In chemistry, a reaction coordinate is an abstract one-dimensional coordinate system which represents progress along a reaction pathway .
These coordinates can sometimes represent a real coordinate system, although, for more complex reactions especially, this can be difficult. Reaction coordinates are often plotted against enthalpy (often but erroneously plotted against free energy) to demonstrate in some schematic form the energy landscape of the reaction.
For this reason, reaction coordinates usually represent continuous pathways which are locally minimal in terms of energy. Frequently exaggerated diagrams are used, to accommodate superimposed diagrams or to make a discussion point about the enthalpy along the process.
For example, in the homolytic dissociation of hydrogen, an apt coordinate system to choose would be the coordinate corresponding to the interatomic radius .