Phase transitions reversible, entropy change

Entropy in this sense is different from the thermodynamic state function 5, which has a large reversible component, for instance as defined at phase transitions. An entropy change in the system is compensated for by an almost equal, but opposite change in the surroundings. 

Here q is an infinitesimal quantity of heat absorbed from the surroundings by the system and T is measured in kelvins (K). For a reversible phase transition such as the melting of ice at constant pressure and temperature, the change in entropy of the H20 is just AH/T. 

The entropy of the phase transition, AtiansiS , taking place at a fixed temperature, can be calculated using equation (13.14), Frame 13 and since pressure is constant state function, the enthalpy of transition, Alrans// will be identical both for reversible and irreversible changes, so that ... 

For a reversible isothermal process the entropy change of the phase transition follows from q, via to AS = q/T. By way of illustration, values for q and AS thus... 

Retention in chromatography is controlled by thermodynamic equilibria. The partition ofthe analyte between the mobile and the stationary phase is in control of the retention factor. This partition can be described by the laws of reversible thermodynamics. Therefore, we also borrow the thermodynamic description of the temperature dependence of equilibria. This is the so-called van t Hoff equation, which is the quantitative expression of the Le Chatelier principle. According to this, the temperature dependence of the retention factor k can be described by 2.9, with R being the general gas constant, AH° the molar enthalpy (heat tone) related to the transition of the analyte from mobile to stationary phase, AS° the molar entropy change for this transition, andj( the so-called phase ratio of the packed stationary phase in the column. 

Each such reversible phase transition requires positive heat trs- Because the pressure is constant, the heat is equal to the enthalpy change (Eq. 5.3.8). The ratio trs/n is called the molar heat or molar enthalpy of the transition, AtrsT/ (see Sec. 8.3.1). Because the phase transition is reversible, the entropy change during the transition is given by Atrs5 = trs/ Ttrs where Ttrs is the transition temperature. 

Interaction of long-chain n-alkylammonium halides with a halide of a divalent metal leads to the formation of salts of the general formula (RNH3)2MX4 with peculiar phase changes in the solid state [65]. Bis(n-alkylammonium)bromo zinc-ates with n = 10-16 display two reversible high entropy solid crystalline-solid crystalline phase transitions and a solid crystalline-liquid crystalline (smectic phase) transition up to at least 450 K. 

Additionally, we would expect a lipid melting transition to be accompanied by a change in the degree of molecular order and hence accompanied by a change in entropy. For a reversible two-state transition, T represents the temperature at which both phases are in equilibrium and hence the free energy change (AG) associated with the process is zero. The expression... 

The enthalpy of the reasonably reversible, first order transition from the orthorhombic to the hexagonal condis phase of polyethylene is 3.71 kJ/mol at about 500 MPa pressure which is about 80% of the total heat of fusion. The entropy of disordering is 7.2 J/(K mol). The volume change at the transition is... 

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