where ∆ f H i (T ) is a critical thermochemical quantity since it captures the majority of the influence of chemical bonds. The enthalpy of formation is a relative quantity, meaning it is always defined relative to other species or reference states, whereas entropy and heat capacity are properties of an individual species. Defining a set of reference species is necessary to obtain a set of internally consistent ∆ f H for all species. The thermodynamic community has defined the elements in their standard configuration as the standard reference species , so the enthalpy of formation of a species is defined as a formation reaction from the elements in their most stable form at a constant temperature. For example, for C 2 H 6