  We construct an information-geometric structure for chemical thermodynamics,
applicable to a wide range of chemical reaction systems including non-ideal and
open systems. For this purpose, we explicitly construct dual affine coordinate
systems, which completely designate an information-geometric structure, using
the extent of reactions and the affinities of reactions as coordinates on a
linearly-constrained space of amounts of substances. The resulting structure
induces a metric and a divergence (a function of two distributions of amounts),
both expressed with chemical potentials. These quantities have been partially
known for ideal-dilute solutions, but their extensions for non-ideal solutions
and the complete underlying structure are novel. The constructed geometry is a
generalization of dual affine coordinates for stochastic thermodynamics. For
example, the metric and the divergence are generalizations of the Fisher
information and the Kullback-Leibler divergence. As an application, we identify
the chemical-thermodynamic analog of the Hatano-Sasa excess entropy production
using our divergence.
