This document discusses the thermodynamics of water splitting to produce hydrogen. It explains that water decomposition requires a large positive change in free energy, so exergy equal to the change in Gibbs free energy as well as thermal energy must be supplied to shift the reaction equilibrium. The standard thermodynamic properties of the water splitting reaction are provided. Direct thermal decomposition of water is not feasible at an industrial level today due to the very high temperatures required to achieve significant dissociation.