Recent developments in shock-wave experimental techniques have allowed the 100 GPa pressure range in hydrogen to be probed. In recently reported single-shock-wave laser-driven experiments, the principal Hugoniot was determined up to 300 GPa. It has been shown that dissociation of hydrogen molecules becomes operative under such ultrahigh pressures. Various models have been developed which treat pressure dissociation approximately. In this paper we generalize standard fluid variational theory to a two-component system with a reaction (dissociation). From the free energy, other thermodynamic functions such as the internal energy and entropy are derived. Comparison with other approaches and the shock-wave data is performed. (C) 2000 American Institute of Physics. [S0021-9606(00)52306-X].