A simple model is constructed to calculate the potential-energy surface of dissociative adsorption and associative desorption reactions at the metal/solution interface; The model is based on an extension of the Anderson-Newns Hamiltonian and has three reaction coordinates; the bond length or the distance between the fragments, the distance from the surface; and-the generalized solvent coordinate familiar from the classical, theory of electron-transfer reactions. The properties of the three-dimensional potential energy surfaces are studied and the activation energy for dissociative adsorption is calculated as a function of the applied potential and the metal-work function, In the observed trends, the absorption energy and hence the electrosorption valency of the fragments play an important role. For certain "extreme" values of the bonding or antibonding energy levels, molecular ions may become metastable and affect the reaction mechanism.