The boiling point is defined as that temperature where the vapor pressure of the material matches the ambient pressure. Recall that substances can be made to boil at a large range of temperatures depending on that substance's phase diagram as long as we increase or reduce the ambient pressure. If we want to know the entropy associated with the vaporization of a material at a temperature other than that at standard pressure, we can use the procedure of calculating the entropy associated with bringing the material from its non standard boiling point to its standard (normal) boiling point, calculating the entropy associated with this vaporization phase change at that standard (normal) boiling point using the enthalpy of vaporization as q rev, then calculating the entropy change associated with cooling that material back down to the non standard (non normal) boiling temperature we are interested in. All rights reserved.We need molar heat capacities because they are used to determine q which in turn comprises the numerator of our definition of macroscopic entropy change dS = q rev/T. On behalf of the United States of America. Shall not be liable for any damage that may result fromįor NIST Standard Reference Data products. However, NIST makes no warranties to that effect, and NIST Uses its best efforts to deliver a high quality copy of theĭatabase and to verify that the data contained therein haveīeen selected on the basis of sound scientific judgment. The National Institute of Standards and Technology (NIST) One is the fact that the enthalpy of vaporization of a substance is always higher than its enthalpy of fusion.
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