Geoid: Difference between revisions
en>Dexbot m r2.7.1) (Robot: Adding fa:زمینواره |
en>Mhdarin mNo edit summary |
||
| Line 1: | Line 1: | ||
In [[chemistry]], the '''standard molar entropy''' is the [[entropy]] content of one [[mole (unit)|mole]] of substance, under standard conditions (not standard temperature and pressure [[Standard conditions for temperature and pressure|STP]]). | |||
The standard molar entropy is usually given the symbol ''S''°, and as units of [[joule]]s per mole [[kelvin]] (J mol<sup>−1</sup> K<sup>−1</sup>). Unlike [[standard enthalpy change of formation|standard enthalpies of formation]], the value of ''S''° is an absolute. That is, an element in its standard state has a nonzero value of ''S''° at room temperature. The entropy of a pure [[crystalline]] structure can be 0 J mol<sup>−1</sup> K<sup>−1</sup> 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. This residual entropy is often quite negligible. | |||
==Thermodynamics== | |||
If a [[mole (unit)|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: | |||
:<math>S^\circ = \sum_{k=1}^N \Delta S_k =\sum_{k=1}^N \int \frac{dq_k}{T} \, dT</math> | |||
Here, ''dq<sub>k</sub>''/''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 (thermodynamics)|reversible]] process. | |||
==Chemistry== | |||
The standard molar entropy of a [[gas]] at [[Standard conditions for temperature and pressure|STP]] includes contributions from:<ref>{{cite book | last = Kosanke| first = K. | coauthors = | title = Pyrotechnic chemistry | publisher = Journal of Pyrotechnics | year = 2004| isbn = 1-889526-15-0 | chapter = Chemical Thermodynamics | page = 29 }}</ref> | |||
* The [[heat capacity]] of one mole of the [[solid]] from 0 K to the [[melting point]] (including heat absorbed in any changes between different [[crystal structure]]s) | |||
* The [[latent heat of fusion]] of the solid. | |||
* The heat capacity of the [[liquid]] from the melting point to the [[boiling point]]. | |||
* The [[latent heat of vaporization]] of the liquid. | |||
* The heat capacity of the [[gas]] from the boiling point to room [[temperature]]. | |||
Changes in entropy are associated with [[phase transitions]] and [[chemical reactions]]. [[Chemical equations]] make use of the standard molar entropy of [[reactants]] and [[Product (chemistry)|products]] to find the standard entropy of reaction:<ref>{{cite book | last = Chang| first = Raymond | coauthors = Brandon Cruickshank | title = Chemistry | publisher = McGraw-Hill Higher Education | year = 2005 | isbn = 0-07-251264-4 | chapter = Entropy, Free Energy and Equilibrium | page = 765 }}</ref> | |||
: Δ''S''°<sub>rxn</sub> = ''S''°<sub>products</sub> – ''S''°<sub>reactants</sub> | |||
The standard entropy of reaction helps determine whether the reaction will take place [[spontaneous process|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: | |||
: Δ''S''<sub>total</sub> = Δ''S''<sub>system</sub> + Δ''S''<sub>surroundings</sub> > 0 | |||
==See also== | |||
*[[Entropy]] | |||
*[[Heat]] | |||
*[[Gibbs free energy]] | |||
*[[Helmholtz free energy]] | |||
*[[Third law of thermodynamics]] | |||
==References== | |||
{{reflist}} | |||
==External links== | |||
*[http://users.humboldt.edu/rpaselk/C110/C110Notes/C110_lec06.htm Free Energy and Chemical Reactions] - Course notes for General Chemistry (R. Paselk, Humboldt State University) | |||
[[Category:Chemical properties]] | |||
[[Category:Thermodynamic entropy]] | |||
Revision as of 15:24, 1 November 2013
In chemistry, the standard molar entropy is the entropy content of one mole of substance, under standard conditions (not standard temperature and pressure STP).
The standard molar entropy is usually given the symbol S°, and as units of joules per mole kelvin (J mol−1 K−1). Unlike standard enthalpies of formation, the value of S° is an absolute. That is, an element in its standard state has a nonzero value of S° at room temperature. The entropy of a pure crystalline structure 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. This residual entropy is often quite negligible.
Thermodynamics
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.
Chemistry
The standard molar entropy of a gas at STP includes contributions from:[1]
- The heat capacity of one mole of the solid from 0 K to the melting point (including heat absorbed in any changes between different crystal structures)
- The latent heat of fusion of the solid.
- The heat capacity of the liquid from the melting point to the boiling point.
- The latent heat of vaporization of the liquid.
- The heat capacity of the gas from the boiling point to room temperature.
Changes in entropy are associated with phase transitions and chemical reactions. Chemical equations make use of the standard molar entropy of reactants and products to find the standard entropy of reaction:[2]
- ΔS°rxn = S°products – S°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
See also
References
43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.
External links
- Free Energy and Chemical Reactions - Course notes for General Chemistry (R. Paselk, Humboldt State University)
- ↑ 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
My blog: http://www.primaboinca.com/view_profile.php?userid=5889534 - ↑ 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
My blog: http://www.primaboinca.com/view_profile.php?userid=5889534