Name: boiling point and vapor pressure
Uploaded: 2020-03-26T07:22:27.000Z
Duration: 4 min 25 s
Channel: JoVE

K12 Standards

Georgia Standards of Excellence

9-12

Physical Science

SPS2.

Physical Properties

Boiling Points

Flüssigkeiten werden durch schwache Wechselwirkungen zwischen ihren Molekülen zusammengehalten. Um in die Gasphase überzugehen, muss ein Molekül genügend Energie aus Wärme oder anderen Quellen gewinnen, um diese Wechselwirkungen zu überwinden.
Betrachten wir einen geschlossenen Behälter mit einer reinen Flüssigkeit bei Raumtemperatur. Einige der Flüssigphasenmoleküle an der Oberfläche gewinnen genug Energie, um zu verdampfen, und ein Teil dieses Dampfes verliert Energie und kondensiert wieder zu Flüssigkeit. Wenn die Verdampfungs- und Kondensationsraten gleich sind, erreichen Flüssigkeit und Dampf ein Gleichgewicht. Zu diesem Zeitpunkt ist der Raum über der Flüssigkeit mit Dampf gesättigt, und es gibt keine Nettoveränderung in der Menge an Flüssigkeit und Dampf im Behälter. Der vom Dampf ausgeübte Druck wird als Dampfdruck bezeichnet.
Stellen Sie sich nun einen offenen Behälter mit Flüssigkeit vor. Hier kann der Dampf entweichen, so dass der Bereich über der Flüssigkeit nicht gesättigt wird. So könnte die Verdunstung so lange fortgesetzt werden, bis keine Flüssigkeit mehr vorhanden ist. Wenn die Temperatur einer Flüssigkeit steigt, steigt auch die Verdampfungsrate und damit auch der Dampfdruck. Wenn genügend Wärme zugeführt wird, beginnen Moleküle, die tiefer in der Flüssigkeit liegen, zu verdampfen, was wir als sich bildende Dampfblasen sehen.
Dies wird als Sieden bezeichnet und beginnt, wenn der Dampfdruck der Verbindung dem atmosphärischen Druck entspricht. Die Temperatur, bei der dies geschieht, ist der Siedepunkt. Da jede reine Substanz unterschiedliche Arten und Stärken intermolekularer Wechselwirkungen aufweist, haben verschiedene Substanzen unterschiedliche Siedepunkte.
Werfen wir nun einen Blick darauf, wie sich intermolekulare Wechselwirkungen auf den Siedepunkt auswirken. Viele intermolekulare Wechselwirkungen betreffen Bereiche mit ungleichmäßiger Elektronendichte, die als Dipole bezeichnet werden. Alle Moleküle weisen kurzzeitige Schwankungen in ihrer Elektronenverteilung auf, so dass auch unpolare Moleküle zeitweise Dipole aufweisen können. Als Reaktion darauf bilden sich gegensätzliche Dipole in benachbarten Molekülen.
Die Anziehungskräfte zwischen temporären und induzierten Dipolen werden als Londoner Dispersionskräfte bezeichnet und sind die dominierenden Wechselwirkungen zwischen unpolaren Molekülen wie Kohlenwasserstoffen. Die Londoner Dispersionskräfte sind zwischen großen Molekülen mit vielen Elektronen stärker, da diese Moleküle ihre Elektronen schwach halten, was es für benachbarte Moleküle einfacher macht, ihre Elektronenverteilung zu beeinflussen.
Zum Beispiel hat ein unverzweigtes Alkan mit hohem Molekulargewicht einen höheren Siedepunkt als ein unverzweigtes Alkan mit niedrigerem Molekulargewicht. Darüber hinaus hat ein unverzweigtes Alkan aufgrund seiner größeren Oberfläche einen höheren Siedepunkt als ein verzweigtes Alkan mit vergleichbarem Molekulargewicht.
Moleküle mit polaren Bindungen haben permanente Dipole, die regelmäßige Dipol-Dipol-Wechselwirkungen zwischen ihnen ermöglichen. Dipol-Dipol-Wechselwirkungen sind stärker als die Londoner Dispersionskräfte, so dass zum Kochen polarer Flüssigkeiten in der Regel mehr thermische Energie benötigt wird als unpolare Flüssigkeiten. Zum Beispiel liegt der Siedepunkt eines Chlorpentans, eines polaren Moleküls, bei 108 °C. Hexan, das in der Oberfläche ähnlich ist, aber keinen Dipol hat, hat einen niedrigeren Siedepunkt.
Moleküle, bei denen sowohl ein Wasserstoff kovalent an ein elektronegatives Atom gebunden ist, als auch ein elektronenziehendes Atom mit einem einsamen Elektronenpaar sind in der Lage, Wasserstoffbrückenbindungen einzugehen. Wasserstoffbrückenbindungen sind stärker als Dipol-Dipol-Wechselwirkungen und Londoner Dispersionskräfte, so dass noch mehr thermische Energie benötigt wird, um sie zu überwinden. Zum Beispiel haben Propan und Ethanol ähnliche Molekulargewichte und Oberflächen. Der Siedepunkt von Propan ist jedoch viel niedriger als der von Ethanol, da Ethanolmoleküle miteinander Wasserstoffbrückenbindungen eingehen können, während Propan dies nicht kann.
In diesem Labor verwenden Sie die Kapillarmethode, um den Siedepunkt von zwei organischen Lösungsmitteln zu bestimmen.

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Siedepunktbestimmung
Ähnlich wie der Schmelzpunkt ist auch der Siedepunkt eine physikalische Eigenschaft. Wenn es sich bei der Probe um eine reine Verbindung handelt, kann der Siedepunkt verwendet werden, um die Identität der Verbindung zu bestimmen. Letztendlich ist die experimentelle Bestimmung des genauen Siedepunkts eine Herausforderung. Wie die Schmelzpunkte werden auch die experimentellen Siedepunkte als Bereich angegeben und weichen um einige Grad vom tatsächlichen Literaturwert ab.
Dampfdruck
Um zu verstehen, warum ein Lösungsmittel siedet, was durch das bekannte Blasen der Lösung gekennzeichnet ist, ist es wichtig, die Dynamik zwischen der flüssigen und der gasförmigen Phase zu verstehen. Stellen Sie sich eine reine flüssige Verbindung in einem verschlossenen Behälter vor. Einige der Moleküle an der Oberfläche der Flüssigkeit haben genug Energie, um intermolekulare Kräfte zu überwinden und in die gasförmige Phase überzugehen. Moleküle in der Gasphase können aber auch Energie verlieren und wieder zu einer Flüssigkeit kondensieren. Daher gibt es in diesem System zwei konkurrierende Prozesse: Verdampfung und Kondensation.
Wenn die Verdunstungsrate gleich der Kondensationsrate ist, hat das System einen Gleichgewichtszustand erreicht. Das bedeutet, dass für jedes Molekül, das in die Gasphase eintritt, ein anderes zur flüssigen Phase kondensiert und es keinen Nettogewinn oder -verlust der Flüssigkeits- oder Gasmenge im Behälter gibt. Sobald das Gleichgewicht hergestellt ist, wird der Druck, den der Dampf über der Flüssigkeit ausübt, als Dampfdruck bezeichnet. Die Tendenz, dass eine Flüssigkeit verdampft, wird als Volatilität bezeichnet. Eine flüchtigere Flüssigkeit hat einen höheren Dampfdruck, während eine weniger flüchtige Flüssigkeit einen niedrigeren Dampfdruck hat.
Der Dampfdruck variiert je nach Temperatur. Wenn Sie die Temperatur der Lösung erhöhen, haben mehr Moleküle genügend Energie, um der flüssigen Phase zu entkommen, und somit steigt der Dampfdruck. Wenn genügend Wärme zugeführt wird, gehen Moleküle, die sich nicht an der Grenzfläche zwischen Flüssigkeit und Gas befinden, in die Gasphase über und bilden die bekannten Blasen, die wir mit dem Sieden in Verbindung bringen.
Der Siedepunkt einer Flüssigkeit ist erreicht, wenn der Gesamtdampfdruck der Flüssigkeit dem Atmosphärendruck entspricht. Die Temperatur, bei der dies geschieht, wird als Siedepunkt bezeichnet. In höheren Lagen und damit bei niedrigerem Atmosphärendruck siedet eine Flüssigkeit bei einer niedrigeren Temperatur, da weniger Hitze benötigt wird, um den Dampfdruck auf den atmosphärischen Druck zu erhöhen. Darüber hinaus wirkt sich auch die Flüchtigkeit oder die Fähigkeit eines Lösungsmittels zu verdampfen auf den Dampfdruck aus. Lösungsmittel mit hoher Flüchtigkeit haben einen höheren Dampfdruck als Lösungsmittel mit geringerer Flüchtigkeit.
Faktoren, die den Siedepunkt beeinflussen
Die Ähnlichkeit zwischen Schmelzpunkten und Siedepunkten bedeutet, dass die gleichen Faktoren, die den Schmelzpunkt einer Verbindung beeinflussen, auch den Siedepunkt beeinflussen. Daher beeinflussen die Stärke und die Art der intermolekularen Kräfte, die in der flüssigen Verbindung zu finden sind, den Siedepunkt. Denken Sie daran, dass es drei Arten von molekularen Kräften gibt: Wasserstoffbrückenbindungen, Dipol-Dipol-Wechselwirkungen und Londoner Dispersionskräfte. Jede von ihnen hat eine unterschiedliche Anziehungskraft und erfordert unterschiedliche Mengen an Energie, um sie zu überwinden. Verbindungen, die Wasserstoffbrückenbrücken binden können, haben höhere Siedepunkte als Verbindungen, die nur durch Londoner Dispersionskräfte interagieren können. Eine weitere Überlegung bei den Siedepunkten betrifft den Dampfdruck und die Flüchtigkeit der Verbindung. Je flüchtiger eine Verbindung ist, desto niedriger ist in der Regel ihr Siedepunkt.
Die Kapillarmethode zur Bestimmung des Siedepunkts
Eine einfache Methode zur Bestimmung des Siedepunkts einer organischen Verbindung ist die Kapillarmethode. Bei diesem Aufbau wird ein leeres Glaskapillarrohr in einen Behälter mit der reinen Verbindung in der flüssigen Phase invertiert. Wenn die Flüssigkeit erhitzt wird, steigt der Dampfdruck der Probe und gasförmiger Dampf beginnt in das Glaskapillarrohr einzudringen. Dadurch wird die im Inneren eingeschlossene Luft herausgedrückt und es entstehen Blasen aus dem Boden des Kapillarrohrs. Zu diesem Zeitpunkt wird die Flüssigkeit abkühlen gelassen. Sobald der Dampfdruck der Probe dem atmosphärischen Druck im Glaskapillarrohr entspricht, beginnt die Flüssigkeit in das Röhrchen einzudringen. Die Temperatur der Lösung, wenn dieses Phänomen auftritt, ist der Siedepunkt der flüssigen Verbindung.

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boiling point and vapor pressure

Boiling point and vapor pressure are key physical properties of a liquid. For a pure compound, the boiling point can help identify the substance. In experiments, the boiling point is usually recorded as a range because exact values are hard to measure and may differ by a few degrees from the literature value.

Vapor pressure explains why a liquid boils. In a sealed container, some molecules at the liquid surface gain enough energy to escape into the gas phase. At the same time, some gas molecules lose energy and condense back into the liquid. When evaporation and condensation happen at the same rate, the system is at equilibrium. The pressure from the vapor above the liquid is the vapor pressure.

Volatility is the tendency of a liquid to vaporize. A more volatile liquid has a higher vapor pressure, while a less volatile liquid has a lower vapor pressure. Vapor pressure also changes with temperature. As temperature rises, more molecules can escape the liquid, so vapor pressure increases. With enough heat, molecules inside the liquid also change to gas and form the bubbles seen during boiling.

A liquid boils when its total vapor pressure matches atmospheric pressure. That temperature is the boiling point. Because atmospheric pressure changes with elevation, a liquid boils at a lower temperature at higher elevations. Less atmospheric pressure means less heat is needed for the vapor pressure to reach that level.

Intermolecular forces also affect boiling point. These forces include hydrogen bonding, dipole-dipole interactions, and London dispersion forces. They differ in strength, so different amounts of energy are needed to overcome them. Compounds that can hydrogen bond usually have higher boiling points than compounds that rely only on London dispersion forces. Volatility matters too. In general, more volatile compounds have lower boiling points.

One simple way to measure a boiling point is the capillary method. An empty glass capillary tube is placed upside down in the liquid sample. As the sample is heated, its vapor pressure rises and vapor enters the tube. This pushes out trapped air and causes bubbles to appear from the bottom of the capillary tube. The sample is then allowed to cool. When liquid begins to rise into the tube, the vapor pressure inside the tube matches the atmospheric pressure. The temperature at that moment is the boiling point of the compound.

Boiling Point and Vapor Pressure

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Molecular Compounds: Formulas and Nomenclature

Molecular compounds or covalent compounds result when atoms share electrons to form covalent bonds. Since there is no electron transfer, molecular ...

Chemical Symbols

A chemical symbol is an abbreviation that is used to indicate an element or an atom of an element. For example, the symbol for mercury is Hg. We use the ...

Chemical Formulas

A chemical formula presents information about the proportions of atoms constituting a particular chemical compound or molecule, mainly using symbols of ...

Chemical Names and Chemical Formulas

Obtain, evaluate, and communicate information to explain how atoms bond to form stable compounds.

Boiling Point and Vapor Pressure

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