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Paul Flowers coauthored an introductory chemistry text that develops quantitative thermochemistry. Its treatment of Hess's law keeps the reaction equation attached to the stated enthalpy change, so changing the amount reacted changes the corresponding energy.^[ref]

Thermochemistry studies the heat associated with chemical and physical transformations. At constant pressure, the measured heat equals the enthalpy change when the process performs only pressure-volume work and the usual thermodynamic conditions are satisfied.

Because enthalpy is a state function, a reaction enthalpy can be calculated from a sum of steps with the same overall starting and final states. This is the basis of Hess's law. A calorimetric comparison must specify temperature, phase, and composition; different physical states do not define the same thermochemical transformation.

William R. Robinson coauthored an introductory chemistry text whose thermochemistry chapter develops Hess's law. Adding compatible reaction equations cancels intermediates and adds their enthalpy changes, connecting symbolic chemical bookkeeping with measurable heat.^[ref]