Galvani or internal

In thermochemistry, the chemical potential of an uncharged species i is defined by the following equation  

Therefore represents the contribution of species (relative to one mole) to the system s Gibbs energy, i.e., in interaction with other species in the medium in question. 

This is aiso the work required for the reversibie transport of one moie of species i to switch from infinite distance in vacuum to the buikof the system, at Tand P constant. 

The activity of species i, a, (a dimensioniess number), is defined in terms of its chemicai potentiai by thefoiiowing equation  

A standard state corresponding to ai= 1 shouid be defined for each species. Except in the case of soiutions (see beiow), most of the time the standard state of species i at temperature Tcorresponds to the pure compound in its physicai state (gas, soiid, iiquid) at the same temperature and under the standard pressure of P°= 1 bar. A standard state of reference is aiso defined for each pure compound, and this corresponds to the standard state of the pure compound in its most stabie physicai state at the temperature in question. 

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