 Unequal-concentration second-order integration

For a one-to-one elementary reaction with concentrations x and h, the difference $\Delta =h_0-x_0$ remains constant. Integration gives $\ln (h{x}_0/(x{h}_0))=k\Delta t$ when $\Delta \ne 0$; the equal-initial-concentration limit is $1/x-1/x_0=kt$.