Standard Enthalpy of Formation|Definition|Examples|Application of Standard Enthalpy of Formation

Standard Enthalpy of Formation (ΔHf°):

Definition:
The standard enthalpy of formation of a compound is the change in enthalpy that occurs when one mole of the compound is formed from its constituent elements in their standard states.

Examples:

- ΔHf° (H2O) = -285.8 kJ/mol (heat is released when hydrogen and oxygen combine to form water)
- ΔHf° (CO2) = -393.5 kJ/mol (heat is released when carbon and oxygen combine to form carbon dioxide)

Application of Standard Enthalpy of Formation:

1. Calculating the enthalpy of reaction (ΔH°rxn): ΔH°rxn = ΣΔHf°(products) - ΣΔHf°(reactants)
2. Determining the spontaneity of a reaction: If ΔH°rxn is negative, the reaction is spontaneous.
3. Calculating the heat of combustion: ΔHc° = ΔHf°(products) - ΔHf°(reactants)
4. Determining the energy change in a physical transformation: ΔH° = ΔHf°(final state) - ΔHf°(initial state)
5. Calculating the enthalpy of solution: ΔHsol° = ΔHf°(solute in solution) - ΔHf°(pure solute)

Note: Standard states are:
* Elements: most stable form at 298 K and 1 atm
* Compounds: most stable form at 298 K and 1 atm

ΔHf° values are usually given in kJ/mol.


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