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

Ciecze są utrzymywane razem przez słabe oddziaływania między ich cząsteczkami. Aby przejść do fazy gazowej, cząsteczka musi uzyskać wystarczającą ilość energii z ciepła lub innych źródeł, aby przezwyciężyć te interakcje.
Rozważmy zamknięty pojemnik z czystą cieczą w temperaturze pokojowej. Niektóre cząsteczki fazy ciekłej na powierzchni uzyskują wystarczającą ilość energii, aby odparować, a część tej pary traci energię i skrapla się z powrotem do cieczy. Gdy szybkość parowania i kondensacji jest taka sama, ciecz i para osiągają równowagę. W tym momencie przestrzeń nad cieczą jest nasycona parą i nie ma zmiany netto ilości cieczy i pary w pojemniku. Ciśnienie wywierane przez parę nazywane jest ciśnieniem pary.
Teraz rozważ otwarty pojemnik z płynem. W tym miejscu para może się ulatniać, zapobiegając nasyceniu obszaru nad cieczą. W ten sposób parowanie może trwać do momentu, gdy nie pozostanie płyn. Wraz ze wzrostem temperatury cieczy wzrasta szybkość parowania, a co za tym idzie, rośnie również prężność pary. Jeśli doda się wystarczającą ilość ciepła, cząsteczki znajdujące się głębiej w cieczy zaczynają parować, co widzimy jako tworzące się pęcherzyki pary.
Nazywa się to gotowaniem i rozpoczyna się, gdy prężność pary związku jest równa ciśnieniu atmosferycznemu. Temperatura, w której to następuje, to temperatura wrzenia. Ponieważ każda czysta substancja ma różne rodzaje i siłę oddziaływań międzycząsteczkowych, różne substancje mają różne temperatury wrzenia.
Przyjrzyjmy się teraz, jak interakcje międzycząsteczkowe wpływają na temperaturę wrzenia. Wiele oddziaływań międzycząsteczkowych obejmuje obszary o nierównomiernej gęstości elektronów zwane dipolami. Wszystkie cząsteczki mają krótkie wahania w rozkładzie elektronów, więc nawet cząsteczki niepolarne mogą tymczasowo mieć dipole. W odpowiedzi w pobliskich cząsteczkach tworzą się przeciwstawne dipole.
Siły przyciągania między tymczasowymi i indukowanymi dipolami nazywane są siłami dyspersji londyńskiej i są dominującymi oddziaływaniami między cząsteczkami niepolarnymi, takimi jak węglowodory. Londyńskie siły dyspersyjne są silniejsze między dużymi cząsteczkami z wieloma elektronami, ponieważ cząsteczki te słabo utrzymują swoje elektrony, co ułatwia sąsiednim cząsteczkom wpływanie na ich rozkład elektronów.
Na przykład nierozgałęziony alkan o dużej masie cząsteczkowej ma wyższą temperaturę wrzenia niż nierozgałęziony alkan o niższej masie cząsteczkowej. Dodatkowo nierozgałęziony alkan ma wyższą temperaturę wrzenia niż rozgałęziony alkan o porównywalnej masie cząsteczkowej ze względu na większą powierzchnię.
Cząsteczki z wiązaniami polarnymi mają stałe dipole, umożliwiające regularne oddziaływania dipol-dipol między nimi. Oddziaływania dipol-dipol są silniejsze niż siły dyspersji Londona, więc zwykle potrzeba więcej energii cieplnej do zagotowania cieczy polarnych niż cieczy niepolarnych. Na przykład temperatura wrzenia jednego chloropentanu, cząsteczki polarnej, wynosi 108 °C. Heksan, który ma podobną powierzchnię, ale nie ma dipola, ma niższą temperaturę wrzenia.
Cząsteczki, w których zarówno wodór jest kowalencyjnie związany z bardziej elektroujemnym atomem, jak i atom wycofujący elektrony z samotną parą elektronów, są zdolne do wiązania wodorowego. Wiązania wodorowe są silniejsze niż oddziaływania dipol-dipol i siły dyspersji Londona, więc do ich pokonania potrzeba jeszcze więcej energii cieplnej. Na przykład propan i etanol mają podobną masę cząsteczkową i powierzchnię. Ale temperatura wrzenia propanu jest znacznie niższa niż etanolu, ponieważ cząsteczki etanolu mogą tworzyć ze sobą wiązania wodorowe, podczas gdy propan nie.
W tym laboratorium użyjesz metody kapilarnej do określenia temperatury wrzenia dwóch rozpuszczalników organicznych.

temperatury wrzenia

Oznaczanie temperatury wrzenia
Podobnie jak temperatura topnienia, temperatura wrzenia jest właściwością fizyczną. Jeśli próbka jest czystym związkiem, wówczas temperatura wrzenia może być użyta do określenia tożsamości związku. Ostatecznie eksperymentalne określenie dokładnej temperatury wrzenia jest trudne. Podobnie jak temperatury topnienia, eksperymentalne temperatury wrzenia są podawane jako zakres i różnią się o kilka stopni od rzeczywistej wartości literaturowej.
ciśnienie pary
Aby zrozumieć, dlaczego rozpuszczalnik wrze, który charakteryzuje się znajomym bulgotaniem roztworu, ważne jest zrozumienie dynamiki między fazą ciekłą i gazową. Rozważ czysty płynny związek w szczelnie zamkniętym pojemniku. Niektóre cząsteczki na powierzchni cieczy będą miały wystarczającą ilość energii, aby pokonać siły międzycząsteczkowe i wejść w fazę gazową. Jednak cząsteczki w fazie gazowej mogą również tracić energię i kondensować się z powrotem w ciecz. Dlatego w tym systemie występują dwa konkurencyjne procesy: parowanie i kondensacja.
Gdy szybkość parowania jest równa szybkości kondensacji, system osiągnął stan równowagi. Oznacza to, że na każdą cząsteczkę, która wchodzi do fazy gazowej, inna skrapla się do fazy ciekłej i nie ma zysku ani utraty netto ilości cieczy lub gazu w pojemniku. Po ustaleniu równowagi ciśnienie wywierane przez parę nad cieczą nazywa się prężnością pary. Skłonność cieczy do parowania nazywana jest jej lotnością. Bardziej lotna ciecz ma wyższą prężność pary, podczas gdy mniej lotna ciecz ma niższą prężność pary.
Prężność par zmienia się w zależności od temperatury. Jeśli zwiększysz temperaturę roztworu, więcej cząsteczek ma wystarczająco dużo energii, aby uciec z fazy ciekłej, a tym samym wzrasta prężność pary. Ostatecznie, jeśli zostanie zastosowana wystarczająca ilość ciepła, cząsteczki, które nie znajdują się na granicy faz między cieczą a gazem, przejdą do fazy gazowej i utworzą znajome pęcherzyki, które kojarzymy z wrzeniem.
Temperatura wrzenia cieczy zostaje osiągnięta, gdy całkowita prężność pary cieczy jest równoważna ciśnieniu atmosferycznemu. Temperatura, w której to zachodzi, nazywana jest temperaturą wrzenia. Na wyższych wysokościach, a tym samym przy niższym ciśnieniu atmosferycznym, ciecz zagotuje się w niższej temperaturze, ponieważ do podniesienia ciśnienia pary do ciśnienia atmosferycznego potrzeba mniej ciepła. Ponadto lotność, czyli zdolność rozpuszczalnika do parowania, również wpływa na prężność pary. Rozpuszczalniki o dużej lotności mają wyższą prężność par niż rozpuszczalniki o niższej lotności.
Czynniki wpływające na temperaturę wrzenia
Podobieństwo między temperaturami topnienia i wrzenia oznacza, że te same czynniki, które wpływają na temperaturę topnienia związku, będą również wpływać na temperaturę wrzenia. Dlatego siła i rodzaje sił międzycząsteczkowych, które znajdują się w ciekłym związku, będą miały wpływ na temperaturę wrzenia. Przypomnijmy, że istnieją trzy rodzaje sił molekularnych: wiązania wodorowe, oddziaływania dipol-dipol i siły dyspersji londyńskiej. Każdy z nich ma inną siłę przyciągania i wymaga innej ilości energii do pokonania. Związki, które mogą wiązać się wodorem, będą miały wyższą temperaturę wrzenia niż związki, które mogą oddziaływać tylko poprzez siły dyspersji Londona. Dodatkowym czynnikiem wpływającym na temperaturę wrzenia jest prężność pary i lotność związku. Zazwyczaj im bardziej lotny jest związek, tym niższa jest jego temperatura wrzenia.
Metoda kapilarna do określania temperatury wrzenia
Prostą metodą określania temperatury wrzenia związku organicznego jest zastosowanie metody kapilarnej. W tej konfiguracji pusta szklana rurka kapilarna jest odwracana do pojemnika z czystym związkiem w fazie ciekłej. W miarę podgrzewania cieczy wzrasta prężność pary próbki, a para gazowa zaczyna dostawać się do szklanej rurki kapilarnej. To wypycha powietrze uwięzione w środku i powoduje powstawanie pęcherzyków z dna rurki kapilarnej. W tym momencie płyn pozostawia się do ostygnięcia. Gdy prężność pary próbki będzie taka sama jak ciśnienie atmosferyczne wewnątrz szklanej rurki kapilarnej, ciecz zacznie dostawać się do probówki. Temperatura roztworu, w którym występuje to zjawisko, jest temperaturą wrzenia ciekłego związku.

Temperatury wrzenia

Oznaczanie temperatury wrzenia
Podobnie jak temperatura topnienia, temperatura wrzenia jest właściwością fizyczną. Jeśli próbka jest czystym związkiem, to wrzący ...

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JoVE Lab Manual

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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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Reaction Rate, Arrhenius, and Catalysts

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galvanic cells

Battery Power from Redox Reactions

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electrolytic cells

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proper lab notebook keeping

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melting points

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boiling points

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...

recrystallization

Purifying Crystals by Solubility

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extraction

Separating Mixtures with Solvent Extraction

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simple distillation

Separating Liquids by Boiling Point

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steam distillation

Steam Distillation and Low-Boiling Separation

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thin-layer chromatography

TLC Separation by Polarity

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column chromatography

Separating Mixtures by Polarity

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Ester Formation and Everyday Fragrances

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identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

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How Dyes Absorb Visible Light

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chemistry

Comprehensive, curriculum-focused videos for both instructors and students in introductory chemistry lab courses.

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Explore how vapor pressure, volatility, and intermolecular forces affect boiling point, and learn how the capillary method measures it in the lab.

Boiling Point and Vapor Pressure | JoVE K12

Przygotuj się do nauczania Temperatury wrzenia dzięki temu 1-minutowemu przewodnikowi dla instruktorów. Obejmuje kluczowe kroki przygotowawcze dla laboratorium Chemistry na JoVE Lab Manual.

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Boiling Point and Vapor Pressure

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