The negative derivative of the chemical potential with respect to the reaction coordinate is called reaction elec- tronic ux and has recently focused a wide interest to better understand chemical reactions at molecular level. After much consideration, it is now well accepted that positive REF val- ues are associated with spontaneous processes, while negative REF ones translate unspontaneous phenomena. These char- acteristics of the REF are based on a thermodynamic analogy and have been shown right through computational results. In this paper, we develop two analytical expressions of the REF in both the canonical and the grand canonical ensembles. The connection between both equations is established. They are then analyzed, and some arguments are put forward to support the alleged characteristic of the REF and its ability to properly discriminate spontaneous from unspontaneous phenomena.