In this work the differences between the formulation of reaction rate equations for equilibrium limited reactions in homogeneous and heterogeneous systems are investigated. A rigorous derivation for equilibrium limited gas-solid reactions is proposed based on the core ideas of homogeneous gas phase reactions. The resulting reaction rate expression allows for two distinct reaction rate mechanisms for the forward and reverse reaction direction and is, therefore, valid in both reaction directions. The thermodynamic consistency and the validity of the proposed reaction rate expression is shown using a simple reactor model. The results are compared to traditional reaction rate expressions and it is explained why they are not able to appropriately describe the true reaction behavior in some cases. The implications and use cases for process modeling are investigated and suggestions are proposed for solving numerical challenges associated with the proposed reaction rate equations. (C) 2019 Elsevier Ltd. All rights reserved.