The heat of fusion is the heat that is absorbed to transform a substance from its solid state to its liquid state at constant, that is, to melt the solid substance. In common usage, the heat of fusion is used in place of the more precise term the enthalpy of fusion, which has the symbol ()Hfus. The enthalpy of fusion is the heat of fusion for melting of one mole of the substance under three specific conditions: (1) the pressure remains constant, (2) the only possible work that occurs is expansion against the atmosphere (so-called P()V work) and (3) the temperature remains constant during the process. A heat of fusion for a substance is only valid for conversion of the pure solid to the pure liquid state of the substance.
Unlike many other types of enthalpy changes, enthalpies of fusion for a large number of substances can be measured directly. Both the solid and liquid states are condensed phases and relatively small changes in volume occur as a substance is melted. In addition, if heat is applied slowly, a very nearly constant temperature can be maintained. To help maintain a constant temperature, it is useful to finely divide the solid substance to produce a large surface area to volume ratio. In this way, heat is readily distributed within the small particles.
For those cases in which it is not feasible to carry out a solid to liquid transformation without increasing the temperature of the system, the enthalpy of fusion can be determined by subtracting the heat absorbed to increase the temperature of the resulting liquid from the total heat absorbed in the process. The heat absorbed by the liquid is equal to the change in temperature multiplied by the molar heat capacity and the number of moles of solid melted.
Enthalpies of fusion are often specifically designated to show the conditions under which they apply. The temperature at which a substance melts depends upon the external pressure on the solid. If the pressure is other than one atmosphere, a subscript indicating the pressure should be used, for example, ()Hfus, 2 atm for a reaction at twice atmospheric pressure. One atmosphere is the pressure of the atmosphere at sea level, previously defined by the atmospheric pressure that would support a 760 mm column of mercury in a tube closed at the top (a barometer) and now defined in the metric system or Syteme International, SI as 101,325 Pascals where one Pascal is one kilogram meter per seconds squared. Other pressure units that are commonly used are the torr (equal to 1.760 of a standard atmosphere), the Pascal, the bar (equal to 0.9869 atmospheres or 100,000 Pascals) and pounds per square inch (used principally by American engineers). The units typically used for heats of solution are kilojoules per mole or kilocalories per mole.
Enthalpies of fusion are positive because the solid state affords the molecules or ions that compose a pure substance the opportunity to maximize their attractive interactions with each other. When a substance is melted, the substance not only becomes fluid on the macroscopic scale that we can see, but within the material the molecules or ions are more mobile. In the liquid state they do not maintain the best geometric arrangement for maximizing their attraction for each other, as they do within the solid, but continually shift their orientations to neighboring molecules or ions.
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