In
chemistryChemistry is the science concerned with the composition, behavior, structure, and properties of matter, as well as the changes it undergoes during chemical reactions...
, the
standard molar entropy is the
entropyEntropy is a concept of information maintaining great importance in physics, chemistry, and information theory...
content of one
moleThe mole is a unit of amount of substance: it is an SI base unit, and one of the few units used to measure this physical quantity. The name "mole" was coined in German by Wilhelm Ostwald in 1893, although the related concept of equivalent mass had been in use at least a century earlier...
of substance, under standard conditions (not standard temperature and pressure
STP-Commercial:*Straight Through Processing, banking term where a financial transaction is automatically completed without manual intervention*STP , motor oil company, its initialism meaning "Scientifically Treated Petroleum"...
).
The standard molar entropy is usually given the symbol
S°, and the units
jouleThe joule , named for James Prescott Joule, is the derived unit of energy in the International System of Units. It is the energy exerted by a force of one newton acting to move an object through a distance of one metre...
s per mole
kelvinThe kelvin is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero kelvin...
(J mol
−1 K
−1). Unlike
standard enthalpies of formationThe standard enthalpy of formation "standard heat of formation" of a compound is the change of enthalpy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states...
, the value of
S° is an absolute. That is, an element in its standard state has a nonzero value of
S° at room temperature.
In
chemistryChemistry is the science concerned with the composition, behavior, structure, and properties of matter, as well as the changes it undergoes during chemical reactions...
, the
standard molar entropy is the
entropyEntropy is a concept of information maintaining great importance in physics, chemistry, and information theory...
content of one
moleThe mole is a unit of amount of substance: it is an SI base unit, and one of the few units used to measure this physical quantity. The name "mole" was coined in German by Wilhelm Ostwald in 1893, although the related concept of equivalent mass had been in use at least a century earlier...
of substance, under standard conditions (not standard temperature and pressure
STP-Commercial:*Straight Through Processing, banking term where a financial transaction is automatically completed without manual intervention*STP , motor oil company, its initialism meaning "Scientifically Treated Petroleum"...
).
The standard molar entropy is usually given the symbol
S°, and the units
jouleThe joule , named for James Prescott Joule, is the derived unit of energy in the International System of Units. It is the energy exerted by a force of one newton acting to move an object through a distance of one metre...
s per mole
kelvinThe kelvin is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero kelvin...
(J mol
−1 K
−1). Unlike
standard enthalpies of formationThe standard enthalpy of formation "standard heat of formation" of a compound is the change of enthalpy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states...
, 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 an element can be 0 J mol
−1 K
−1 only at 0 K, according to the
third law of thermodynamicsThe third law of thermodynamics is a statistical law of nature regarding entropy and the impossibility of reaching absolute zero of temperature...
. 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
moleThe mole is a unit of amount of substance: it is an SI base unit, and one of the few units used to measure this physical quantity. The name "mole" was coined in German by Wilhelm Ostwald in 1893, although the related concept of equivalent mass had been in use at least a century earlier...
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
reversibleIn thermodynamics, a reversible process, or reversible cycle if the process is cyclic, is a process that can be "reversed" by means of infinitesimal changes in some property of the system without loss or dissipation of energy. Due to these infinitesimal changes, the system is at rest throughout the...
process.
Chemistry
The standard molar entropy of a
gasThis page is about the physical properties of gas as a state of matter. For the uses of gases, and other meanings, see Gas .A gas is one of four states of matter. Near absolute zero, a substance exists as a solid...
at
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includes contributions from:
- The heat capacity of one mole of the solid from 0 K to the melting point
The melting point of a solid is the temperature range at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. When considered as the temperature of the reverse change from liquid to solid, it is referred to as the freezing point...
(including heat absorbed in any changes between different crystal structureIn mineralogy and crystallography, a crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
s)
- The latent heat of fusion of the solid.
- The heat capacity of the liquid
Liquid is one of the principal states of matter. A liquid is a fluid that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material. The surface is a free surface where the liquid is not constrained by a container....
from the melting point to the boiling pointThe boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
.
- The latent heat of vaporization of the liquid.
- The heat capacity of the gas from the boiling point to room temperature
In physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
.
Changes in entropy are associated with phase transitions and chemical reactions. Chemical equations make use of the standard molar entropy of reactants and
productsA product is a substance that forms as a result of a biological- or chemical reaction. While the end product of some chemical reactions may be the result of a relatively rapid reaction, nanoseconds to seconds, chemical equilibria in complex systems may require years or even centuries to be...
to find the standard entropy of reaction:
- ΔS°rxn = S°products – S°reactants
The standard entropy of reaction helps determine whether the reaction will take place
spontaneouslyA spontaneous process is the time-evolution of a system in which it releases free energy and moves to a lower, more thermodynamically stable, energy state...
. According to the
second law of thermodynamicsThe second law of thermodynamics is an expression of the universal principle of entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium, and that the entropy change dS of a system undergoing any...
, a spontaneous reaction always results in an increase in total entropy of the system and its surroundings:
- ΔStotal = ΔSsystem + ΔSsurroundings > 0
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