Post on 26-Dec-2015
3-minute exercise
Is each of the following a spontaneous change?• Water evaporates from a puddle• A small amount of sugar dissolves in hot
tea• Methane burns in air• A hamburger becomes uncooked
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Thermodynamics
First Law: Law of Conservation of Energy
3
Second Law: Systems change towards more disorder
Internal E of a system
wqE
heat
work
surrsys EE
Limitation:
Explains change, but not direction
Spontaneous Change
• Change that occurs without continuous E input
• Change can only be spontaneous in one direction, under a given set of conditions
4
Enthalpy (∆H):• heat gained or lost at constant P• Sign of ∆H
• Exothermic or endothermic• No information about spontaneity
Entropy (∆S):• Freedom of particle motion (dispersed E of
motion)
Energy Levels• Each atom or molecule has quantized E
levels– Electronic– Kinetic
• Vibrational, rotational, translational
• Microstate: combined E at any given point– each microstate is equally possible (equal E)
for a given set of conditions
5
WkS lnentrop
y
Number of microstates
Boltzmann constant= R/NA = 1.38 x 10-23
J/K
(J/K)
Entropy Change: Heat Changes
7
T
qS rev
sys
• Remove 1 grain of sand• Gas does work on piston
• absorbs heat to maintain E• Works for tiny changes (totally reversible)
Always Increasing EntropyAll real processes occur spontaneously in
direction that increases the entropy of the universe.
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surrsysuniv SSS 0
Perfect crystal at T = 0 K has S = 0• 1 microstate 1lnkS 0
Standard Molar Entropy (S°) • S increase from 0 to standard state
• 1 atm (gases)• 1 M (solutions)• Pure substance, most stable form
(liquids/solids)
What affects S°?
1. Temperature change↑ T ↑ S
2. Phase changeabsorb heat ↑ S
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As # of microstates (or kinetic E) increases, S increases
ofus
ovap SS
gl ls
10
4. Atomic Size• heavier atoms• allotropes
5. Molecular Complexitymore complex
closer E levels
more microstateso
graphiteodiamond SS
more types of movement more
microstatesOnly applies to molecules in same physical state
3. DissolutionIons: increased S, except small, highly charged ionsMolecules (solid or liquid): ∆S ≈ 0 Gases: decreased S
3-minute practice
What is the sign of ∆Ssys?
• A pond freezes in winter
• Atmospheric CO2 dissolves in the ocean
• 2 K (s) + F2 (g) 2KF (s)
11
Standard Entropy of Reaction Increasing disorder:
orxnS
0 orxnS
Predict ∆Srxn:
orxt
oprod
orxn nSmSS
Decreasing disorder:
0 orxnS
N2 (g) + 3H2 (g) 2 NH3 (g)
orxnS )2(
3
oNHS )31(
22
oH
oN SS
Change in # moles of gas
Calculate ∆Srxn:
∆Suniverse
Decrease in ∆Ssys only if greater increase ∆Ssurr
System acts as heat sink or drain1.Exothermic
2.Endothermic
0sysq 0surrq 0 surrS
0sysq 0surrq 0 surrS
T
qS sys
surr T
HS sys
surr
at constant P
Measure ∆Hsys to determine ∆Ssurr
Spontaneous at 298 K?N2 (g) + H2 (g) NH3 (g)3 21. Balance equation!
2. Calculate ∆Ssys
3. Calculate ∆Hsys
4. Calculate ∆Ssurr
5. Calculate ∆Suniv
osysS )31(
22
oH
oN SS )2(
3 oNHS
1932 )6.13035.191( KJ197
osysH 0 o
fNHH
32 0 kJ8.91
T
HS
osys
surr
K
kJ
298
8.91 KJ308
surrosysuniv SSS KJ111
Entropy at Equilibrium
Approaching equilibrium: At equilibrium:
No net change 0 univS
? sysS surrsys SS
0 univS
revfwd SS
5-minute Practice
Calculate ∆Ssys for the combustion reaction of ammonia (producing nitrogen dioxide and water vapor).
Gibbs Free Energy• Measure of spontaneity• Combines enthalpy and entropy
surrosysuniv SSS
T
HSS syso
sysuniv
sysosysuniv HSTST
osyssyssys STHG
Spontaneous if…∆Suniv > 0
univsys STG
T∆Suniv > 0
-T∆Suniv < 0 ∆Gsys < 0
∆G0G Spontaneous process
0G Nonspontaneous process
0G Process at equilibrium
Standard Free Energy Change osys
osys
osys STHG
Standard Free Energy of Formation :
of
of
orxn rxtprod
nGmGG
ofG
E change when 1 mol of compound is made from its elements in standard states
Element in standard state: 0 ofG
• Nonspontaneous process:• Process may occur if work is done to the system• How much work is needed?
∆G and work (constant T & P)
• Spontaneous process:• ∆G = maximum useful work done by the
system
w = ∆G
• wmax only if process is totally reversible
• Actually does less w < wmax
• Extra E lost as heat
Useful Work• Excludes work done by or on
atmosphere• Some free energy is always lost to heat
∆Hsys?
)(O2
25)(HC 2188 gl )( OH9)(CO 22 gg
∆Ssys?
∆Gsys?
< 0
> 0
< 0
∆Gsys = wmax that can be done by system
∆Gsys > w actually done by system
In some multistep reactions, ∆G from one reaction can cause an otherwise nonspontaneous reaction to occur. “coupling of reactions”
What about T?
• Typically ∆H > T∆S• For ∆G to be negative, need ∆H to be…
• What about at high T?
osys
osys
osys STHG
negative
• T∆S term can dominate• Sign of ∆S becomes important
1. ∆H < 0 & ∆S > 0
2. ∆H > 0 & ∆S < 0
3. ∆H > 0 & ∆S > 0
4. ∆H < 0 & ∆S < 0
4 situations:
∆G > 0 at low T; ∆G < 0 at high T
∆G < 0 at low T; ∆G > 0 at high T
∆G < 0
∆G > 0
Equilibria and ∆G
• If Q < K, reaction…
• If Q > K, reaction…
• If Q = K, at equilibrium
→
←
∆G < 0
∆G > 0
∆G = 0
Q/K
Q/K
Q/K
< 1
> 1
= 1
0ln KQ
0ln KQ
0ln KQ
ln ln
K
QRTG ln
proportional
KRTQRTG lnln
KRTGo ln
Make Q standard state (all values = 1)
0
oGQRTG ln