The standard entropy of a substance (its entropy at 1 atmospheric
pressure) helps determine if a reaction will take place spontaneously.
LEARNING OBJECTIVE [ edit ]
Define standard entropy.
KEY POINTS [ edit ]
Entropies generally increase with molecular weight; for noble gases this is a direct reflection of
the principle that translational quantum states are more closely packed in heavier molecules.
Entropies can also show the additional effects of rotationalquantum levels
(in diatomic molecules) as well as the manner in which the atoms are bound to one another
(solids).
There is a general inverse correlation between the hardness of a solid and its entropy.
The entropy of a solid is less than the entropy of a liquidwhich is much less than the entropy of
a gas.
TERM [ edit ]
standard entropy
Entropy of a substance at 1 atm pressure.
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Defining Standard Entropy
The standard entropy of a substance is its entropy at 1 atm pressure. The values found in the
J
table are normally those for 298K, and are expressed in units of K×mole
. This table shows
some typical values for gaseous substances.
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Standard Entropy
Standard entropies of some gases at 298K, J K–1 mol–1
Scientists conventionally set the energies of formation ofelements in their standard states to
zero. Entropy, however, measures not energy itself, but its dispersal among the
various quantum states available to accept it, and these exist even in pure elements.
Comparing Entropy
It is apparent that entropies generally increase with molecular weight. For the noble gases,
this is a directreflection of the principle that translational quantum states are more closely
packed in heavier molecules, allowing them to be occupied. The entropies of
the diatomic and polyatomicmolecules show the additional effects of rotational quantum
levels.
Entropy in Solids
The entropies of solid elements are strongly influenced by the type of atom packing in the
solid. Although both diamondand graphite are types of carbon, their entropies differ
significantly. Graphite, which is built up of loosely­bound stacks of hexagonal sheets, soaks
up thermal energy twice as well as diamond. The carbon atoms in diamond are tightly locked
in a three­dimensional lattice, preventing them from vibrating around
their equilibrium positions.
There is an inverse correlation between the hardness of a solid and its entropy. For example,
sodium, which can be cut with a knife, has almost twice the entropy of iron, which cannot be
easily cut. These trends are consistent with the principle that the more disordered a
substance, the greater its entropy.
Entropy in Gases and Liquids
Gases, which serve as efficient vehicles for spreading thermal energy over a large volume of
space, have much higher entropies than condensed phases. Similarly, liquids have higher
entropies than solids because molecules in a liquid can interact in many different ways.
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 systemand its surroundings:
ΔS total = ΔS system + ΔS surroundings