This work focuses on the development of State-to-State and reduced-order models for dissociation and energy transfer in aerothermodynamics. The reduction is realized by grouping the population of elementary states into energy bins based on Maxwell-Boltzmann distributions. Different grouping strategies are investigated. Kinetic and thermodynamic data are taken from the rovibrational ab-initio database for the N(4Su)-N2(1Σ+ g) system developed at NASA Ames research center. Applications consider the steady expanding flow within the nozzle of the Electric Arc Shock Tube (EAST) facility at NASA Ames Research Center. Numerical solutions are obtained by using a decoupled implicit method. Results show that the population of high-lying vibrational and rotational states depart from the local equilibrium (i.e. Boltzmann distribution). The comparison between the State-to-State and reduced-order model solutions shows that the macroscopic re-combination can be predicted by using only three energy groups.