6.2. Measurement of freezing point depression

Transcription

1 6.2. easureent f freezng pnt depressn I. asks: Deternatn f nentratn f urea slutn y easurng freezng pnt depressn. Investgatn f freezng pnt depressn n ase f ttal and partal dssatn. Iprtant nepts Phase equlru, therdyna ndtns f phase equlru, phase equlru n ultpnent systes, deal slutns, ault`s law, heal ptental, heal ptental as the funtn f pstn, llgatve prpertes, freezng pnt depressn, lng pnt elevatn, st pressure, rysp nstant. II. Freezng pnt depressn - thery Dsslvng (nn-vlatle) sld ateral n pure slvent the freezng pnt f the slutn s always lwer than the freezng pnt f the slvent, f the sld phase s fred fr the pure slvent nly (the slute and the slvent d nt x n sld phase). he dfferene f the freezng pnts s alled freezng pnt depressn. he ause f the phenenn s that the dsslved ateral dereases the heal ptental f the slvent. he freezng pnt depressn f dlute slutns prprtnal t the nentratn f the slute. () where s the lal freezng pnt depressn (rysp nstant) f the slvent, the ault nentratn r lalty f the present partles (nuer f les f the slute / ass f the slvent n l/kg unt). he lal freezng pnt depressn ( ) depends nly n the physal nstants f the slvent (enthalpy f eltng, eltng pnt), and des nt depends n the prpertes f the slute. he freezng pnt depressn ( ) depends n nly the aunt f the present dsslved partles, therefre t s alled llgatve quantty (slar as st pressure, lng pnt elevatn, pressure f deal gas). *2 A, (2) fush where s the lar gas nstant, fus H s the enthalpy f fusn f the slvent, * s the eltng pnt f the pure slvent and A s the lar ass f the slvent. lal freezng pnt depressn s hgh f the enthalpy f eltng s sall and the eltng pnt s hgh. lal freezng pnt depressns f slvents slvent freezng pnt / C / (K kg l ) water enzene naphthalene aphr Fr easurng the freezng pnt depressn we an alulate the lar ass f the slute f we knw ts nentratn O knwng the lar ass we an alulate the nentratn f the slutn. If the slute dssates the freezng pnt s lwer than we expet ased n the nentratn alulated fr lar ass. he freezng pnt depressn depends n the nuer f les f the spees, s knwng the lar ass the alulatns wll gve the etve lalty (the ttal lalty f the spees present n the slutn). fpd_206_s

2 [ ( ν ) ] α (3) where ν s the nuer f the fred ns fr the pund, s the van t Hff fatr. Fr the etve lalty ( ) and the alulated lalty fr lar ass ( ) we an alulate the degree f dssatn (α). Durng a freezng pnt experent a lqud slutn ay e led dwn far elw ts equlru freezng teperature t a nu superled teperature value, s wng t ertan sthast features f the rystallzatn press. he degree f superlng and the aunt f pure slvent that freezes ut are dfferent fr ne experent t the ther. he atual freezng pnt f slutn slghtly depends n the aunt f the phase frzen ut (n dluted slutns t s the slvent), sne the slutn n equlru wth the sld phase s re nentrated (a part f the slvent s frzen ut) and we easure the freezng pnt f ths slutn. he ass f sld phase here e an e estated usng the degree f superlng (δ s ). he aunt f e s just enugh t prdue as uh heat as t an nrease the teperature f the superled syste t the freezng pnt. We an draw the fllwng splfed equatn aut ths press: q δ where s the ass f e, s the spef heat apaty f water (4,86 J/(g K)), q s the spef heat f eltng f e (333,62 J/g), s the ass f slutn (30 g). he left sde f the equatn nsders the heat prdued durng the freezng, the rght sde the heat need fr the teperature nrease. Nte that we have assued that the spef heat apates f water and e are equal and we have negleted the heat apaty f the ther parts f the apparatus, the heat apaty f the thereter and furtherre the heat lss s neglgle due t the lw heat ndutvty. Expressng the ass f e: δ (4) q he superlng s advantageus fr pratal pnt f vew, sne ths prdues fne dstruted sld phase and sharp, well reprdule freezng. he large surfae ntat f the sld and lqud phases s a pratal ndtn f phase equlru. III. Experental predure he apparatus We use the apparatus shwn n Fgure fr deternatn f the freezng pnt. Besde ths we use anther nsulated ntaner t freeze the lquds. Fr the easureent f teperature we use the eletr resstane f a teperature sensr whh s prprtnal t the teperature. fpd_206_s 2

3 Fgure Cryspe: an apparatus fr the deternatn f the freezng pnt depressn n dluted slutn Steps f the experent. Preparatn f the lng xtures Durng the easureent we use a lng xture etween 6 and 8 C. he xtures n the nsulated ntaners are ade f grund e, lean water and salt. By addng salt and/r e (and a lttle water) set the teperature f the xture. Str the e-water-salt freezng xture ntnuusly, and hek the teperature several tes durng the preparatn. Use lean water prvded n a plast alln fr the preparatn f the lng xture, and dstlled water fr the servatn f the freezng pnt. Put the nsulatr tue n the lng xture. 2. Clng the saples, detetn f the freezng pnt Fll 30 3 f dstlled water nt a test tue. Plae the strrer and the teperature sensr n the slutn. Ierse the test tue dretly nt the ler freezng xture and let t l wth nstantly slw strrng. Start the data lletn usng the prvded puter prgra (see tehnal detals n Appendx 2 SCOPEVIEW_prgra)! When the lqud starts t freeze the gradually drppng resstane suddenly jups up t the resstane value f the equlru teperature. At ths pnt take the test tue ut f the lng xture, dry the uter wall f the test tue qukly, and plae t nt the nsulatr tue whh s ersed n the freezng xture. Keep strrng the slutn slwly and ntnuusly. Cntnue the data lletn tll the rerded data (seen n the sreen) d nt shw any tendeny (2 3 nutes). elt the e rystals pletely y warng the test tue wth yur hand, and then repeat the experent 2 re tes. If e rystals rean n the egnnng f the experent superlng annt e reahed and the freezng pnt deternatn wll e less aurate and takes uh lnger! When yu have fnshed the three experents wth dstlled water dry the freezng tue and easure the freezng pnt f yur unknwn slutn (gven y the tehnan havng exatly 30 3 vlue), the 0.5 l/kg urea slutn and fnally the 0.5 l/kg KCl slutn. epeat the freezng experent 3 tes wth ALL lquds. Whle easurng, always take are f the leanness f the laratry tls! Whle hangng slutns, always pur the new slutn nt a lean, dry test tue. fpd_206_s 3

4 he rrespndng naes f the datafles and frzen lquds shuld appear n yur la reprt! he fles an e dwnladed fr the fllwng lnk: IV. Evaluatn f the easured data. ake the average f the data n the equlru state. In ase f water we an take the average f the freezng pnt at the dfferent experents (the frzen e des nt nfluene the freezng pnt f the pure slvent). In ase f slutn we take the averages separately (dfferently nentrated slutns have dfferent freezng pnt). Calulate the ass f the utfrzen e ased n the equatn 4. Calulatn f teperature dfferene usng eletrn thereter he resstane vs. teperature funtn f the teperature sensr has a slpe value f the sensr prvded n the la ust e used n the alulatns. S Ω / C. he slpe Yur data easured wth the sensr: Freezng pnt f dstlled water: 0 Freezng pnt f slutn: s Superlng pnt f slutn: s he freezng pnt depressn: he degree f superlng: 0 s S δ s S s 2.a We an alulate the ntal ass perentage f the unknwn slutn: w ( ) 000 (4) he dervatn f ths equatn an e fund n Appendx a n detals. We use urea as slute, therefre the lar ass f the slute s 60,06 g/l. Sne the slutns are dlute the asses f slutns are apprxately 30 g n all ases. 2. Fr the study f dssatn we alulate the van`t Hff fatr: B (5) he dervatn f ths equatn an e fund n Appendx n detals. fpd_206_s 4

5 If we use urea as slute the lar ass f the slute s kg/l. If we use ptassu hlrde as slute the lar ass f the slute s kg/l. Sne the slutns are dlute the asses f slutns are apprxately kg n all ases. Use the asses n kg, the lar ass n kg/l. Calulate the degree f dssatn (eq.(3)). Dsuss the results! V. esults (ust e nluded n the la reprt) the nae f the fles ntanng the easure data, the nuer f yur unknwn the resstanes rrespndng t the freezng pnt f dstlled water and ther average the resstanes f the superlng pnts and the freezng pnts fr the unknwn nentratn urea slutn (separately fr eah freezng experent) the ass fratn f urea n the unknwn slutn wth 95% nfdene nterval the resstanes f the superlng pnts and the freezng pnts fr the knwn nentratn KCl and urea slutn (separately fr eah freezng experent) the freezng pnt depressns fr the knwn nentratn KCl and urea slutn and the nsequenes an e drawn the degree f dssatn fr the KCl slutn nludng the steps f alulatn (assung that the atvty ents are unts) fpd_206_s 5

6 fpd_206_s 6 Appendx a Dervatn f the equatn used fr the alulatn f the pstn We wuld lke t get the nentratn f the slutns n ass fratn: w Let us express the ass f the slute ( ) usng the easured teperature dfferene ( ), the ass f the slutn ( ) and the e frzen ut ( )! Urea des nt dssate, therefre. Fr the easureent f the freezng pnt depressn: he lalty f the slutn: water / n he ass f the slute an e expressed: / / We an wrte the lalty alulated fr the freezng pnt depressn n the equatn and rearrange t: Let us susttute ak ths t ur frst equatn: w Sne th the nueratr and the dennatr ntans quanttes wth ass densn ther unts are artrary, just the sae shuld e used fr th. he lar ass shuld e susttuted n kg/l. If we wuld lke t use lar ass n g/l the hangng fatr appears n the equatn: kg 000g / w

7 fpd_206_s 7 Appendx. Calulatn f the van`t Hff fatr fr the freezng pnt depressn takng nt aunt the ass f the utfrzen e he etve lalty n the rgnal slutn s: water n akng nt aunt the dsstatn (see thery equatn (3) the lar ass fr appendx.a an e wrtten n the fllwng fr: By the help f the equatn ave the etve ault nentratn an e wrtten as: Usng : Dvdng th sdes y : Use the asses n kg, the lar ass n kg/l.

8 Appendx 2. Usage f the EEX SpeVew prgra n the easureent f freezng pnt depressn. Be sure that the EEX ultéter s turned n and the funtn dal s n the 2 koh resstane easureent. 2. Start prgra SpeVew. Fr a shrt te a wele sreen appears and we get the an enu sn. 3. Cnnet the deve t the prgra lkng n the Pwer uttn. fpd_206_s 8

9 4. Clkng n the Spe n we selet the Spe de. he setup sreen f the de appears: 5. Set the fllwng values: Vertal fled: - turn ff the Aut Sale (deselet the hekx efre the Aut Sale, f needed) - Unts/dv: Offset:.5 e Base feld: - Saple Every Se, rgger feld: - turn ff the rgger (deselet the hekx efre the rgger, f needed) Sweep de feld: - set t t epettve - set Expand X2 at Sweep agnfy If we set up everythng well the fllwng sreen an e seen: fpd_206_s 9

10 6. Clk n the "erd" uttn, and set the fle nae f the data lletn n the "Fle Nae" feld ardng ths shee: (nuer f wrkplae -6)(last dgt f year)(nth 0-2)(day 0-3)(experent nuer 0-99).txt Befre the frst experent set the dretry t C:\FPCSADA. Clkng n OK we aept the settngs and g ak t the prevus wndw. 7. Clkng n the "Spe" uttn we get the SpeVew Output graphal sreen. We wll see the results f ur easureent here. he resstane f the teperature sensr appears n the tp left rner. We an start the lletn f date lkng n un. If the data lletn s runnng we an stp t lkng n the Stp uttn appearng nstead f the run uttn. At the end f the easureent push the Clse uttn. hs takes us nt nly ak t the settng sreen, ut stres the easured data. 8. epeat steps 6-7 fr eah freezng experent. Hw t press the SpeVew datafles: he fles ntan 20 lnes header; ths shuld e skpped when we prt the fles. he frst lun s the resstane f the sensr n koh. e nfratn s ssng fr the fle; therefre we have t reate a te lun durng the date pressng (the lletn f the datapnts was unfr n te; ne pnt was easured n every send). fpd_206_s 0