A new model is presented, which consistently combines equations of state in terms of the Helmholtz energy with excess Gibbs energy models. It enables the description of reactive mixtures by solving the phase and chemical equilibrium based on Helmholtz energy equations of state, that are commonly used to describe non-reactive systems. The consistent calculation of all thermodynamic properties in the complete fluid region, including gas, liquid, and supercritical states as well as phase equilibria, is possible. The fundamental property is the Helmholtz energy with the independent variables temperature, density, and composition. The new model reproduces well experimental vapor-liquid equilibrium, homogeneous density, and speciation data of the binary system of monoethanolamine + carbon dioxide as well as of the ternary system of water + monoethanolamine + carbon dioxide.