A factor involving activity coefficients that appears as a multiplier in the rate equation. For a bimolecular reaction the factor is \[\frac{y_{\text{A}}\ y_{\text{B}}}{y_{\ddagger}}\] where y A, y B and y ‡ are the activity coefficients for the reactants A and B and the activated complex, respectively. For a unimolecular reaction the factor is \[\frac{y_{\text{A}}}{y_{\ddagger}}\] and for a trimolecular reaction it is \[\frac{y_{\text{A}}\ y_{\text{B}}\ y_{\text{C}}}{y_{\ddagger}}\]