (DH?_f)of a compound is the change in enthalpy that accompanies the formation of 1 mole of that substance from its elements, with all substances in their standard states

The standard enthalpy of formation for ethanol (C₂H₅OH) is the enthalpy change for the following reaction