Standard enthalpy of fusion

Here, AHfu° is the standard enthalpy of fusion and T, is the melting point. All entropies of fusion are positive, and so they are normally reported without their positive sign. 

Figure 1.3 Standard enthalpy of aluminium relative to 0 K. The standard enthalpy of fusion (Afugffm) is significantly smaller than the standard enthalpy of vaporization (A vap// ).

AfusH° standard enthalpy of fusion per mole (solid - liquid) Afus ... 

The standard enthalpy change that accompanies a change in physical state is called the standard enthalpy of transition. The standard enthalpies of fusion and vaporization are two examples of enthalpies of transition. 

The highest correlation coefficient (0.98) is for enthalpy of vaporization and standard enthalpy of atomization. For metals, the latter one is actually the standard sublimation enthalpy and is given, according to Hess law, by the sum of the standard enthalpies of fusion and vaporization. The enthalpies of vaporization are those at the boiling temperature and, therefore, lower than the standard ones. Other very close correlations are those between the boiling... 

Another common phase transition is fusion, or melting, as when ice melts to water. The change in molar enthalpy that accompanies fusion under standard conditions (pure solid at 1 bar changing to pure liquid at 1 bar) is called the standard enthalpy of fusion, A H. Its value for water at 0°C is +6.01 kJ mol . As for enthalpies of vaporization, all enthalpies of fusion are positive, and the sign is not written explicitly in tables. Notice that the enthalpy of fusion of water is much less than its enthalpy of vaporization. In vaporization the molecules become completely separated from each other, whereas in melting the molecules are merely loosened without separating completely (Fig. 1.21). 

Notice how we must use the enthalpy of fusion at the melting temperature and that this expression appHes only at the melting temperature. We get the standard entropy of fusion, AjuS, if the solid and liquid are both at 1 bar we use the melting temperature at 1 bar and the corresponding standard enthalpy of fusion at that temperature. All enthalpies of fusion are positive (melting is endothermic it requires heat), so all entropies of fusion are positive too disorder increases on melting. The entropy of water, for example, increases when it melts because the orderly structure of ice collapses as the liquid forms (Fig. 2.6). 

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