- Standard molar entropy
chemistry, the standard molar entropy is the entropycontent of one mole of substance, under standard conditions (not standard temperature and pressure STP).
The standard molar entropy is usually given the symbol "S"o, and the units
joules per mole kelvin(J mol−1 K−1). Unlike standard enthalpies of formation, the value of "S"o is an absolute. That is, an element in its standard state has a nonzero value of "S"o at room temperature. The entropy of an element can be 0 J mol−1 K−1 only at 0 K, according to the third law of thermodynamics. However, this presupposes that the material forms a 'perfect crystal' without any frozen in entropy (defects, dislocations), which is never completely true because crystals always grow at a finite temperature. Fortunately this residual entropy is often quite negligible.
If a mole of substance were at 0 K, then warmed by its surroundings to 298 K, its total molar entropy would be the addition of all "N" individual contributions:
Here, dqk/T represents a very small exchange of heat energy at temperature "T". The total molar entropy is the sum of many small changes in molar entropy, where each small change can be considered a reversible process.
The standard molar entropy of a
gasat STPincludes contributions from: [cite book | last = Kosanke| first = K. | coauthors = | title = Pyrotechnic chemistry | publisher = Journal of Pyrotechnics | year = 2004| id = ISBN 1-889526-15-0 | chapter = Chemical Thermodynamics | page = 29 ]
heat capacityof one mole of the solid from 0 K to the melting point(including heat absorbed in any changes between different crystal structures)
latent heat of fusionof the solid.
* The heat capacity of the
liquidfrom the melting point to the boiling point.
latent heat of vaporizationof the liquid.
* The heat capacity of the gas from the boiling point to room
Changes in entropy are associated with
phase transitionsand chemical reactions. Chemical equationsmake use of the standard molar entropy of reactantsand productsto find the standard entropy of reaction:
[cite book | last = Chang| first = Raymond | coauthors = Brandon Cruickshank | title = Chemistry | publisher = McGraw-Hill Higher Education | year = 2005 | id = ISBN 0-07-251264-4 | chapter = Entropy, Free Energy and Equilibrium | page = 765 ]
: ΔS°rxn = So(products) - So (reactants)
The standard entropy of reaction helps determine whether the reaction will take place spontaneously. According to the
second law of thermodynamics, a spontaneous reaction always results in an increase in total entropy of the system and its surroundings:
: ΔStotal = ΔSsystem + ΔSsurroundings > 0
Gibbs free energy
Third law of thermodynamics
* [http://www.ualberta.ca/~jplambec/che/data/index.htm Compendium of Chemical Data] - Chemical properties of various substances (James A. Plambeck, University of Alberta)
* [http://www.humboldt.edu/~rap1/C110/C110Notes/C110_lec06.htm Free Energy and Chemical Reactions] - Course notes for General Chemistry (R. Paselk, Humboldt State University)
Wikimedia Foundation. 2010.
Look at other dictionaries:
Entropy — This article is about entropy in thermodynamics. For entropy in information theory, see Entropy (information theory). For a comparison of entropy in information theory with entropy in thermodynamics, see Entropy in thermodynamics and information… … Wikipedia
Standard state — In chemistry, the standard state of a material is its state at 1 bar (100 kilopascals exactly). This pressure was changed from 1 atm (101.325 kilopascals) by IUPAC in 1990. [GoldBookRef | file = S05910 | title = standard conditions for gases] The … Wikipedia
Measuring instrument — Captain Nemo and Professor Aronnax contemplating measuring instruments in Twenty Thousand Leagues Under the Sea … Wikipedia
Orders of magnitude (data) — Computer memory types can be ordered into a magnitude hierarchy: memory at the top is fastest to access but holds little data, while each lower type holds more, but is slower to access An order of magnitude is generally a factor of ten. A… … Wikipedia
Cyclohexane (data page) — This page provides supplementary chemical data on cyclohexane. Contents 1 Material Safety Data Sheet 2 Structure and properties 3 Thermodynamic properties 4 Vap … Wikipedia
Dichloromethane (data page) — Please find below supplementary chemical data about dichloromethane. Contents 1 MSDS sheets 2 Structure and properties 3 Thermodynamic properties 4 Vapor pressure of liquid … Wikipedia
Methanol (data page) — This page provides supplementary chemical data on methanol. Contents 1 Material Safety Data Sheet 2 Structure and properties 3 Thermodynamic properties 4 Spectral data … Wikipedia
Diethyl ether (data page) — This page provides supplementary chemical data on diethyl ether. Contents 1 Material Safety Data Sheet 2 Structure and properties 3 Thermodynamic properties 4 V … Wikipedia
Chloroform (data page) — This page provides supplementary chemical data on chloroform. Contents 1 Material Safety Data Sheet 2 Structure and properties 3 Thermodynamic properties 4 Vapor … Wikipedia
Propan-1-ol (data page) — This page provides supplementary chemical data on n propanol. Contents 1 Material Safety Data Sheet 2 Structure and properties 3 Thermodynamic properties 4 … Wikipedia