physicaltransformationsofpuresubstances-fadil-aldev.ppt

7/26/2019 physicaltransformationsofpuresubstances-fadil-aldev.ppt

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Physical Chemistry


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1.1 Stabilities of Phases

The phase with the lowest chemical potential will be the most stablephase

Phaseis a form of matter that is uniform throughout, not only in

chemical composition but also in physical state Phase Transition is spontaneous conversionof one phase into

another, occurring at a characteristic temperature for a given pressure. For example:

Consier water atp ! 1 atm T! Temperature "!Chemical potential

#ce is stable phase when T $ % because "ice $ "li&ui water when T $ %

'i&ui water is stable phase when T ( % because "ice ( "li&ui water

when T ( % Transition Temperature)Ttrs* is the temperature when chemical

potentials are e&ual+ for example, "ice ! "li&ui water at T ! %


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1. -ates of Phase Transitions

istinguish between Spontaneity of transition and Rate of transition:

Spontaneity of transition Thermoynamics may preict the spontaneity of the physicaltransformation, but not the rate at which it occurs.

Rate of transition /inetics preicts the rate at which a transition occurs, but not the

spontaneity.

Consier the phase transition from iamon to graphite: 0ormal pressure an temperature, "iamon ( "graphite This example shows the chemical potential of graphite lower than the chemical potential of

iamon at normal pressure an temperature. #f we loo for the stability of phase the iamonchange become graphite. 2ut it oes not happen because for changing lie that the C atoms mustexchange positions, which is an increibly slow process for solids )except at hightemperatures*not in normal temperature an pressure.

#n conclusion the gases an the li&uis, these changes can tae place rapily, but in solis,

thermodynamic instability may be inherent in the system

Metastable phasesThermoynamically unstable phases that persist ue to inetic hinrance


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1.3 Phase 2ounaries

Phase Diagram is the iagram that shows regions ofpressure an temperature where phases arethermoynamically stable.

Pressureis pressure of a gas in e&uilibrium with theli&ui phase.

Sublimation Vapor Pressure is pressure of gas ine&uilibrium with soli phase

Phase Boundaries separate regions an showp anTwheretwo phases exist ine&uilibrium

Triple Point is the point that shows the temperaturean the pressure when the substance in all phase)soli, li&ui an gas*

Critical point is the point where the pressure an the

temperature mae the phase of the substance cannotifferentiate between li&ui phase or the vapor phase.


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1.4 Critical Points an 2oiling Points

Free apori!ation or boiling occurs throughout theli&ui, an vapor can expan freely into the surrounings. #n an open vessel the temperature where the vapor pressureis e&ual to the external pressure

The normal boiling point, Tb, is the temperature of freevapori5ation at a pressure of 1.% atm

The standard boiling point is the temperature of freevapori5ation at a pressure of 1.% bar )%.678 atm* )e.g.,water: 66.9oC for Tb ! 1%%oC*

#n a closed essel, boiling oes not occur rather thevapor pressure an ensity of the vapor increase with

1. increasing temperature e&uilibrium. increasing ensity

3. Two ensities e&ual, surface btw. phases isappears Critical temperature )Tc* an critical pressure)pc*:

where interphase surface isappears Supercritical fluid: fills container an interface no

longer exists )figure c*


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1.; ater, T3 = 273.16 K and 611 Pa (6.11 mbar, 4.58 Torr)


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C?, Fire =xtinguishers @ Coffee

Positive slope of soliAli&ui bounary ischaracteristic of most substances )m.p. increasesas pressure is increase*

Triple point lies above 1 atm, implying that li&uiC? cannot exist at normal pressures )solisublimes at normal pressures, hence the name,Bdry ice"#

To obtain li&ui C?,p = 5.11 atm is theminimum. For a cyliner of C?)l* at ;oC, thepressure of the gas must be greater than 98 atm Agas comes out of a ouleAThomson throttle,

emerging intop !1 atm, conenses into a snow$li%e solid

Supercritical C? )highly compresse* is use insupercritical flui chromatography whichseparates lipis an phospholipis, fuel oils intocomponents an to decaffeinate coffee


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Phase iagram of Delium )4De*

Et low temperatures, helium behaves unusually A soli an gasare never in e&uilibrium: De atoms are so light that they

vibrate with high amplitue motions which Bshae the soliapart

Soli helium can only be obtaine at low temperature an veryhigh pressure hcp an bcc enote he&agonal closepac%ing anbody$centred cubic pac%ing

The 7$line mar%s a special phase transitionwhere heatcapacity becomes infinite, maring the fluiAsuperfluidtransition of li&ui Delium

The DeA## li&ui phase is a superfluid' as it flows withoutany viscosity

4De an 3De have ifferent phase iagrams+ in fact, theentropy of 3De )l* is less than that of 3De)s*, so melting is

e&othermic


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()* Close Pac%ing in Solids

#n a soli, atoms pac together in ifferent arrangementsto occupy as much space as possible A close pac%ing

=ach atom E is surroune by six other E atoms in onelayer, atoms are then pile on top in the 2 an C Bholes

Etoms that pac in the arrangment E2C.E2C.E2C... Ere

sai to have cubic close pac%ing +ccp# which is alsocalle face$centred cubic +fcc# pac%ing +e)g)' gold#

Pacing in the arrangement E2.E2 or EC.EC results inhe&agonal close pac%ing +hcp# $ there is oftencompetition between ccp an hcp phases, epenent uponlong range forces of the atoms )e.g., low temp 0a*

Thebody$centred cubic +bcc# pac%ing arrangementis slightly less close pace than fcc an hcp, with oneatom at the centre, an other atoms at the corners A it isoften the high temperature form of metals that are closeApace at lower temperatures )e.g., iron either ccp or bcc,heating*


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(), Phase Stability and Phase Transitions

The thermodynamic criterion of e-uilibrium isAt e!i"ibri!m, the#hemi#a" potentia" o$ a s!bstan#e is the same thro!%ho!t a samp"e,re%ard"ess o$ ho& man' phases are present

For example, when soli an li&ui phases are in e&uilibrium, neither has ahigher chemical potential )same throughout the soli an the li&ui*

#f amo!nt dn is trans$erred $rom 1 to 2, ibbs ener%' changes by A"1dn in

"o#ation 1, and b' *2dn in "o#ation 2d = (*2 + *1)dn

#f "1 ( ", * is ne%atie, pro#ess is spontaneo!s #f "1 ! ", no change in *, s'stem at e!i"ibri!m #f *-s's )-s!r = , s'stem at e!i"bri!m


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(). Temperature Dependence of Phase Stability

Temperature epenence of Gibbs energy is expresse interms of entropy, ) / T)p = +-. -in#e the #hemi#a" potentia"o$ a p!re s!bstan#e is the same as the molar energy

Es T raised, * "o&ered, -m 0 , so, s"ope o$ * s. T is ne%atie(see the next slie*

Plot of " vs T is steeper $or %ases than "i!ids, sin#e the -m(%)0 -m(").

For solis an li&uis, usually-m(s) -m(")Solids and /i-uids0Steep negative slope of ")l* falls below

")s* when the temperature is high enough )li&ui becomesstable phase an soli melts*

/i-uids and 1ases0")g* plunges ownwars as

temperature is raise )-m(%) hi%h), so the gas is the stablephase an the li&ui vapouri5es

ST

G

p

=


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()2 Melting Response to 3pplied Pressure


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()(4Pressure and Chemical Potential

5&ample0 Calculate the effect of chemicalpotentials of ice and water of increasing thepressure from 1.%% bar to .%% bar at %oC. Theensity of ice is ! %.618 g cmA3, an of water is !%.666 g cmA3

" ! m p $or #han%e in #hemi#a" potentia" o$ anin#ompressib"e

substance when pressure is change by *p th!s,

M p

=


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")ice* rises more sharply than ")water*, so if they are at

e&uilbrium at 1.%% bar, there is a tenency for ice to meltat .%% bar )This is the reason applying pressure to ice atconstant pressure results inmelting to li&ui phaseJ*

-2 -1 5-1

-3

(1.802 x 10 kg mol ) x (1.00 x 10 Pa)( ) +1.97 J mol

917 kg mice = =

-2 -1 5-1

-3

(1.802 x 10 kg mol ) x (1.00 x 10 Pa)( ) +1.80 J mol

999 kg mwater = =


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()((3pplied Pressure and Vapour Pressure

>hen pressure is applie to a conense phase, the vapour pressurerises )molecules are s&uee5e out an escape as a gas* Pressure canbe exerte on the conense phase: )a* mechanically or )b* subHectingit to an inert pressuri5ing gas

#n case )b*, vapour pressure is the partial pressure of the vapour ine&uilibrium with the conense phase A this is the partial apourpressure of the substance +it is possible that some of the inertgas can issolve in the conense phase if it is a li&ui, or perhapssolate some of the li-uid molecules but wewill not treat thesecomplications*

The relationship between vapour pressure,p, o$ a "i!id, and aapo!r pressure,p, a$ter an app"ied press!re #han%e o$ )P, is

Kapour pressure increases when pressure acting on conense phaseincreases

Vm P/RTp p e =


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6ustification

Et e&uilibrium, ")l* ! ")g*+ so, any change thatpreserves e&uilibrium, the resulting change in ")l* must

be e&ual to that in ")g* )d*(") = d*(%))>hen pressurePon "i!id is in#reased b' dP, d*(") = m(") dP, and

d*(%) = m(%)dp (dp = #han%e in apo!r press!re). $apo!r is a per$e#t gas, m(%) = T/p, and

#f changes in chemical potentials of vapour an li&uiare e&uate:

RT dpd (g)=p


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The pressureP eperien#ed b' the "i!id is e!a"to the norma" apo!r pressure,p so &hen P =

p, p = p as &e"". 9hen there is additiona"vapourpressure *P on the "i!id, P = p )P, the apo!r

press!re is p )which we are trying to fin* A replacep in p )P b' p itse"$

RT dpVm(l) dP

p=

( )

p p p

m

p p

dpRT v l dp

p

+

= = l! ( ) mp

RT v l pp

=


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5stimating 5ffect of Pressure


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/ocation of Phase Boundaries


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Slopes of Phase Boundaries


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Solid$/i-uid Boundary


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Solid$/i-uid Boundary' 7


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/i-uid$Vapour Boundary


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/i-uid$Vapour Boundary' 7


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Solid$Vapour Boundary' 7


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Than you........

physicaltransformationsofpuresubstances-fadil-aldev.ppt

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