Entropy and Gibbs Energy

Ch 15 – Entropy and Gibbs Energy


Section 15.1 – Spontaneous Processes

AG = ( )
What is a spontaneous process? Give an example. A process that occurs under a specific set of conditions

Ex) Ice melting at room temperature

DG (t) What is a nonspontaneous process? Give an example. A process that
= does not occur under a
specific set of
conditions .
Ex) water freezing at room temperature
Does the sign of ΔH alone determine whether a process is spontaneous or nonspontaneous?
a negative SH (exothermic) tends to be
spontaneous but the sign of
, Al alone cannot predict spontaneity
in
every circumstance



Section 15.2 – Entropy

The entropy (S) of a system is a measure of: how
spread out or


dispersed the system's energy is .




The mathematical definition for entropy is S = klnW

• What is k? (Note: This is NOT a rate constant.)Boltzmann constant (1 38x10-23(/K) .




• What is W? # of energetically equivalent ways the molecules in a system can be arranged
• Based on this equation, what happens to S when W increases?
W also increases


Section 15.3 – Entropy Changes in a System

What is standard entropy? (S )
%




the absolute of
entropy a substance at lati


The entropies of substances (elements and compounds) are always ___________.
positive

solid Liquid gas
List the states of matter in order of increasing entropy: ________<________<________

same state same temp
higher thetemp greater the entropy
,
,




List the solids Au, Al,
-
98
C in order of increasing entropy at 25oC:
26 12 01
196 9
. .
.




C Al Al
________<________<________. Explain why you chose this order.
lightest
- heaviest

/for monatomic
species the one w/ ,
the larger molar mass
has a greater standard entropy (
1

, O3(g) and F2(g) are in the same phase has have relatively similar molar masses. Which one has
the greater standard entropy? Explain your answer. Oz will have the greater standard entropy .
For 2 substances w/ similar molar masses in the same phase ,
the substance with the more complex
molar structure has the greater So .

For the reaction aA + bB → cC + dD, write the equation you would use to calculate the standard
entropy change of the reaction (ΔS°), given the standard entropies of the components of the
reaction: products reactants -




15 xn
=
[25 °
(a) +
dSCD)] -

[aS °
(A) + bS
°

(B)]
Work through Worked Example 15.2. Once you understand the steps needed to solve this type
of problem, challenge yourself with the 3 Practice Problems (Attempt, Build, Conceptualize).
This should seem familiar to you, since you did something similar in General Chemistry I with
enthalpy (ΔH) values!

When the temperature of a system increases, the entropy ______________.
increases

When a substance dissolves in a solvent, the entropy often ________________.
increases




Review pages 696-697, then summarize the 6 main factors that influence the entropy of a
system:

• volume change
-


when volume increased ,
more energy levels become available within
which the energy can disperse
a vol =
↑ entropy (direct

•
Temperature change -
at higher temp molecules
, have a greater Kinetic energy making more energy
levess accessible
Temp
↑
=

Pentropy (direct)

• MolecularComplexity the greater a molecules
complexity the it can rotate vibrate
-




more ways +
,


which results
energy levels for system to
disperse
in more a


complexity =

Tentropy (direct)

• Molar
↑
mass-
molar mass =


q entropy (direct



• PhaseChange (s -1)
entropy increases when a molecule melted vaporized Is eg) ,
-


is
,


or sublimed(S-9) bc entropy increases in this order : S
,
1,.
9


• Chemical reaction -
when a chemicalrxn produces more
gas molecules than it consumes,

of (w)
possible arrangements
the # of diff molecules increases & entropy increases
2

(When Phase changes from above occur in chew rxn .
, entropy increases