CHM1 12 Standard States Collection

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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 `DeltaH°_(rxn)` for a reaction: reactants `->` products
refers to the `DeltaH` 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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