Editing Thermodynamic activity (section)

==Use==
Chemical activities should be used to define [[chemical potential]]s, where the chemical potential depends on the [[temperature]] {{mvar|T}}, [[pressure]] {{mvar|p}} and the activity {{mvar|a<sub>i</sub>}} according to the [[formula]]:
:<math>\mu_i = \mu_i^{\ominus} + RT\ln{a_i}</math>
where {{mvar|R}} is the [[gas constant]] and {{math|''μ''{{su|b=''i''|p=<s>o</s>}}}} is the value of {{mvar|μ<sub>i</sub>}} under standard conditions. Note that the choice of concentration scale affects both the activity and the standard state chemical potential, which is especially important when the reference state is the infinite dilution of a solute in a solvent. Chemical potential has units of joules per mole (J/mol), or energy per amount of matter. Chemical potential can be used to characterize the specific [[Gibbs free energy]] changes occurring in chemical reactions or other transformations.

Formulae involving activities can be simplified by considering that:
* For a chemical solution:
** the [[solvent]] has an activity of unity (only a valid approximation for rather dilute solutions)
** At a low concentration, the activity of a solute can be approximated to the ratio of its concentration over the standard concentration: <math display="block">a_i = \frac{c_i}{c^{\ominus}}</math>
Therefore, it is approximately equal to its concentration.

* For a mix of [[gas]] at low pressure, the activity is equal to the ratio of the [[partial pressure]] of the gas over the standard pressure: <math display="block">a_i = \frac{p_i}{p^{\ominus}}</math> Therefore, it is equal to the partial pressure in atmospheres (or bars), compared to a standard pressure of 1 atmosphere (or 1 bar).
* For a solid body, a uniform, single species solid has an activity of unity at standard conditions. The same thing holds for a pure liquid.

The latter follows from any definition based on Raoult's law, because if we let the solute concentration {{math|''x''<sub>1</sub>}} go to zero, the vapor pressure of the solvent {{mvar|p}} will go to {{mvar|p*}}. Thus its activity {{math|1=''a'' = {{sfrac|''p''|''p''*}}}} will go to unity. This means that if during a reaction in dilute solution more solvent is generated (the reaction produces water for example) we can typically set its activity to unity.

Solid and liquid activities do not depend very strongly on pressure because their molar volumes are typically small. [[Graphite]] at 100&nbsp;bars has an activity of only 1.01 if we choose {{math|1=''p''<sup><s>o</s></sup> = 1 bar}} as standard state. Only at very high pressures do we need to worry about such changes. Activity expressed in terms of pressure is called [[fugacity]].