Entropy change reversible phase changes

Enthalpy-entropy compensation has been investigated in reversed-phase HPLC with octylsilica stationary phase [77]. The compensation temperatures were determined for this system, and the results show that their change with the composition of the mobile phase is almost similar to that with octadecylsilica stationary phase. It can be concluded that the retention mechanisms of the separation of alkyl benzenes is the same in both systems with the mobile phase exceeding 20% water content. 

Eor the heat exchange with the surroundings to occur reversibly (so that we can calculate the entropy change in the surroundings), we can imagine the gas to be in a vessel immersed in a large two-phase system (for example, solid-hquid) at... 

The entropy, Spontaneous vs non-spontaneous, Reversible and irreversible processes, Calculation of entropy changes (Isothermal, isobaric, isochoric, adiabatic), Phase changes at equilibrium, Trouton s rule, Calculation for irreversible processes... 

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 ... 

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. 

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... 

Irreversible processes of phase transfer and chemical reaction within a closed system, whether homogeneous (a single phase) or heterogeneous (more than one phase), lead to T djS > 0. At equilibrium, T djS = 0. For fixed S and V constraints, dE = —T djS. A reversible process corresponds to zero internal entropy change and a minimum in dE. 

The entropy of vaporization of water at 85 °C may be carried out through a series of three reversible steps. Namely, reversibly heating the reactants to 100 °C, carrying out the phase change at this temperature, and finally... 

The temperature-entropy diagram in fig. 1 clearly shows that fluids of low molar specific heat, e.g. water, can only partly be evaporated or condensed by reversible adiabatic processes. Fluids of high molar specific heat, e.g. perfluoro-n-hexane (C6F14), however, make complete adiabatic phase changes feasible. Physically, the difference between... 

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