= Thermochemistry
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<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.
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