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Chemistry, The Central Science, 10th edition
Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten
Review Unit 7 (Chp 5,8,19):
Thermodynamics
o
(∆H ,
John D. Bookstaver
St. Charles Community College
St. Peters, MO
 2006, Prentice Hall, Inc.
o
∆S ,
o
∆G ,
K)
Changes in Internal Energy
• Energy is transferred between
the system and surroundings,
as either heat (q) or work (w). E = q + w
E = ?
E = (–) + (+)
Surroundings
System
E = q + w
q in (+)
q out (–)
w on (+)
w by (–)
E = +
Energy (E)
Enthalpy (H)
(kJ)
+
ΔH = ΔE + PΔV
internal work by
energy system
(KE + PE) (–w)
ΔE = q + w
PΔV = –w
Entropy (S)
(J/K)
(disorder)
microstates
–T∆Suniv as: ΔHsys
& ΔSsys at a T
dispersal of
matter &
energy at T
max work done
by favorable rxn
(at constant P)
∆Suniv = +
ΔH = q
ΔS = ΔH
T
(heat)
=
Free Energy (G)
(kJ)
K > 1 means
–∆Gsys & +∆Suniv
ΔG = ΔH – TΔS
Big Idea #5: Thermodynamics
Bonds break and form
to lower free energy (∆G).
Systems are driven by a
decrease in G (–∆G) by:
• a decrease in enthalpy (–∆H), or
• an increase in entropy (+∆S), or
• both.
Chp. 5,8: Calculate ∆H (4 Ways)
1) Bond Energies
(NOT H
rxn = (BEreactants)  (BEproducts)
given)
(+ broken)
(– formed)
2) Hess’s Law
(NOT H
overall = Hrxn1 + Hrxn2 + Hrxn3 …
given)
3) Standard Heats of Formation (Hf )
(given)

H  = nHf(products) – nHf(reactants)
4) Calorimetry (lab)
(given)
q = mc∆T (surroundings or thermometer)
–q = ∆H ∆H/mol = kJ/mol (molar enthalpy)
Entropy (S) (Molecular Scale)
S : dispersal of matter & energy at T
S(s) < S(l) < S(aq) < S(g)
solid
H2O(g)
gas
T
V
(s) + (l)
 (aq)
+∆S
(dispersal)
more
Temperature
microstates  Volume
H2O(g)
 Particle mixing
 Particle number
 Particle size
So = So(products) – So(reactants)
(given)
Thermodynamically Favorable
Chemical and physical processes are driven by:
•
decrease in enthalpy (–∆Hsys) causes (+∆Ssurr)
•
increase in entropy (+∆Ssys)
(+) + S (+)
Suniv = Ssystem
surroundings > 0
•
Thermodynamically Favorable: (defined as)
increasing entropy of the universe (∆Suniv > 0)
∆Suniv > 0
(+Entropy of the Universe)
(∆Suniv) ↔ (∆Gsys)
For all thermodynamically favorable reactions:
Suniverse = Ssystem + Ssurroundings > 0
Suniverse = Ssystem +
(Boltzmann)
Hsystem (Clausius)
T
multiplying each term by T:
–TSuniverse = –TSsystem + Hsystem
rearrange terms:
–TSuniverse = Hsystem – TSsystem
Gsystem = Hsystem – TSsystem
(Gibbs free energy equation)
(∆Suniv) & (∆Gsys)
–TSuniv = Hsys – TSsys
Gsys = Hsys – TSsys
(Gibbs free energy equation)
• Gibbs defined TSuniv as the change in
free energy of a system (Gsys) or G.
• Free Energy (G) is more useful than
Suniv b/c all terms focus on the system.
• If –Gsys , then +Suniverse . Therefore…
–G is thermodynamically favorable.
“Bonds break & form to lower free energy (∆G).”
o
(∆G )
Standard Free Energy
and Temperature (T)
(on
equation
sheet)
(consists G = H – TS
of 2 terms)
free
enthalpy entropy units
energy
term
term
convert
(kJ/mol) (kJ/mol) (J/mol∙K) to kJ!!!
max energy
energy
energy
used for
transferred dispersed
work
as heat
as disorder
The temperature dependence of free energy
comes from the entropy term (–TS).
o
(∆G )
Standard Free Energy
and Temperature (T)
G
Thermodynamic
o
∆G
Favorability
(fav. at high T) (high T) –
(unfav. at low T) (low T) +
(unfav. at ALL T)
+
(fav. at ALL T)
–
(unfav. at high T) (high T) +
(fav. at low T) (low T) –
= H  TS
= (∆Ho) – T(∆So)
( + ) –T( + )
= ( + ) – T( + )
= ( + ) – T( – )
= ( – ) – T( + )
– ) –T( – )
(
= ( – ) – T( – )
Calculating ∆Go (4 ways)
1) Standard free energies of formation, Gf :
G = Gf (products) – G
f (reactants)
(given equation)
2) Gibbs Free Energy equation:
G = H – TS
(given equation)
(may need to calc. ∆Ho & ∆So first)
3) From K value (next few slides) (given equation)
4) From voltage, Eo (next Unit) (given equation)
Free Energy (∆G) & Equilibrium (K)
G = –RT ln K
(on equation sheet)
–1∙K–1
R
=
8.314
J∙mol
If G in kJ,
then R in kJ……… = 0.008314 kJ∙mol–1∙K–1
–∆Go = ln K
RT
Solved
for K :
–∆Go
RT
K = e^
(NOT on
equation sheet)
Free Energy (∆G) & Equilibrium (K)
G = –RT ln K
∆Go = –RT(ln K)
– = –RT ( + )
K
@ Equilibrium
> 1 product favored
(favorable forward)
+ = –RT ( – ) < 1
reactant favored
(unfavorable forward)