24.8: Activities are Calculated with Respect to Standard States
- Page ID
- 14510
Need to define a new variable. The thermodynamic activity, \(a\), is the effective concentration of a species in a mixture. It is a dimensionless quantity that are calculated with respect to standard states. For a gas, this would be related to the fugacity and for a solution, to the concentration. The activity for a real gas:
\[a_i=\frac{f_i}{P^{\circ}}=\frac{\phi_iP_i}{P^{\circ}}=\frac{\phi_i(y_iP)}{P^{\circ}}\]
For systems where we treat the gases as ideal:
\[ \phi_i=1 \]
\[ a_i=\frac{P_i}{P^{\circ}}=y_i\frac{P}{P^{\circ}} \]
The activity for a solution:
\[ a_i=\gamma i\sf\frac{[A]}{1\;\underline{M}} \]
General chemistry and organic chemistry use ideal reactants where \(\gamma_i=1\):
\[ a_i=\sf\frac{[A]}{1\;\underline{M}} \]
The activity for a solid or liquid:
\[ a_i=1 \]