Download Physical)transforma=on)) of)pure)substances)

2015-1, E00267 Physical Chemistry 1
Department of Chemistry, University of Ulsan
Chapter)4.))
Physical)transforma=on))
of)pure)substances)
Physical)Chemistry)1)
Department)of)Chemistry,))
University)of)Ulsan)
)
Prof.)Jaehoon)Jung)
Outline)
2/15)
E00267)Physical)Chemistry)1:)Ch.)4.)Physical)transforma=on)of)pure)substances)
•  Phase)diagram))
!"Stability"of"phase""
!"Phase"boundaries""
!"Three"representa6ve"phase"diagrams""
"
•  Thermodynamic)aspects)of)phase)transi=ons)"
!"The"dependence"of"stability"on"the"condi6ons""
!"The"loca6on"of"phase"boundaries""
!"The"Ehrenfest"classifica6on"of"phase"transi6ons"
Phase)
3/15)
E00267)Physical)Chemistry)1:)Ch.)4.)Physical)transforma=on)of)pure)substances)
•  Phase))
is"a"form"of"ma=er"that"is"uniform"throughout"in"chemical"composi6on"and"
physical"state."(e.g.,"gas,"liquid,"and"various"solid"phases)""
•  Number)of)phases,)P))
•  Dispersion))
!"Miscibility"of"substances"in"mixture"
"""!"fully"miscible:"P = 1 "
"""!"immiscible:"P ≠ 1
• 
• 
• 
• 
• 
• 
• 
"
• 
• 
Gaseous"mixture:"P = 1
Fully"miscible"liquid"mixture:"P = 1"
Crystal"of"one"substance:"P = 1 "
Solu6on"of"sodium"chloride"in"water:"P = 1 "
Ice:"P = 1
Slurry"of"Ice"and"water:"P = 2
Thermal"decomposi6on"of"CaCO3:"P = 3
CaCO3 (s) → CaO(s) + CO 2 (g)
Immiscible"two"metal"mixture:"P = 2
Miscible"two"metal"mixture"(alloy):"P = 1
!"a"homogeneous"mixture
Phase)transi=on)
!"uniform"in"macroscopic"scale""
!"not"uniform"in"microscopic"scale"
"
"
!"The"ability"to""
control"the"micro!"
structure"in"dispersion""
resul6ng"from"phase""
equilibria"makes"it""
possible"to""tailor""
the"mechanical"proper6es"of"the"materials"
to"a"par6cular"applica6ons.""
4/15)
E00267)Physical)Chemistry)1:)Ch.)4.)Physical)transforma=on)of)pure)substances)
•  Phase)transi=on)))
is"the"spontaneous"conversion"of"one"phase"into"another"phase."
!"Phase"transi6on"occurs"at"a"characteris6c"temperature,"i.e.,"transi6on"temperature"
(Ttrs),"for"a"given"pressure.""
G = H − TS
dG = dH − TdS
dGT , p ≤ 0
at"1"atm,""
!"T < 0 °C,"water"!"ice"(G < 0)""
!"T > 0 °C,"ice"!"water"(G < 0)""
•  Transi=on)temperature,)Ttrs
is"the"temperature"at"which"the"two"phases"are"in"equilibrium"and"the"Gibbs"energy"of"
the"system"is"minimized"at"the"prevailing"pressure.""
Phase)transi=on)
5/15)
E00267)Physical)Chemistry)1:)Ch.)4.)Physical)transforma=on)of)pure)substances)
•  Phase)transi=on)))
is"the"spontaneous"conversion"of"one"phase"into"another"phase."
!"Phase"transi6on"occurs"at"a"characteris6c"temperature,"i.e.,"transi6on"temperature"
(Ttrs),"for"a"given"pressure.""
•  Observa=on)of)phase)transi=on))
!"Thermal"analysis""
"""!"measuring"the"heat"evolved"or""
""""""""absorbed"during"transi6on"
"""!"useful"for"solid!solid"transi6ons"
!"X!ray"diffrac6on""
"""!"change"in"diffrac6on"pa=ern"
Chem.&Commun."48,"10237"(2012)""
Phase)transi=on)
6/15)
E00267)Physical)Chemistry)1:)Ch.)4.)Physical)transforma=on)of)pure)substances)
•  Metastable)phase))
is"thermodynamically"unstable"phase,"but"persists"because"the"transi6on"is"kine6cally"
hindered."
Diamond""""""""""""""""""""""Graphite"
!"metastable"phase"
!"more"stable"at"SATP"
from&wikepedia.org"
Phase)transi=on)
7/15)
E00267)Physical)Chemistry)1:)Ch.)4.)Physical)transforma=on)of)pure)substances)
•  Thermodynamic)criteria)of)phase)stability))
At"equilibrium,"the"chemical"poten6al"of"substance"is"the"same"throughout"a"sample,"
regardless"of"how"many"phases"are"present.""
•  Chemical)poten=al,)µ))
!"molar"Gibbs"energy"for"a"one!component"system"
µ = Gm
Transfer"of"substance""
from"loca6on"1"to"2"toward"equilibrium,""
1)
2)
−µ1dn
+µ 2 dn
dG = (µ 2 − µ1 ) dn
at equilibrium, µ 2 = µ1 → dG = 0
Phase)diagram)
8/15)
E00267)Physical)Chemistry)1:)Ch.)4.)Physical)transforma=on)of)pure)substances)
•  Phase)diagram)of)pure)substance))
shows"the"regions"of"p"and"T"at"which"its"various"phases"are"thermodynamically"stable."
•  Phase)boundaries)(or)coexistence)curve))
show"the"values"of"p"and"T"at"which"two"phases"
coexist"in"equilibrium"and"their"chemical"poten6al"
are"equal."
•  Triple)point)
indicates"a"set"of"condi6ons,"characteris6c"p"and"
T"of"substance,"under"which"three"different"
phases"of"a"substance"all"simultaneously"coexist"
in"equilibrium.""
Phase)diagram)
9/15)
E00267)Physical)Chemistry)1:)Ch.)4.)Physical)transforma=on)of)pure)substances)
•  Characteris=c)proper=es)related)to)phase)transi=ons"
• 
Vapor)pressure))
is"the"pressure"of"a"vapor"in"
equilibrium"with"the"liquid."
• 
Sublima=on)vapor)pressure))
is"the"pressure"of"a"vapor"in"
equilibrium"with"the"solid."
• 
Boiling)point,)Tb)&)Freezing)(=)Mel=ng))point,)Tf))
!"normal"condi6on:"at"external"p"of"1"atm"
!"standard"condi6on:"at"external"p"of"1"bar"
"
!"Boiling:"equilibrium"between"gas"and"liquid"
!"Freezing)(=)Mel=ng):"equilibrium"between"liquid"and"solid"""
"
Phase)diagram)
10/15)
E00267)Physical)Chemistry)1:)Ch.)4.)Physical)transforma=on)of)pure)substances)
•  Characteris=c)proper=es)related)to)phase)transi=ons"
• 
Cri=cal)point:))
(a)"A"liquid"in"equilibrium"with"its"vapor."!""
(b)"When"a"liquid"is"heated"in"a"rigid,"sealed"container,"
the"density"of"vapor"increases"and"that"of"liquid"
decreases"slightly."!"""
(c)"Two"densi6es"are"equal"and"the"interface"between"
the"fluids"disappears"(!"supercri6cal"fluid)"
Supercri=cal)fluid)of)CO2)
from&h3p://chem.leeds.ac.uk&
!  Cri6cal"temperature:"Tc"
!  Cri6cal"pressure: pc"
Phase)rule)
11/15)
E00267)Physical)Chemistry)1:)Ch.)4.)Physical)transforma=on)of)pure)substances)
•  Phase)rule)
is"a"general"rela6on"between"the"variance,"F,"the"number"of"components,"C,"and"
the"number"of"phases"at"equilibrium,"P,"for"a"system"of"any"composi6ons.""
"
F =C−P+2
"
"
"!  Component"is"a"chemically&independent&
" cons6tuent"of"a"system."
!  Variance"(or"number"of"degrees"of"freedom)"is"
the"number"of"intensive"variables"that"can"be"
changed"independently"without"disturbing"the"
number"of"phases"in"equilibrium.""""
(C = 1, P = 1) → F = 2
(C = 1, P = 2) → F = 1
(C = 1, P = 3) → F = 0
Phase)rule)
12/15)
E00267)Physical)Chemistry)1:)Ch.)4.)Physical)transforma=on)of)pure)substances)
•  Phase)rule)
is"a"general"rela6on"between"the"variance,"F,"the"number"of"components,"C,"and"
the"number"of"phases"at"equilibrium,"P,"for"a"system"of"any"composi6ons.""
"
F =C−P+2
"
"
Typical)phase)diagram))
C = 1 → F = C − P + 2 = 3− P
of)oneZcomponent))
"
"
•  When"only"one"phase"resents,"" P = 1 → F = 2
!  p"and"T"can"be"varied"independently."
!  A"single"phase"is"represented"by"an"area"in"the"phase&diagram."
•  When"two"phases"are"in"equilibrium,""P = 2 → F = 1
!  At"given"T,"p"is"not"freely"variable.""
!  The"equilibrium"of"two"phases"is"represented"by"a"line."
•  When"three"phases"are"in"equilibrium,""P = 3 → F = 0
!  System"is"invariant,"i.e.,"p"and"T"are"characteris6c"of"substance."
!  The"equilibrium"of"three"phases"is"represented"by"a"point."
•  Four"phases"cannot"be"in"equilibrium"in"one!component"system"
because"F"cannot"be"nega6ve.""
Representa=ve)phase)diagrams)
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E00267)Physical)Chemistry)1:)Ch.)4.)Physical)transforma=on)of)pure)substances)
•  Carbon)dioxide,)CO2)
"
"
"
!  Posi6ve"slope"of"the"solid!liquid"boundary"
!"Tf"(or"Tm)"of"solid"CO2"rises"as"p"increases.""
!  Triple"point"lies"above"1"atm.""
!"The"liquid"cannot"exist"at"normal"atmospheric"p."
!"The"solid"at"normal"atmospheric"p:"dry"ice"
!  At"25"°C,"if"both"and"gas"are"present"in"
equilibrium,"a"vapor"pressure"is"67"atm.""
!"CO2"gas"cannot"be"liquefied"unless"one"applies"
high"pressure,"which"implies"the"weakness"of"the"
intermolecular"forces"between"the"nonpolar"carbon"
dioxide"molecules.""
Representa=ve)phase)diagrams)
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E00267)Physical)Chemistry)1:)Ch.)4.)Physical)transforma=on)of)pure)substances)
•  Water,)H2O)
"
"
"
!  Steep"nega6ve"slope"of"the"solid!liquid"boundary"
!"Tf"(or"Tm)"of"solid"H2O,"i.e.,"ice,"falls"as"p"increases."
!"It"is"related"to"the"decrease"in"volume"at"mel6ng.""
"""""="it"is"favorable"for"the"solid"to"transform"into"the""
""""""""liquid"as"p"is"raised.""
ice!I"
!  Hydrogen)bonding""
!"high"Tb"for"a"molecule"of"its"molar"mass"
!"high"Tc"and"pc."
!  Many"different"solid"phases"other"than"ordinary"
ice"(ice!I):"different"arrangement"of"H2O"molecules"
Representa=ve)phase)diagrams)
15/15)
E00267)Physical)Chemistry)1:)Ch.)4.)Physical)transforma=on)of)pure)substances)
•  HeliumZ4,)4He)
"
"
"
!  Solid"and"gas"phases"of"helium"are"never"in"
equilibrium.""
!  Two"different"liquid"phases"
!"He!I:"normal"liquid"
!"He!II:"superfluid""
""""""""""""""""(no"viscosity)"
Lambda)point)transi=on:""
As"the"liquid"is"cooled"down"from"
4.2"K"to"2.2"K,"the"boiling"suddenly"
becomes"violent"for"a"moment.""
from&wikipedia.org&
!  Due"to"a"quantum"mechanical"reason,"i.e.,"
different"nuclear"spins"of"the"isotopes,"helium!3,"
3He,"shows"the"different"phase"diagram.""