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Standard States
From UCDavis Chemwiki
The definition of the standard enthalpy of formation is the change in enthalpy when one mole of a substance, in the standard state of 1 atm of pressure and temperature of 298.15 K, is formed from its pure elements under the same conditions. The thermodynamic standard state of a substance is its most stable pure form under standard pressure (1 atm) and at some specific temperature.
Keep in mind the following:
- For a pure substance in the liquid or solid phase, the standard state is the pure liquid or solid.
- For a gas, the standard state is the gas at a pressure of 1 atm; in a mixture of gases, its partial pressure must be 1 atm.
- For a substance in solution, the standard state refers to 1 M concentration.
The standard enthalpy change ΔH°rxn for a reaction: reactants → products
refers to the ΔH when the specified number of moles of reactants, all at standard states, are converted completely to the specified number of moles of products, all at standard states.
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