Fusion, enthalpy change

Gibbs free energy of formation Latent heat of fusion Enthalpy change at the triple point Heat of vaporization Enthalpy of vaporization Volume change for liquid-solid transformation... 

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

Energy must also be provided to melt a solid substance. This energy is used to overcome the intermolecular forces that hold molecules or ions in fixed positions in the solid phase. Thus, the melting of a solid also has characteristic energy and enthalpy changes. The heat needed to melt one mole of a substance at its normal melting point is called the molar heat of fusion, Ai/fas... 

The terms enthalpy of fusion, enthalpy of vaporization, enthalpy of combustion, and many more cause some students to believe that there are many different kinds of enthalpies. There are not. These names merely identify the processes with which the enthalpy term is associated. Thus, there are processes called fusion (melting), vaporization, sublimation, combustion, and so forth. The corresponding enthalpy changes are called by names that include these descriptions. 

Ans. (a) The change of a liquid into a gas. (b) The change of a solid into a liquid, U) The enthalpy change accompanying a vaporization process, (d) The enthalpy change accompanying a melting (fusion) process. 

For each of the phase transitions, there is an associated enthalpy change or heat of transition. For example, there are heats of vaporization, fusion, sublimation, and so on. 

Nagaoka s equation (Nagaoka et al, 1955) is an extension of Plank s model and takes into account the time required to reduce the temperature from an initial temperature T, above the freezing poinf. The lafenf heat of fusion in equafion 3.3 is replaced by the total enthalpy change A/i which includes the sensible heat which must be removed in reducing the temperature from an initial T and in addition an empirical correction factor is included. Thus... 

Addnl Refs A) C. Lenchitz, "Ice Calorimeter Determination of Enthalpy and Specific Heat of Eleven Organometallic Compounds , PATR 2224 ( 1955) B) R. Veliky et al, "Enthalpy Change, Heat of Fusion, and Specific Heat of Basic Explosives , PATR 2504(1959) c) L.M. Fingerson, "Research on the Development and Evaluation of a Two-Sensor.Enthalpy Probe , Thermo Systems Inc Rept ARL 64-161 (Oct 1964) [Contract AF 33(657)-9917] D) D.E. Adams,... 

The area of a DSC peak can be used to estimate the enthalpy of transition, AH, provided the thermal history of the sample is considered [29]. Calibration with respect to enthalpy requires an area that corresponds to a well-defined enthalpy change—a heat of fusion AH(Tm) is commonly used, especially that of indium [3]. 

Velicky ec si 2 deed 3t icA.rsn " Enthalpy Change, Heat of Fusion and Specific Heat of Basic Explosives"... 

If a substance undergoes a transformation from one physical stale to another, such as a polymorphic transition, the fusion or sublimation of a solid, or the vaporization of a liquid, the heat adsorbed hy the substance during the transformation is defined as the latent heat of transformation (transition, fusion, sublimation or vaporization). It is equal in the enthalpy change of the process, which is the difference between the enthalpy of the substance in the two states at (he temperature of the transformation. For the purpose of thcrmochemical calculations, i( is usually reported as a molar quantity with die units of calories (or kilocalories) per mule (or gram formula weight). The symbol L or AH. with a subscript i.f (or in), s. and n is commonly used and the value is usually given at the equilibrium temperature of the transformation under atmospheric pressure, or at 25 C. 

Those heal effects can be easily calculated when the enthalpies of formation and the enthalpy-temperature relations are available for the substances considered. Usually, the heat of reaction is defined as the heat evolved by the process, and it is equal to the enthalpy change but opposite in sign, while heats of fusion or vaporization always refer to ihe heat adsorbed, and for heals of solution the usage varies. In order to avoid any confusion, it is recommended to express heat effects of chemical process by reporting the enthalpy change. AH. 

Enthalpy of fusion The enthalpy change that takes place when one mole of a solid is changed to one mole of liquid at the same temperature. 

Fusion is an isothermal phase transformation and hence the enthalpy change of M increases without accompanying a temperature change. AH, represents die heat of fusion of M. 

Entropy of Fusion The fusion of a substance occurs at a definite temp, (i.e., its m.p.) and is accompanied by absorption of heat (enthalpy of fusion). The change in entropy known as entropy of fusion is given by... 

Equation (3.6) illustrates that the solubility of a solid in a liquid depends on the enthalpy change at Tm and the melting temperature of the solid. Equation (3.6) is a valid one when T > Tm because the liquid solute in an ideal solution is completely miscible in all proportions. Table 3.1 shows the ideal solubilities of compounds and their heat of fusion. Equation (3.6) is the equation for ideal solubility. The relationship of In x2 (ideal or nonideal solubility) vs. 1/T is shown in Figure 3.1. 

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. 

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