The reactivity of dissolved O-2, a ubiquitous impurity in aqueous solutions, with H-terminated Si(111) surfaces was quantified using a new technique that exploits kinetic competition between an impurity and an etchant to produce impurity-concentration-dependent changes in the steady-state etch morphology. This method is applicable to any impurity that alters the etch morphology. The impurity-induced changes are quantified using scanning tunneling microscopy measurements and atomistic, concentration-dependent kinetic Monte Carlo simulations. The site-specific reactivity of O-2(aq) is surprisingly anisotropic. Oxidation of the highly strained dihydride step site is 4 times faster than oxidation of the relatively unstrained monohydride step site. Both steps are 104 times more reactive than terrace sites. The observed site-specific reaction rates are highly correlated to local strain. FTIR measurements of the Si-H stretch vibration showed that dissolved O-2 inserts O atoms into surface Si-Si back-bonds without removing, the H-termination. Dissolved O-2 does not attack Si-H bonds, because neither Si-H consumption nor silanol production is observed.