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We investigate equilibrium chemistry between molten metal and silicate, and ahydrogen-rich envelope using 18 independent reactions among 25 phase componentsfor sub-Neptune-like exoplanets. Both reactive metal and unreactive metalsequestered in an isolated core are modeled. The overarching effects ofequilibration are oxidation of the envelope and reduction of the mantle andcore. Hydrogen and oxygen typically comprise significant fractions of metalcores at chemical equilibrium, leading to density deficits that offer apossible alternative explanation for the low densities of the Trappist-1planets. Reactions with the magma ocean produce significant amounts of SiO andH$_2$O in the envelopes directly above the magma ocean. Molar concentrations inthe envelopes of planets with reactive metal areH$_2>\rm{SiO}>CO\sim\rm{Na}\sim\rm{Mg}>\rm{H}_2\rm{O}>>\rm{CO}_2\sim\rm{CH}_4>>\rm{O}_2$while for the unreactive metal case H$_2$O becomes the second most abundantspecies, after H$_2$, providing an arbiter for the two scenarios amenable toobservation. The water abundances in the atmospheres exceed those in themantles by at least an order of magnitude in both scenarios. The waterconcentrations in the silicate mantles are $\sim 0.01$ wt\% and $\sim 0.1$ wt\%in the reactive and unreactive metal core cases, respectively, limiting theH$_2$O that might be outgassed in a future super-Earth. Less dissolved water inthe reactive core case is due to sequestration of H and O in the Fe-rich metal.The total hydrogen budget of most sub-Neptunes can be, to first order,estimated from their atmospheres alone, as the atmospheres typically containmore than 90\% of all H.

Chemical equilibrium between Cores, Mantles, and Atmospheres of  Super-Earths and Sub-Neptunes, and Implications for their Compositions,  Interiors and Evolution