As in Ref 17, we suggest that adsorption processes can be considered within the framework of the capacitor model. While an ab-initio reaction path would be missing, we choose a simplified reaction path, where all atoms participating in the reaction move simultaneously along the axis perpendicular to the surface. For example, in the case of the CO 2 adsorption reaction, ω would correspond to a transition from a bent CO 2 * molecule to a linear CO 2 . Figure shows that this approximation leads to q which is slightly underestimated from the corresponding value obtained by fitting, likely a consequence of assuming an idealised reaction path.