Melting point enthalpy change

The thermal properties of four samples were listed in Table 10.3. All of samples showed better thermal stability below 703K. The melting point, enthalpy change of the same series samples were downtrend with the increase of the substituted hydroquinone content because of copolymerization [45]. The DSC results indicated that there are multi-first order phase transition temperatures in heating process of samples. The samples became viscous, and could be sheared and occur strong birefraction above the low phase transition temperature. The samples became the isotropic liquid when the temperature was over high phase transition temperature. 

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... 

Table 11-4 lists values of A, A / vap, melting points, and boiling points for different chemical substances. Example provides practice in using enthalpies of phase changes. 

Figure 5 Changes in volume, V, energy, E, and enthalpy, H, during cooling or heating of the liquid, crystalline, and glassy (vitreous) forms of a substance. Tm is the melting point, and Ts is the glass transition temperature. (Adapted with permission from Ref. 14.)...

The melting points and enthalpies of the three isomeric l-butyl-3-methylimida-zolium hexafluorophosphate salts [BMIMJlPFg] [47] increase with the degree of chain branching, reflecting the changes in efficiency of the crystal packing as free-rotation volume decreases and atom density is increased. The same effects are also observed for the two isomers of l-propyl-3-methyHmidazoHum hexafluorophosphate [PMlM][PFd [24]. 

The molar enthalpy for the transition from a solid to a supercooled liquid is not a constant with respect to temperature. The molar heat capacities of the solid and supercooled liquid forms of the solute inLuence its magnitude at temperatures below the melting point. It is frequently assumed that either the molar heat capacity of the solid at constant prestiiiippnd the molar heat capacity of its liquid form at constant pressure,pi, are nearly constant or that they change at the same rate with a change in temperature. In either case, the molar differential heat capacity, deLned as... 

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. 

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