The linear free energy relation: \[\log _{10}(\frac{k_{\rm{s}}}{k_{0}}) = m\ Y\] expressing the dependence of the rate of solvolysis of a substrate on ionizing power of the solvent. The rate constant \(k_{0}\) applies to the reference solvent (ethanol–water, 80:20, v/v) and \(k_{\rm{s}}\) to the solvent s, both at \(25\ ^{\,\unicode{x26ac}}\rm{C}\). The parameter \(m\) is characteristic of the substrate and is assigned the value unity for tert-butyl chloride. The value \(Y\) is intended to be a quantitative measure of the ionizing power of the solvents. The equation was later extended to the form: \[\log _{10}(\frac{k_{\rm{s}}}{k_{0}}) = m\ Y+l\ N\] where \(N\) is the nucleophilicity of the solvent and \(l\) its susceptibility parameter. The equation has also been applied to reactions other than solvolysis.