  We present an embedding technique for metallic systems that makes it possible
to model energy dissipation into substrate phonons during surface chemical
reactions from first principles. The separation of chemical and elastic
contributions to the interaction potential provides a quantitative description
of both electronic and phononic band structure. Application to the dissociation
of O$_2$ at Pd(100) predicts translationally "hot" oxygen adsorbates as a
consequence of the released adsorption energy (ca. 2.6 eV). This finding
questions the instant thermalization of reaction enthalpies generally assumed
in models of heterogeneous catalysis.
