Activity (chemistry) - Use

Use

Chemical activities should be used to define chemical potentials, where the chemical potential depends on the temperature T, pressure p and the activity a_i according to the formula:

where R is the gas constant and µ_io is the value of µ_i 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.

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:

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:

Therefore, it is equal to the partial pressure in bars (compared to a standard pressure of 1 bar).

• For a solid body, a uniform, single species solid at one bar has an activity of unity. 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 x₁ go to zero, the vapor pressure of the solvent p will go to p*. Thus its activity a = p/p* will go to unity. This means that if during a reaction in dilute solution more solvent is generated (the reaction produces water e.g.) 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 bars has an activity of only 1.01 if we choose po = 1 bar as standard state. Only at very high pressures do we need to worry about such changes.Changes can also come as a result of too much dilution of solution.