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

Lorsqu’un mélange idéal de deux liquides miscibles est chauffé à ébullition, la solution bout à une température comprise entre les points d’ébullition de chaque composant. Si ces liquides ont des points d’ébullition très différents, lorsque le mélange commence à bouillir, la vapeur est riche en molécules du composant le plus volatil. Ce phénomène est souvent utilisé pour séparer les mélanges à l’aide d’une distillation simple, où un mélange de deux liquides miscibles est chauffé et la vapeur est ensuite condensée en liquide et recueillie.
Au fur et à mesure que la vapeur riche en composant le plus volatil est collectée sous forme de distillat, la phase liquide devient riche en molécules du composant le moins volatil. Cependant, cette technique nécessite que la solution soit chauffée au moins jusqu’au point d’ébullition du composé le plus volatil et souvent au-delà.
Dans le cas des composés organiques sensibles à la température, cette température élevée pourrait entraîner la décomposition des molécules organiques en quelque chose d’autre. Alors, comment pouvons-nous séparer ces types de composés ? Tout d'abord, prenons un peu de recul.
Rappelons que la pression d’une vapeur en équilibre avec sa phase condensée est appelée pression de vapeur. Les composants d’un mélange de liquides ont chacun leur propre pression de vapeur, que nous appelons leur pression partielle. Nous savons qu’une solution bout lorsque la pression de vapeur totale de la solution est égale à la pression atmosphérique. La pression de vapeur totale est égale à la somme des pressions partielles des composants.
Pour un mélange de liquides miscibles, c’est-à-dire que toute combinaison de liquides forme une solution homogène, les pressions partielles sont calculées à partir des pressions de vapeur des composés purs multipliées par leurs fractions molaires dans la solution. Cependant, pour un mélange hétérogène de liquides non miscibles, c’est-à-dire que les liquides sont insolubles les uns dans les autres, les pressions partielles sont simplement les pressions de vapeur des composés purs.
Étant donné que chaque composant du mélange hétérogène contribue à la pression de vapeur totale indépendamment des autres composants, le mélange bout lorsque la pression de vapeur totale, qui est la somme des pressions partielles, est égale à la pression atmosphérique. Cela se produit à une température plus basse que les points d’ébullition individuels de chaque composant, car la pression de vapeur totale augmente avec la température beaucoup plus rapidement que ce à quoi on pourrait s’attendre, même pour le composant le plus volatil.
Nous pouvons exploiter ce phénomène pour effectuer la distillation à la vapeur d’eau, qui est utilisée pour isoler un composé organique sensible à la température qui se décompose à haute température et est insoluble dans l’eau à partir de substances non volatiles. La configuration de distillation à la vapeur est similaire à une configuration de distillation simple avec l’ajout d’un réservoir d’eau pour reconstituer l’eau tout au long du processus.
Au fur et à mesure que le mélange bout, l’eau et le composé organique d’intérêt sont vaporisés. L’eau et les vapeurs de composés organiques se déplacent dans le condenseur, sont condensées en liquide et collectées. Les liquides non miscibles sont ensuite séparés. Seuls de l’eau et des matériaux non volatils sont laissés dans le mélange dans le ballon.
Dans ce laboratoire, vous mettrez en place et effectuerez une expérience de distillation à la vapeur pour extraire l’huile essentielle des composants non volatils d’une peau d’orange. Vous utiliserez ensuite l'extraction liquide-liquide pour extraire l'huile essentielle de l'eau en un solvant organique.

distillation à la vapeur

Steam Distillation

<h4>Distillation à la vapeur </h4>
La distillation à la vapeur est une technique de séparation qui exploite la propriété du point d’ébullition bas des mélanges non miscibles. Il est principalement utilisé pour séparer les molécules organiques sensibles à la température d’un contaminant non volatil. La molécule organique doit être non miscible dans l’eau.
Dans la distillation à la vapeur d’eau, le mélange non miscible est chauffé à ébullition, provoquant la distillation de l’eau et des composés organiques volatils. Cela signifie que le mélange gazeux se déplace vers le haut vers un condenseur, qui condense ensuite la vapeur en liquide afin qu’elle puisse être collectée. Contrairement à la distillation simple, la distillation à la vapeur utilise un réservoir d’eau pour reconstituer l’eau du mélange chauffé tout au long du processus. Le composant organique non miscible est distillé lentement avec l’eau, tandis que le composant non volatil reste dans le mélange chauffé. Une fois le composant organique distillé, il peut ensuite être séparé de l’eau par extraction liquide-liquide.
<h4>Pression de vapeur d’un mélange</h4>
Pour un mélange miscible qui forme une solution homogène, la pression de vapeur de chaque composant dépend de la pression de vapeur du composant pur et de sa fraction molaire dans le mélange liquide selon la loi de Raoult.
pA = pA*xA
où pA est la pression de vapeur d’un composant liquide dans un mélange liquide miscible, pA* est la pression de vapeur du liquide pur, et xA est la fraction molaire de ce liquide dans le mélange, qui est égale à nA/nt. nA est le nombre de moles du liquide individuel dans le mélange, et nt est le nombre total de moles de tous les liquides dans le mélange.
La pression de vapeur totale au-dessus du mélange de liquide miscible est égale à la somme de la pression de vapeur partielle de chaque composant de celui-ci, ce qui est connu sous le nom de loi de Dalton. La pression de vapeur d’un liquide augmente avec la température à mesure que davantage de molécules gagnent de l’énergie cinétique pour échapper de la phase liquide à la phase gazeuse. Dans un mélange miscible contenant deux liquides, la pression totale peut être décrite comme suit :
P = pA + pB
où pA et pB sont les pressions de vapeur du liquide A liquide B, respectivement, au-dessus du mélange. P est la pression de vapeur totale des deux liquides au-dessus du mélange. La combinaison des équations décrit la relation entre la pression de vapeur totale de la solution et la fraction molaire des composants individuels :
P = pA*xA + pB*xB
Dans un mélange non miscible, où les composants forment un mélange hétérogène, les pressions de vapeur de chaque composant contribuent indépendamment à la pression de vapeur totale. Ainsi, la pression de vapeur totale est égale à la somme des pressions de vapeur pure individuelles. Dans un mélange non miscible composé de deux liquides, la pression totale est définie comme la pression de vapeur du premier liquide plus la pression de vapeur du second liquide.
P = pA* + pB*
<h4>Point d’ébullition d’un mélange non miscible</h4>
Lorsqu’un liquide est chauffé, la pression de vapeur augmente. Chaque composant d’un mélange a son propre point d’ébullition. Dans un mélange de liquides miscibles, l’ébullition se produit à une température comprise entre les points d’ébullition des liquides constitutifs.
Pour un mélange non miscible, l’ébullition se produit à une température beaucoup plus basse que les points d’ébullition des composants individuels. Comme chaque composant individuel contribue indépendamment, moins de chaleur est nécessaire pour augmenter la pression de vapeur totale à la pression atmosphérique.
Par exemple, considérez le mélange non miscible de benzène et d’eau. Le point d’ébullition du benzène à pression atmosphérique normale est de 80,1 °C et le point d’ébullition de l’eau à pression atmosphérique normale est de 100 °C. La solution bout lorsque la pression de vapeur totale atteint 760 mm Hg (pression atmosphérique normale). À 69,3 °C, la pression de vapeur de l’eau est de 227 mm Hg et la pression de vapeur du benzène est de 533 mm Hg, ce qui équivaut au total aux 760 mm Hg nécessaires pour faire bouillir. C’est bien en dessous du point d’ébullition de l’un ou l’autre des composants individuels.
<h4>références</h4>
<ol>
	<li style="font-size :17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chimie et réactivité chimique. Belmont, Californie : Brooks/Cole, Cengage Learning.</li>
	<li style="font-size :17px">Silderberg, M.S. (2009). Chimie : la nature moléculaire de la matière et du changement. Boston, MA : McGraw Hill.</li>
</ol>

Distillation à la vapeur

Distillation à la vapeur 
La distillation à la vapeur est une technique de séparation qui exploite la propriété du point d’ébullition bas des mélanges non miscibles. Il s’agit principalement ...

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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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Learn how steam distillation separates temperature-sensitive organic compounds from water-insoluble mixtures using vapor pressure and low boiling points.

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Steam Distillation and Low-Boiling Separation

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