SAGAR SINGH - IIT DELHI
Last Activity: 14 Years ago
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 (DHf )
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
