Name: steam distillation and low-boiling separation
Uploaded: 2020-03-26T07:22:28.000Z
Duration: 3 min 54 s
Channel: JoVE

Когда идеальная смесь двух смешивающихся жидкостей нагревается до кипения, раствор закипает при температуре между точками кипения каждого компонента. Если эти жидкости имеют очень разные температуры кипения, когда смесь начинает кипеть, пар богат молекулами более летучего компонента. Это явление часто используется для разделения смесей с помощью простой дистилляции, когда смесь двух смешивающихся жидкостей нагревается, а затем пар конденсируется обратно в жидкость и собирается.
По мере того как пар, богатый более летучим компонентом, собирается в виде дистиллята, жидкая фаза становится богатой молекулами менее летучего компонента. Однако этот метод требует, чтобы раствор был нагрет, по крайней мере, до точки кипения более летучего соединения, а часто и выше.
В случае чувствительных к температуре органических соединений эта высокая температура может привести к разложению органических молекул на что-то другое. Итак, как мы можем разделить эти типы соединений? Во-первых, давайте сделаем шаг назад.
Напомним, что давление пара, находящегося в равновесии с его конденсированной фазой, называется давлением пара. Каждый компонент смеси жидкостей имеет свое собственное давление пара, которое мы называем их парциальным давлением. Мы знаем, что раствор закипает, когда общее давление пара раствора равно атмосферному. Общее давление пара равно сумме парциальных давлений компонентов.
Для смеси смешивающихся жидкостей, что означает, что любая комбинация жидкостей образует однородный раствор, парциальные давления рассчитываются на основе давления паров чистых соединений, умноженного на их молярные доли в растворе. Однако для гетерогенной смеси несмешивающихся жидкостей, что означает, что жидкости нерастворимы друг в друге, парциальные давления являются просто давлением паров чистых соединений.
Поскольку каждый компонент гетерогенной смеси вносит свой вклад в общее давление пара независимо от других компонентов, смесь закипает, когда общее давление пара, которое является суммой парциальных давлений, равно атмосферному давлению. Это происходит при более низкой температуре, чем отдельные точки кипения каждого компонента, потому что общее давление пара увеличивается с температурой гораздо быстрее, чем можно было бы ожидать даже для самого летучего компонента.
Мы можем использовать это явление для проведения паровой дистилляции, которая используется для выделения чувствительного к температуре органического соединения, которое разлагается при высокой температуре и нерастворимо в воде из нелетучих веществ. Установка паровой дистилляции похожа на простую установку дистилляции с добавлением резервуара для воды для пополнения воды на протяжении всего процесса.
Когда смесь закипает, вода и представляющее интерес органическое соединение испаряются. Вода и пары органических соединений попадают в конденсатор, конденсируются в жидкость и собираются. После этого несмешивающиеся жидкости отделяются. В смеси в колбе остаются только вода и нелетучие материалы.
В этой лаборатории вы поставите и проведете эксперимент по паровой дистилляции для извлечения эфирного масла из нелетучих компонентов апельсиновой кожуры. Затем вы будете использовать жидкостную экстракцию для извлечения эфирного масла из воды в органический растворитель.

Паровая дистилляция

Steam Distillation

<h4>Паровая дистилляция </h4>
Паровая дистилляция — это метод разделения, который использует свойство низкой температуры кипения несмешивающихся смесей. В основном он используется для отделения чувствительных к температуре органических молекул от нелетучих загрязнителей. Органическая молекула должна быть несмешиваемой в воде.
При паровой дистилляции несмешивающаяся смесь нагревается до кипения, что приводит к перегонке как воды, так и летучих органических соединений. Это означает, что газообразная смесь поднимается вверх к конденсатору, который затем конденсирует пар в жидкость, чтобы его можно было собрать. В отличие от простой дистилляции, при паровой дистилляции используется резервуар для воды для пополнения запасов воды в нагретой смеси на протяжении всего процесса. Несмешивающийся органический компонент медленно перегоняется вместе с водой, в то время как нелетучий компонент остается в нагретой смеси. После дистилляции органического компонента его можно отделить от воды с помощью жидкостной экстракции.
<h4>Давление паров смеси</h4>
Для смешиваемой смеси, которая образует однородный раствор, давление пара каждого компонента зависит от давления пара чистого компонента и его молярной доли в жидкой смеси в соответствии с законом Рауля.
pA = pA*xA
где pA – давление паров одного компонента жидкости в смешиваемой жидкой смеси, pA* – давление пара чистой жидкости, а xA – молярная доля этой жидкости в смеси, которая равна nA/nt.n A – число молей отдельной жидкости в смеси, а nt — общее количество молей всех жидкостей в смеси.
Общее давление пара над смешиваемой жидкой смесью равно сумме парциального давления пара каждого компонента в ней, что известно как закон Дальтона. Давление паров жидкости увеличивается с температурой по мере того, как все больше молекул набирают кинетическую энергию для перехода жидкой фазы в газовую. В смешиваемой смеси, содержащей две жидкости, общее давление можно описать следующим образом:
P = pA + pB
где pA и pB – давление паров жидкости A жидкости B, соответственно, над смесью. P — общее давление паров обеих жидкостей над смесью. Объединение уравнений описывает соотношение между общим давлением пара раствора и молярной долей отдельных компонентов:
P = pA*xA + pB*xB
В несмешивающейся смеси, где компоненты образуют гетерогенную смесь, давление пара каждого компонента вносит свой вклад в общее давление пара независимо друг от друга. Таким образом, общее давление пара равно сумме отдельных давлений чистых паров. В несмешивающейся смеси, состоящей из двух жидкостей, общее давление определяется как давление пара первой жидкости плюс давление пара второй жидкости.
P = pA* + pB*
<h4>Температура кипения несмешивающейся смеси</h4>
По мере нагревания жидкости давление пара увеличивается. Каждый компонент в смеси имеет свою точку кипения. В смеси смешивающихся жидкостей кипение происходит при температуре между точками кипения составляющих жидкостей.
Для несмешивающейся смеси кипение происходит при гораздо более низкой температуре, чем точки кипения отдельных компонентов. Поскольку каждый отдельный компонент вносит свой вклад независимо, требуется меньше тепла для увеличения общего давления пара до атмосферного давления.
Для примера рассмотрим несмешивающуюся смесь бензола и воды. Температура кипения бензола при нормальном атмосферном давлении составляет 80,1 °С, а температура кипения воды при нормальном атмосферном давлении — 100 °С. Раствор закипает, когда общее давление пара достигает 760 мм рт.ст. (нормальное атмосферное давление). При температуре 69,3 °C давление паров воды составляет 227 мм рт. ст., а давление паров бензола — 533 мм рт. ст., что в сумме равно необходимым для кипения 760 мм рт. ст. Это значительно ниже точки кипения каждого отдельного компонента.
<h4>Ссылки</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Химия и химическая реакционная способность. Бельмонт, Калифорния: Брукс/Коул, Cengage Learning.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). Химия: молекулярная природа материи и изменения. Бостон, Массачусетс: Макгроу Хилл.</li>
</ol>

Паровая дистилляция 
Паровая дистилляция — это метод разделения, который использует свойство низкой температуры кипения несмешивающихся смесей. Он преимущественно ...

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Химия

steam distillation and low-boiling separation

Steam distillation is a separation method for temperature-sensitive organic compounds. It works best when the organic compound is immiscible in water, meaning the two liquids do not mix. The method helps remove the organic molecule from a non-volatile contaminant without using the higher temperatures needed for many other separations.

In steam distillation, the immiscible mixture is heated until both water and the volatile organic compound distill together. The vapor rises to a condenser, where it cools and becomes a liquid that can be collected. Unlike simple distillation, this setup uses a water reservoir to keep replacing water in the heated mixture during the process.

The organic component is carried over slowly with the water, while the non-volatile component stays behind in the heated flask. After distillation, the organic product can be separated from the water by liquid-liquid extraction. This makes steam distillation useful for isolating sensitive organic materials from unwanted impurities.

The same low-boiling behavior can be explained by vapor pressure. For a miscible mixture, each liquid follows Raoult’s law, so its vapor pressure depends on the pure liquid vapor pressure and its mole fraction. The mole fraction is the number of moles of one liquid divided by the total number of moles in the mixture. The total vapor pressure above a miscible mixture follows Dalton’s law and equals the sum of the partial vapor pressures.

As temperature rises, vapor pressure increases because more molecules gain enough kinetic energy to escape into the gas phase. In a two-liquid miscible mixture, the total pressure can be written as P = pA + pB, or as P = pA*xA + pB*xB when Raoult’s law is included. In an immiscible mixture, the components act independently, so the total pressure is the sum of the vapor pressure of the first liquid and the vapor pressure of the second liquid.

This independent vapor-pressure behavior is why immiscible mixtures boil at lower temperatures than either pure liquid. In a miscible mixture, boiling usually happens between the boiling points of the components. In an immiscible mixture, less heat is needed to reach atmospheric pressure, so the boiling point drops well below the boiling points of the individual liquids.

A benzene-water mixture shows this effect clearly. Benzene boils at 80.1 °C, and water boils at 100 °C at normal atmospheric pressure. The mixture boils when the total vapor pressure reaches 760 mm Hg. At 69.3 °C, water contributes 227 mm Hg and benzene contributes 533 mm Hg, which adds up to 760 mm Hg. That means the mixture boils at a temperature below either pure component.

Steam Distillation and Low-Boiling Separation

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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

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Steam Distillation and Low-Boiling Separation

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