Heat of dilution

In thermochemistry, the heat of dilution, or enthalpy of dilution, refers to the enthalpy change associated with the dilution process of a component in a solution at a constant pressure. If the initial state of the component is a pure liquid (presuming the solution is liquid), the dilution process is equal to its dissolution process and the heat of dilution is the same as the heat of solution. Generally, the heat of dilution is normalized by the amount of the solution and its dimensional units are energy per unit mass or amount of substance, commonly expressed in the unit of kJ/mol (or J/mol). The heat of dilution can be defined from two perspectives: the differential heat and the integral heat. The differential heat of dilution is viewed on a micro scale, which is associated with the process in which a small amount of solvent is added to a large quantity of solution. The molar differential heat of dilution is thus defined as the enthalpy change caused by adding a mole of solvent at a constant temperature and pressure to a very large amount of solution. Because of the small amount of addition, the concentration of dilute solution remains practically unchanged. Mathematically, the molar differential heat of dilution is denoted as: where ∂∆ni is the infinitesimal change or differential of mole number of the dilution. The integral heat of dilution, however, is viewed on a macro scale. With respect to the integral heat, consider a process in which a certain amount of solution diluted from an initial concentration to a final concentration. The enthalpy change in this process, normalized by the mole number of solute, is evaluated as the molar integral heat of dilution. Mathematically, the molar integral heat of dilution is denoted as: If the infinite amount of solvent is added to a solution with a known concentration of solute, the corresponding change of enthalpy is called as integral heat of dilution to infinite dilution. The dilution between two concentrations of the solute is associated to an intermediary heat of dilution by mole of solute.