Fusion, molar enthalpy

Hfus) heat of fusion molar heat of fusion molar enthalpy of fusion. The change in enthalpy when one mole of solid melts to form one mole of liquid. Enthalpies of fusion are always positive because melting involves overcoming some of the intermolecular attractions in the solid. 

If, in going from 0 K to T, a substance undergoes phase changes (fusion, vaporization, etc) at and Tg with molar enthalpies of transition AHy, one can write... 

The normal boiling point of 2-methylthiazole is 17 0= 128.488 0.005°C. The purity of various thiazoles was determined cryometrically by Handley et al. (292), who measured the precise melting point of thiazole and its monomethyl derivatives. Meyer et al. (293, 294) extended this study and, from the experimental diagrams of crystallization (temperature/degree of crystallization), obtained the true temperatures of crystallization and molar enthalpies of fusion of ideally pure thiazoles (Table 1-43). 

Molar enthalpy of fusion of ideally pure sample. 

The molar enthalpy change that accompanies melting (fusion) is called the enthalpy of fusion, AHfus, of the substance ... 

C14-0050. Table lists molar enthalpies of fusion of several substances. Calculate the molar entropy of fusion at its normal melting point for each of the following (a) argon (b) methane (c) ethanol and (d) mercury. 

A molar enthalpy of formation B specific heat C heat of fusion D mass... 

The enthalpy A W((lcUi is the energy required to melt 1 mol of material at constant pressure. We need to be careful when obtaining data from tables, because many books cite the enthalpy of fusion, which is the energy released during the opposite process of solidification. We do not need to worry, though, because we know from Hess s law that AH elt) = — A//( lsion). The molar enthalpy of melting water is +6.0 kJmol-1. 

The term (7/Sig2Si04,meit Mg2Si04,oiivine) is the molar enthalpy of solution of pure forsterite in a pure Mg2Si04 melt, and is equivalent to the molar enthalpy of fusion at r =... 

The molar enthalpy of molten components in the standard reference conditions of r = 298.15 K and P = bar is usually obtained indirectly, by adding first the molar enthalpy of fusion to the molar enthalpy of the crystalline component at its melting point (see also figure 6.10) ... 

In equation 33, the superscript I refers to the use of method I, a T) is the activity of component i in the stoichiometric liquid (si) at the temperature of interest, AHj is the molar enthalpy of fusion of the compound ij, and ACp[ij] is the difference between the molar heat capacities of the stoichiometric liquid and the compound ij. This representation requires values of the Gibbs energy of mixing and heat capacity for the stoichiometric liquid mixture as a function of temperature in a range for which the mixture is not stable and thus generally not observable. When equation 33 is combined with equations 23 and 24 in the limit of the AC binary system, it is termed the fusion equation for the liquidus (107-111). 

Therefore, the mole fraction ideal solubility of a crystalline solute in a saturated ideal solution is a function of three experimental parameters the melting point, the molar enthalpy of fusion, and the solution temperature. Equation 2.15 can be expressed as a linear relationship with respect to the inverse of the solution temperature ... 

Thus, it can be seen that the greater the molar enthalpy of fusion, the greater the increase in solubility as the solution temperature is increased, and a steeper slope would be evident in a plot such as Figure 2.1. 

In the second assumption,Cp is apparently equal to the molar entropy of fusidi f, which can be expressed in terms of the melting point and molar enthalpy of fusion, as in Equation 2.23. This second assumption results in the following expression ... 

We have two alternate developments that depend on whether we choose to use the experimental molar enthalpy of fusion of the solvent or the difference between the molar enthalpy of the pure, liquid species AB and that of the solid. In the first case we must introduce the chemical potential of the pure liquid component and eliminate the chemical potential of the AB species. In order to do so we consider the pure liquid at the experimental temperature T indicated in Equation (11.160). Then... 

The difference between the two standard chemical potentials is determined in terms of the molar enthalpy of fusion by the methods developed in Section 10.12. For real solutions, the equation corresponding to Equation (11.162) is... 

The molar enthalpy of fusion (AEP J is the heat necessary to convert one mole of a solid into a liquid at its normal melting point. The molar enthalpy of vaporization (AH°vap) is the heat required to convert one mole of a liquid to a gas at its normal boiling point. When melting or vaporization occurs at constant pressure, it is acceptable to use heat instead of enthalpy. This is because heat and change in enthalpy are equal to each other under constant pressure conditions. The interested student should consult any physical chemistry textbook for more details. Both AHfm and AHyap are inherently endothermic, and represent an amount of energy that must be added to the sample in order for the phase transition to occur. The heat of fusion represents the amount of energy necessary to overcome the intermolecular forces to the point that the molecules can start to move around each other. The heat of vaporization represents the amount of energy necessary to overcome all intermolecular forces so that the molecules can escape into the gas phase. 

True temperature of crystallization of ideally pure sample. b Molar enthalpy of fusion of ideally pure sample. 

In this expression we have first invoked Eq. (1.20.15b) or, alternatively, (2.7.3) in different notation H and are the molar enthalpies of pure liquid A and of pure solid A respectively. Next, we have defined T/ as the molar heat of fusion of pure A it is not the molar enthalpy change accompanying the transfer of pure A into the solution A + B. Lastly, we invoked Eq. (2.8.1) to obtain the quantity on the right. 

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

Like water, almost all substances can be in the three common states of matter. Table 5 lists the molar enthalpies and entropies of fusion and vaporization for some elements and compounds. Because intermolecular forces are not significant in the gaseous state, most substances have similar values for molar entropy of vaporization, ASy p-... 

Table 5 Molar Enthalpies and Entropies of Fusion and Vaporization... 

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