The Molar Heat of Fusion and Vaporization

It requires heat energy to melt (fuse) a solid or vaporize a liquid. The amount of heat energy required to melt one mole of a solid is called the molar heat of fusion. The amount of heat energy required to vaporize one mole of a liquid is called the molar heat of vaporization. These phase changes are constant temperature processes, and the amount of heat is that involved in the phase change itself. Notice in the following table that the vaporization values are larger than those for fusion. 

Compound Molar Heat of Fusion Molar Heat of Vaporization 

The opposite of melting (fusion) is freezing. If 6.01 kj of heat is absorbed to melt one mole of water, then 6.01 kj of heat will be evolved if one mole of liquid water freezes. 

The same kind of thing holds for the vaporization and condensation of water. 

The heat associated with chemical and physical changes, AH, and the sign of AH are discussed in Chapter 7. Calculations involving molar heats of fusion and vaporization are done in the following examples. 

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Making and Using Graphs The heats required to melt or to vaporize a mole (a specific amount of matter) of matter are known as the molar heat of fusion (Hf) and the molar heat of vaporization (Hy), respectively. These heats are unique properties of each element. You will investigate if the molar heats of fusion and vaporization for the period 2 and 3 elements behave in a periodic fashion. 

Because molecules are more tightly held in a sohd, the vapor pressure of a solid is generally much less than that of the corresponding liquid. Molar heat of sublimation (AHsub) of a substance is the energy (usually in kilojoules) required to sublime one mole of a solid. It is equal to the sum of the molar heats of fusion and vaporization ... 

The molar heats of fusion and vaporization of benzene are 10.9 kJ/mol and 31.0 kJ/mol, respectively. Calculate the entropy changes for the solid —> liquid and liquid —> vapor transitions for benzene. At 1 atm pressure, benzene melts at 5.5°C and boils at 80.1°C. 

The molar heats of fusion and vaporization for water are 6.02 kj/mol and 40.6 kj/mol, respectively, and the specific heat capacity of liquid water is 4.18 J/g °C. What quantity of heat energy is required to melt 25.0 g of ice at 0 °C What quantity of heat is required to vaporize 37.5 g of liquid water at 100. °C What quantity of heat is required to warm 55.2 g of liquid water from 0 °C to 100. °C ... 

Use Table C-6 in Appendix C to look up and record the molar heat of fusion and the molar heat of vaporization for the period 3 elements listed in the table. Then, record the same data for the period 2 elements. 

Calculate the molar entropies of fusion and vaporization for benzene. Having a molecular weight of 78.1, benzene melts at 5.5°C with a heat of fusion of 2350 cal/(g mol). Its normal boding point is 80.1°C, and its heat of vaporization at that temperature is 94.1 cal/g. 

The bond energies recorded in Table VII are based on data for substances in the gaseous state strictly, therefore, they should be used for reactions involving gases only. However, molar heats of fusion and vaporization are usually of the order of 1 to 10 kcal. hence, provided equal numbers of molecules of solids and liquids appear on both sides of the chemical equation, the conventional bond energies may be employed to yield results of a fair degree of accuracy. 

The molar heats of fusion and sublimation of molecular iodine are 15.27 kJ/mol and 62.30 kJ/mol, respectively. Estimate the molar heat of vaporization of liquid iodine. 

The molar heats of fnsion and vaporization of argon are 1.3 kJ/mol and 6.3 kJ/mol, and argon s melting point and boiling point are 190°C and 186°C, respectively. Calcnlate the entropy changes for fusion and vaporization. 

Are changes in state physical or chemical changes Explain. What type of forces must be overcome to melt or vaporize a substance (are these forces in-tramolecular or mtennolecular) Define the molar heat of fusion and molar heat of vaporization. Why is the molar heat of vaporization of water so much larger than its molar heat of fusion Why does the boiling point of a liquid vary with altitude ... 

The heat accompanying the phase change (2) is 1.44 kcal/mole. This is much less than the molar heat of vaporization of water, 10 kcal/mole. Table 5-II contrasts the melting points and the heats of melting per mole (the molar heat of melting, or the molar heat of fusion) of the same pure substances listed in Table 5-1. 

The volatilization process changes the contaminant from a sohd or hquid state, where the molecules are held together by intermolecular forces, into a vapor phase. The molar heats of fusion (A//p, volatilization (AH), and sublimation (AH) are related according to the Bom-Haber cycle by... 

Define the molar enthalpy of fusion and the molar enthalpy of vaporization, and identify them for a substance by using a heating curve. 

Molar heat of fusion and molar heat of vaporization are tiie amounts of heat needed to melt 1 mole of the solid and to vaporize 1 mole of the liquid, respectively. 

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