Burdge/Overby, Chemistry: Atoms First, 2e Ch14 | Page 27

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CHAPTE R 14? Entropy and Free Energy

Chapter Summary
Section 14.1

• A spontaneous process is one that occurs under a specified set
of conditions.
• A nonspontaneous process is one that does not occur under a
specified set of conditions.
• Spontaneous processes do not necessarily happen quickly.

Section 14.2

• Entropy is a thermodynamic state function that measures how dispersed or spread out a system’s energy is.

Section 14.3
• Entropy change for a process can be calculated using standard
entropy values or can be predicted qualitatively based on factors
such as temperature, phase, and number of molecules.
• Whether or not a process is spontaneous depends on the change
in enthalpy and the change in entropy of the system.
• Tabulated standard entropy values are absolute values.

Section 14.4

• According to the second law of thermodynamics, the entropy
change for the universe is positive for a spontaneous process and
zero for an equilibrium process.
• According to the third law of thermodynamics, the entropy of a
perfectly crystalline substance at 0 K is zero.

Section 14.5

• The Gibbs free energy (G) or simply the free energy of a system
is the energy available to do work.
• The standard free energy of reaction (?G° ) for a reaction tells
rxn
us whether the equilibrium lies to the right (negative ?G° ) or to
rxn
the left (positive ?G° ).
rxn
• Standard free energies of formation (?G°) can be used to calf
culate standard free energies of reaction.

Section 14.6
• In living systems, thermodynamically favorable reactions provide the free energy needed to drive necessary but thermodynamically unfavorable reactions.

Key Words
Entropy (S),?572
Equilibrium process,? 584
Free energy,? 588
Gibbs free energy (G),?588

Nonspontaneous process,? 571
Second law of
thermodynamics,?583
Spontaneous process,? 571

Standard entropy,? 575
Standard free energy of
formation (?G°),?590
f

Standard free energy of
reaction (?G° ),?590
rxn
Third law of
thermodynamics,?586

Key Equations
14.1? S 5 k ln W

The entropy S of a system is equal to the product of the Boltzmann constant
(k) and ln of W, the number of possible arrangements of molecules in the
system.

14.2? W 5 XN

The number of possible arrangements W is equal to the number of possible
locations of molecules X raised to the number of molecules in the system N.

14.3? DSsys 5 Sfinal 2 Sinitial

The entropy change in a system DSsys, is equal to final entropy, Sfinal, minus
initial entropy, Sinitial.

V
14.4? DSsys 5 nR ln _____?
?? final???
Vinitial

For a gaseous process involving a volume change, entropy change is
calculated as the product of the number of moles (n), the gas constant (R),
and ln of the ratio of final volume to initial volume [ln(Vfinal/Vinitial)].

14.5? DS° 5 [cS°(C) 1 dS°(D)] 2 [aS°(A) 1 bS°(B)]
rxn

Standard entropy change for a reaction (DS° ) can be calculated using
rxn
tabulated values of absolute entropies (S°) for products and reactants.

14.6? DS° 5 SnS°(products) 2 SmS°(reactants)
rxn

DS° is calculated as the sum of absolute entropies for products minus the
rxn
sum of absolute entropies for reactants. Each species in a chemical equation
must be multiplied by its coefficient.

2DHsys
14.7? DSsurr 5 _______?
??
? ?
?
T

Entropy change in the surroundings (DSsurr) is calculated as the ratio of minus
the enthalpy change in the system (?DHsys) to absolute temperature (T).

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