AH of Phase Changes

Heat flow into a substance does not always raise its temperature. If heat flows into an ice cube at 0°C, for example, the ice will melt to form liquid water at 0°C. (If heat continues to flow into the resulting water, its temperature will begin to rise, of course.) How can the temperature remain constant despite the influx of heat To understand this, we need to remember that intermolecular forces are more extensive in a solid than in a liquid. So as the ice cube melts, energy must be expended to overcome some of the intermolecular attractions. The internal energy of liquid water is higher than that of solid ice, even though both are at the same temperature. 

Here n is the number of moles, as usual. Notice that this relationship does not include AT, in contrast to Equations 9.4 and 9.5. This should make sense if we keep 

For any substance the heat of fusion is typically much smaller than the heat of vaporization. 

Standard molar enthalpies and temperatures for phase changes of water Fusion Freezing Vaporization 

The AHf value in Table 9.3 is in J/mol, so the amount of ice must be converted into moles. Multiplying the number of moles by AHf will provide the desired quantity. 

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Enthalpy Defining Enthalpy AH of Phase Changes Vaporization and Electricity... 

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