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

Жидкости удерживаются вместе за счет слабых взаимодействий между их молекулами. Чтобы перейти в газовую фазу, молекула должна получить достаточно энергии от тепла или других источников, чтобы преодолеть эти взаимодействия.
Рассмотрим закрытую емкость с чистой жидкостью комнатной температуры. Некоторые молекулы жидкой фазы на поверхности получают достаточно энергии для испарения, а часть этого пара теряет энергию и конденсируется обратно в жидкость. Когда скорости испарения и конденсации одинаковы, жидкость и пар достигают равновесия. В этот момент пространство над жидкостью насыщается паром, и количество жидкости и пара в емкости не меняется. Давление, оказываемое паром, называется давлением пара.
Теперь рассмотрим открытую емкость с жидкостью. Здесь пар может выходить, предотвращая насыщение области над жидкостью. Таким образом, испарение может продолжаться до тех пор, пока не останется жидкости. По мере того как температура жидкости увеличивается, увеличивается и скорость испарения, а следовательно, и давление пара. Если добавить достаточное количество тепла, молекулы, находящиеся глубже в жидкости, начинают испаряться, что мы видим как образование пузырьков пара.
Это называется кипячением, и оно начинается, когда давление пара соединения становится равным атмосферному. Температура, при которой это происходит, называется точкой кипения. Поскольку каждое чистое вещество имеет разные типы и силу межмолекулярных взаимодействий, разные вещества имеют разную температуру кипения.
Теперь давайте рассмотрим, как межмолекулярные взаимодействия влияют на температуру кипения. Многие межмолекулярные взаимодействия связаны с областями с неравномерной электронной плотностью, называемыми диполями. Все молекулы имеют кратковременные флуктуации в распределении электронов, поэтому даже неполярные молекулы могут временно иметь диполи. В ответ на это в соседних молекулах образуются противоположные диполи.
Силы притяжения между временными и индуцированными диполями называются лондонскими дисперсионными силами, и они являются доминирующими взаимодействиями между неполярными молекулами, такими как углеводороды. Дисперсионные силы Лондона сильнее между большими молекулами с большим количеством электронов, потому что эти молекулы слабо удерживают свои электроны, что облегчает соседним молекулам влияние на их распределение электронов.
Например, высокомолекулярный неразветвленный алкан имеет более высокую температуру кипения, чем низкомолекулярный неразветвленный алкан. Кроме того, неразветвленный алкан имеет более высокую температуру кипения, чем разветвленный алкан с сопоставимой молекулярной массой, из-за его большей площади поверхности.
Молекулы с полярными связями имеют постоянные диполи, что обеспечивает регулярное диполь-дипольное взаимодействие между ними. Диполь-дипольные взаимодействия сильнее, чем дисперсионные силы Лондона, поэтому для кипячения полярных жидкостей обычно требуется больше тепловой энергии, чем для неполярных. Например, температура кипения одного хлорпентана, полярной молекулы, составляет 108 °C. Гексан, который похож по площади поверхности, но не имеет диполя, имеет более низкую температуру кипения.
К водородной связи способны молекулы как с водородом, ковалентно связанным с более электроотрицательным атомом, так и с атомом, снимающим электроны, с одной парой электронов. Водородные связи прочнее, чем диполь-дипольные взаимодействия и дисперсионные силы Лондона, поэтому для их преодоления требуется еще больше тепловой энергии. Например, пропан и этанол имеют схожую молекулярную массу и площадь поверхности. Но температура кипения пропана намного ниже, чем у этанола, потому что молекулы этанола могут образовывать водородные связи друг с другом, в то время как пропан — нет.
В этой лаборатории вы будете использовать капиллярный метод для определения температуры кипения двух органических растворителей.

Точки кипения

Определение температуры кипения
Подобно температуре плавления, температура кипения является физическим свойством. Если образец является чистым соединением, то для определения идентичности соединения можно использовать температуру кипения. В конечном счете, экспериментальное определение точной точки кипения является сложной задачей. Как и точки плавления, экспериментальные точки кипения задаются в виде диапазона и варьируются на несколько градусов от фактического значения в литературе.
Давление пара
Чтобы понять, почему закипает растворитель, для которого характерно знакомое барботирование раствора, важно понимать динамику между жидкой и газообразной фазами. Рассмотрите чистое жидкое соединение в герметичной емкости. Некоторые молекулы на поверхности жидкости будут обладать достаточной энергией, чтобы преодолеть межмолекулярные силы и перейти в газообразную фазу. Однако молекулы в газовой фазе также могут терять энергию и конденсироваться обратно в жидкость. Таким образом, в этой системе есть два конкурирующих процесса: испарение и конденсация.
Когда скорость испарения равна скорости конденсации, система достигает состояния равновесия. Это означает, что на каждую молекулу, которая входит в газовую фазу, другая конденсируется в жидкую фазу, и нет никакого чистого прироста или потери количества жидкости или газа в контейнере. После установления равновесия давление, оказываемое паром над жидкостью, называется давлением пара. Склонность жидкости к испарению называется ее летучестью. Более летучая жидкость имеет более высокое давление пара, в то время как менее летучая жидкость имеет более низкое давление пара.
Давление пара изменяется в зависимости от температуры. Если вы повышаете температуру раствора, больше молекул имеют достаточно энергии, чтобы выйти из жидкой фазы, и, таким образом, давление пара увеличивается. В конечном счете, если приложить достаточное количество тепла, молекулы, которые не находятся на границе между жидкостью и газом, перейдут в газовую фазу и образуют знакомые пузырьки, которые мы ассоциируем с кипением.
Температура кипения жидкости достигается, когда общее давление пара жидкости эквивалентно атмосферному давлению. Температура, при которой это происходит, называется температурой кипения. На больших высотах и, следовательно, при более низком атмосферном давлении жидкость будет кипеть при более низкой температуре, так как для увеличения давления пара до атмосферного требуется меньше тепла. Кроме того, летучесть, или способность растворителя испаряться, также влияет на давление пара. Растворители с высокой летучестью имеют более высокое давление пара, чем растворители с более низкой летучестью.
Факторы, влияющие на температуру кипения
Сходство между температурами плавления и кипения означает, что те же факторы, которые влияют на температуру плавления соединения, также влияют на температуру кипения. Таким образом, сила и типы межмолекулярных сил, которые находятся в жидком соединении, будут влиять на температуру кипения. Напомним, что существует три типа молекулярных сил: водородные связи, диполь-дипольные взаимодействия и дисперсионные силы Лондона. Каждый из них имеет разную силу притяжения и требует разного количества энергии для преодоления. Соединения, которые могут связываться водородом, будут иметь более высокие температуры кипения, чем соединения, которые могут взаимодействовать только через дисперсионные силы Лондона. Дополнительным фактором для определения температуры кипения является давление пара и летучесть соединения. Как правило, чем летучее соединение, тем ниже его температура кипения.
Капиллярный метод определения точки кипения
Простым методом определения температуры кипения органического соединения является использование капиллярного метода. В этой установке пустая стеклянная капиллярная трубка переворачивается в контейнер с чистым соединением в жидкой фазе. По мере нагревания жидкости давление пара образца увеличивается, и газообразный пар начинает попадать в стеклянную капиллярную трубку. Это вытесняет воздух, попавший внутрь, и приводит к появлению пузырьков, выходящих из нижней части капиллярной трубки. В этот момент жидкости дают остыть. Как только давление пара образца станет таким же, как атмосферное давление внутри стеклянной капиллярной трубки, жидкость начнет поступать в трубку. Температура раствора при возникновении этого явления является температурой кипения жидкого соединения.

Определение температуры кипения
Подобно температуре плавления, температура кипения является физическим свойством. Если образец представляет собой чистое соединение, то кипячение...

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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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synthesis of luminol

Luminol Chemiluminescence in Forensics

Luminol chemiluminescence shows how a chemical reaction can produce light and help with blood detection. Chemical reactions may be endothermic or exothermic, depending on whether they absorb or release heat as reactants become products. The enthalpy of a reaction is the energy difference between reactants and products, or ΔH. A positive ΔH means the reaction is endothermic. A negative ΔH means it is exothermic. Many reactions do not happen in a single step. They can pass through intermediates...

esterification

Ester Formation and Everyday Fragrances

Esters are organic compounds with a carbonyl group attached to an oxygen that connects to another alkyl or aryl group. Their general formula is RCOOR'. They are often made by reacting a carboxylic acid with an alcohol in an esterification reaction. In this process, the hydroxyl group of the acid is replaced by an alkyl or aryl group. Many simple esters have familiar fruit and flower smells because their structures are small and less complex. Formic acid and methanol can combine to form methyl...

identification of unknown aldehydes and ketones

identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

Aldehydes and ketones are carbonyl compounds, which means they contain a C=O functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon. Acetone is the simplest ketone, with two methyl groups. An aldehyde is similar, but it has at least one hydrogen attached to the carbonyl carbon. Formaldehyde is the simplest aldehyde, and acetaldehyde has one hydrogen and one methyl group. The carbonyl carbon in both aldehydes and ketones is electrophilic because the oxygen atom...

uv-vis spectroscopy of dyes

How Dyes Absorb Visible Light

Dyes absorb visible light because their molecular structure lets them interact with specific wavelengths. When light hits a substance, some light is absorbed and the rest is reflected or transmitted. The color we see depends on the wavelength that is reflected. A blue substance reflects blue light, around 430 to 480 nm, and absorbs the complementary orange region, around 590 to 630 nm. Compounds that do not absorb in the visible range appear colorless to the human eye. Light also has energy and...

identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary. Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical...

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

Подготовьтесь к преподаванию Точки кипения с помощью этого 1-минутного руководства для преподавателей. Охватывает ключевые этапы подготовки для лаборатории Chemistry на JoVE Lab Manual.

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Следуйте пошаговым процедурам Точки кипения в этом 6-минутном видео студенческого протокола. Часть коллекции Chemistry на JoVE Lab Manual.

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Types of Chemical Bonds

Chemical bonding theories were pioneered by American chemist Gilbert&#xA0;N. Lewis. He developed a model called the Lewis model to explain the type and ...

Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

Chemical Bonds

Boiling Point Determination
Similar to the melting point, the boiling point is a physical property. If the sample is a pure compound, then the boiling ...

Melting Points

Melting Points in Organic Chemistry
The melting point of a compound is the temperature at which the solid phase transitions into the liquid phase at a ...

Melting Point

Source: Vy M. Dong and Jan Riedel, Department of Chemistry, University of California, Irvine, CA
One of the most important properties of a crystalline ...

Electrolyte and Nonelectrolyte Solutions

Substances that undergo either a physical or a chemical change in solution to yield ions that can conduct electricity are called electrolytes. If a ...

ionic-compounds-formulas-and-nomenclature

Ionic Compounds: Formulas and Nomenclature

An element composed of atoms that readily lose electrons (a metal) can react with an element composed of atoms that readily gain electrons (a nonmetal) to ...

molecular-compounds-formulas-and-nomenclature

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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