Hess's law states that compatible chemical reaction enthalpies add when their equations add. Reversing a chemical reaction reverses its enthalpy change; multiplying the entire chemical reaction equation by a factor multiplies that change by the same factor. The reason is that enthalpy is a state function: intermediate states cancel from the differences. A cycle supplies one independent balance relation; it cannot determine two unknown inputs without further information.
The standard enthalpy of formation is the enthalpy change for forming one mole of a specified chemical substance from its chemical elements in their reference states at the stated temperature. For sodium chloride at , those reactant states are solid sodium and diatomic gaseous chlorine. Standard state specifies conditions; it does not by itself select a temperature.
An enthalpy of atomization is the enthalpy change for making one mole of separated gaseous chemical atoms from a specified starting chemical substance. For sodium, the relevant step is sublimation to monatomic gas; for diatomic chlorine, it is half the compatible molar bond-dissociation energy expressed as an enthalpy.
An enthalpy of solution is an enthalpy difference for dissolution into a specified solvent at stated temperature and final concentration. Keep a measured finite-dilution value distinct from an infinite-dilution reference value.
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Thermochemistry is a branch of chemistry that deals with the study of heat changes that occur during chemical reactions and physical transformations. It involves the measurement and understanding of energy changes associated with various processes, particularly focusing on the heat absorbed or released during these processes. Key concepts in thermochemistry include: 1. **Enthalpy (ΔH)**: A measure of the total heat content of a system at constant pressure.