![]() The heat required per mole to vaporize the liquid at constant pressure is equal to the enthalpy of vaporization, Hvap. For example, the entropy of vaporization, Svap, is the change in entropy per mole of molecules when a substance changes from a liquid into a vapor. It follows that we can replace qrev in the expression for the entropy change with H for the phase change. Because the transition takes place at constant pressure (for instance, 1 atm), the heat supplied is equal to the change in enthalpy of the substance. Provided the external pressure is fixed (at 1 atm, for instance), raising the temperature of the surroundings an infinitesimal amount results in complete vaporization, and lowering the temperature causes complete condensation.ģ. At the temperature of a phase transition, the transfer of heat is reversible. The T in the denominator of the equation, Change in S = qrev/T, is therefore a constant and may be set equal to the transition temperature (in kelvins).Ģ. All the energy supplied is used to drive the phase transition, such as the conversion of liquid into vapor, rather than to raise the temperature. At the transition temperature (such as the boiling point for vaporization), the temperature of the substance remains constant as heat is supplied. Using the equation, Change in S = qrev/T, to calculate the entropy change for a substance undergoing a transition from one phase to another at its transition temperature, we need to note three facts:ġ. ![]() ![]() ![]() I have copied some of the information below! Point Number 3 answers the first part of your question. In the textbook has a really good explanation for the entropy change of phase changes. ![]()
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