lecture 1 handout full
TRANSCRIPT
8/8/2019 Lecture 1 Handout Full
http://slidepdf.com/reader/full/lecture-1-handout-full 1/14
Chem51LA
Winter‘11
Lecture1:MelngPoint
8/8/2019 Lecture 1 Handout Full
http://slidepdf.com/reader/full/lecture-1-handout-full 2/14
OrganicChemistryTechniques
• InChem51LAwewillbuildatoolboxoftechniquesintwomaincategories:
– Purificaon:separangmixturesofcompoundsbasedontheirchemicalorphysicalproperes
– Idenficaon:determiningtheidentyofacompoundthroughphysicalproperesor
structureelucidaon
8/8/2019 Lecture 1 Handout Full
http://slidepdf.com/reader/full/lecture-1-handout-full 3/14
Experiment1:MelngPoint
8/8/2019 Lecture 1 Handout Full
http://slidepdf.com/reader/full/lecture-1-handout-full 4/14
MelngPoint(Idenficaon)
• Themelngpointofacompoundisaphysicalproperty
– Canbeusedtodetermineidentyandpurity.
• Melng:transionfromsolidphasetoliquid – Requiresenergytobreakupcrystallace – Solidheatedtopointwherevaporpressureofsolidequalsvaporpressureofliquid.
8/8/2019 Lecture 1 Handout Full
http://slidepdf.com/reader/full/lecture-1-handout-full 5/14
LoweringtheMelngPoint
• MelngpointofanimpuresubstancewillALWAYSbelowerandbroaderthanthepure
substance!Why?
– Ittakeslessenergytobreakupalacewithcontaminants.
– Vaporpressue!
8/8/2019 Lecture 1 Handout Full
http://slidepdf.com/reader/full/lecture-1-handout-full 6/14
Freezing(Melng)PointDepression
• Solidandliquidarealwaysinequilibrium.• Formelngtooccur,thevaporpressureofthesolidmustequalthevaporpressureoftheliquid.
• Vaporpressuresofsolidsdon’tchangewithcomposion(norealmixing)butdochangewithtemperature.
8/8/2019 Lecture 1 Handout Full
http://slidepdf.com/reader/full/lecture-1-handout-full 7/14
Freezing(Melng)PointDepression(II)
• Assomesolidstartstomelt,thecontaminantsubstance(B)dissolves,loweringthevaporpressureoftheliquid(soluon,A).
• Remember,solidonlyneedstobeheatedenoughtoreachvaporpressureequaltoliquid’svaporpressure.
• SO...Asmorecontaminantdissolvesinliquid,vaporpressureofliquiddecreases,andamountofheatrequiredtomeltsoliddecreases.
LiquidA+DissolvedB
SolidB
8/8/2019 Lecture 1 Handout Full
http://slidepdf.com/reader/full/lecture-1-handout-full 8/14
MelngPointComposionDiagram
h[p://www.chem.arizona.edu/~salzmanr/480a/480ants/2comppd/2comppd.html
Melngpointsarenotloweredindefinitely.
Theeuteccpointrepresentsthelowestpossiblemelngtemperatureforamixtureof
twosubstances.
8/8/2019 Lecture 1 Handout Full
http://slidepdf.com/reader/full/lecture-1-handout-full 9/14
HowCanWeUseMelngPoint?
• Idenficaon:Melngpointcanhelpustoidenfyacompound
– DoALLcompoundshavecompletelydifferentmelngpoints?
• Ex:o-anisicacid(98-100ºC)vs4-tert -butylphenol(98-101ºC)
• Howcanwetellthemapart(usingonlymelngpoint)?
8/8/2019 Lecture 1 Handout Full
http://slidepdf.com/reader/full/lecture-1-handout-full 10/14
MixedMelngPointandIdenficaon
• Scenario:Yourunknowncompoundmeltsat97-100°C.
– Couldbeo-anisicacid(98-100ºC)(A) – Orcouldbe4-tert-butylphenol(98-101ºC)(B)
• Tryamixedmelngpointexperiment! – Mixunknown+o-anisicacid:melngpointislowered – Mixunknown+4-tert-butylphenol:nochange
• Yourcompoundis4-tert -butylphenol!
8/8/2019 Lecture 1 Handout Full
http://slidepdf.com/reader/full/lecture-1-handout-full 11/14
WhatYou’reDoinginLab
1. Calibraoncurve(knowncompounds)2. Determinemelngpointofunknowns3. Tryamixedmelngpoint(known
compounds)4. Usemixedmelngpointtoconfirmidenty
ofunknowns
8/8/2019 Lecture 1 Handout Full
http://slidepdf.com/reader/full/lecture-1-handout-full 12/14
What’saCalibraonCurve?
• Arethermocouplesalwaysaccurate?• Determinemelngpointoftwoknowncompounds.
• Makegraphcomparingtheorecalmelngpointtoobservedmelngpoint.
• Usethisgraphtoadjustalltemperaturereadings!
8/8/2019 Lecture 1 Handout Full
http://slidepdf.com/reader/full/lecture-1-handout-full 13/14
HowtoDetermineMelngPointofan
Unknown
1. Doa“quickanddirty”run.• Quicklymeltasampletodetermineapproximatemelng
point.
2. Doamorecarefulrun.• Heatquicklyunl~20°Cbelowapproximatemelng
point.
• Slowheangto1-2°CperminuteandWATCHCLOSELY!
3. Doitagain.•
Youneedatleast2determinaonswheremelngtemperaturesagree.
8/8/2019 Lecture 1 Handout Full
http://slidepdf.com/reader/full/lecture-1-handout-full 14/14
TipsandTricks
• Grindyoursolidwithaspatulaonawatchglassbeforeloadingintocapillarytube.• Usethe“drop”methodtopacksample(TAwilldemo).
• Mel-Tempshave3slots,sodomorethanonesampleatame!
• Startwithlower-melngsamples.• Useafreshsampleeachme.• Don’tforgettoletyourmel-tempcoolbetweenruns.
• USEYOURCALIBRATIONCURVE!