Based on classical thermodynamic arguments, an equation relating the partial derivatives of the kinetic rate constant with respect to temperature and pressure is derived for a reversible elementary chemical reaction at equilibrium. It is argued that the rate constant is in principle a state function with temperature, pressure and composition as independent variables in non-equilibrium states. It reduces to a state function of temperature and pressure only at equilibrium, or when the rate constant is (explicitly assumed to be) independent of composition. The reaction rate is a state function of temperature and pressure only in the case of equilibrium. On this basis an alternative interpretation of observed correlations between experimental rate data reported in the literature is proposed.