Name: separating liquids by boiling point
Uploaded: 2020-03-26T07:22:28.000Z
Duration: 3 min 13 s
Channel: JoVE

Lorsqu’un liquide est chauffé, il gagne de l’énergie jusqu’à ce que le désordre accru de la phase gazeuse l’emporte sur les interactions intermoléculaires dans la phase liquide. Une fois qu’un nombre suffisant de molécules sont en phase gazeuse, elles s’échappent du liquide sous forme de bulles. Cet effet, appelé ébullition, se produit lorsque la pression de vapeur totale de la substance est égale à la pression atmosphérique.
La pression de vapeur est la pression de la vapeur en équilibre avec sa phase condensée, et elle varie avec la température. Dans un mélange de liquides, chaque composant a sa propre pression de vapeur, que nous appelons sa pression partielle. La pression de vapeur totale du mélange est égale à la somme des pressions partielles. Si les liquides sont miscibles, c’est-à-dire qu’ils forment toujours une solution homogène, la pression partielle de chaque composant est la pression de vapeur du composé pur à cette température multipliée par sa fraction molaire dans le liquide.
La température à laquelle la première bulle de vapeur commence à se former dans un mélange liquide est appelée le point de bulle. Pour un liquide pur, le point de bulle et la température à laquelle la vapeur commence à se condenser, ou point de rosée, sont les mêmes que le point d’ébullition. Cependant, pour un mélange de deux liquides miscibles, le point de bulle et le point de rosée se trouveront entre les points d’ébullition des composants.
Lorsque le mélange bout pour la première fois, la vapeur est riche en composé ayant le point d’ébullition le plus bas, ou le composé le plus volatil. Cela augmente la proportion du composé ayant le point d’ébullition le plus élevé, ou le composé le moins volatil, dans le mélange liquide.
La distillation est une technique de séparation qui tire parti de ce phénomène. Dans une distillation simple, un mélange liquide homogène est bouilli. La vapeur montante pénètre ensuite dans la chambre intérieure d’un condenseur refroidi à l’eau. La vapeur se condense en un liquide, appelé distillat, qui est ensuite recueilli dans un récipient séparé.
Au fur et à mesure que l’ébullition se poursuit, les compositions du liquide et de la vapeur changent à mesure que le composant le plus volatil est éliminé. Ainsi, si nous recueillons le distillat en petites fractions, nous verrons que chaque fraction contient les composés dans un rapport molaire différent.
À mesure que la proportion du composant le moins volatil dans le mélange liquide augmente, le point de bulle et le point de rosée augmentent également. En traçant les points de bulle et de rosée du mélange en fonction des fractions molaires des composants, on obtient un diagramme du point d'ébullition. Une fois que nous avons ce schéma, nous pouvons utiliser la courbe du point de rosée pour déterminer la composition de la vapeur à une température donnée.
Dans ce laboratoire, vous mettrez en place et effectuerez la distillation simple d’un mélange de cyclohexane et de toluène et enregistrerez la température de la vapeur tout au long de l’expérience. Vous utiliserez ensuite le diagramme de point d'ébullition publié pour le cyclohexane et le toluène pour déterminer la composition de la vapeur, ce qui vous permettra d'estimer la composition du mélange liquide tout au long de la distillation.

simple distillation

Simple Distillation

<h4>Distillation d’un mélange miscible</h4>
Certains liquides s’évaporeront entièrement au fil du temps s’ils sont laissés dans un récipient ouvert à température ambiante. Cependant, ce processus d’évaporation peut être considérablement accéléré si le liquide est chauffé. Au fur et à mesure que le liquide est chauffé, les molécules qu’il contient acquièrent l’énergie nécessaire pour s’échapper de la phase liquide et passer à la phase gazeuse sous forme de bulles. Ce phénomène s’appelle l’ébullition.
<h4>Pression de vapeur</h4>
Considérez un récipient fermé de liquide. Dans un premier temps, une partie de ce liquide s’évapore, mais seulement jusqu’à ce que le taux de vaporisation soit égal au taux de condensation. Une fois ce point atteint, il n’y a plus de changement dans le système et le liquide et la vapeur sont à l’équilibre. Une fois que cela a été établi, la pression exercée par la vapeur au-dessus du liquide est appelée pression de vapeur. La tendance d’un liquide à se vaporiser s’appelle sa volatilité. Un liquide plus volatil a une pression de vapeur plus élevée, tandis qu’un liquide moins volatil a une pression de vapeur plus faible.
Lorsqu’un récipient ouvert de liquide est chauffé, une plus grande partie du liquide s’évapore. Si suffisamment de chaleur est ajoutée, des bulles de vapeur se forment dans le liquide et le liquide bout. La température à laquelle la pression de vapeur du liquide est la même que la pression atmosphérique est connue sous le nom de point d’ébullition.
Pour une substance pure, la pression de vapeur est simple à déterminer. Qu’en est-il d’un mélange de différents liquides ? Si les liquides purs forment une solution homogène miscible, chacun d’eux contribuera à la pression de vapeur totale sous forme de pressions partielles. La pression totale d’un mélange de gaz est égale à la somme des pressions individuelles que chaque gaz exercerait isolément. Cette règle est connue sous le nom de loi de Dalton. Par conséquent, pour déterminer la pression de vapeur totale du mélange, vous devez connaître les pressions de vapeur des substances pures et la contribution molaire de chaque liquide au mélange total, une valeur connue sous le nom de fraction molaire. Cette relation est connue sous le nom de loi de Raoult :
pA = pA* xA
où pA est la pression de vapeur d’un composant liquide dans un mélange, pA* est la pression de vapeur du liquide pur isolé, qui peut être référencé dans la littérature, et xA est la fraction molaire du composant liquide dans le mélange liquide. Les fractions molaires sont calculées en divisant le nombre de moles du composant liquide par le nombre total de moles de chaque composant dans le mélange liquide.
La loi de Dalton sur les pressions partielles peut être appliquée une fois que vous connaissez la pression de vapeur de chaque composant individuel.
P = pA + pB
La pression totale (P) est la somme de la pression de vapeur des deux liquides au-dessus du mélange, où pA et pB sont les pressions de vapeur des liquides A et B, respectivement, au-dessus du mélange.
<h4>point d'ébullition</h4>
La température à laquelle une substance organique pure passe de la phase liquide à la phase gazeuse est connue sous le nom de point d’ébullition. Dans un mélange de liquides miscibles, la solution bout lorsque la pression de vapeur totale de la solution est égale à la pression atmosphérique. Ainsi, le point d'ébullition d'un mélange se produit à une température comprise entre les points d'ébullition des deux liquides purs.
Lorsque le mélange est chauffé jusqu’à son point d’ébullition, certaines molécules s’échappent de la phase liquide et entrent dans la phase gazeuse. La température à laquelle les premières bulles commencent à se former dans une solution miscible qui est chauffée est la température du point de bulle. Dans le cas d’un liquide pur, le point de bulle est le même que le point d’ébullition.
La phase gazeuse est riche en molécules du composant le plus volatil, ou du composant avec la pression de vapeur la plus élevée et le point d’ébullition le plus bas. Le nombre de molécules qui s’évaporent augmente à mesure que la chaleur est appliquée. Ainsi, la phase liquide est riche en molécules du composant le moins volatil, ou du composant avec la pression de vapeur la plus basse et le point d’ébullition plus élevé. La température à laquelle les premières gouttes de liquide commencent à se former pendant la distillation est connue sous le nom de température du point de rosée.
<h4>Diagramme d’équilibre vapeur-liquide</h4>
Un diagramme d’équilibre vapeur-liquide est un graphique de la température d’équilibre de la fraction molaire des composants d’un mélange binaire, avec une courbe tracée pour la phase liquide et la phase vapeur. L’axe des x représente la fraction molaire de chacun des deux composants du mélange, et l’axe des y est la température. Ces graphiques sont disponibles dans la littérature pour les mélanges courants et peuvent être utilisés pour identifier les températures du point d’ébullition d’un mélange en fonction de la fraction molaire de chaque composant. Ils sont également utilisés pour déterminer la composition de chaque phase d’une expérience de distillation.
<h4>distillation </h4>
La distillation est une technique de séparation qui tire parti des propriétés du point d’ébullition des mélanges. Pour effectuer la distillation, un mélange miscible de deux liquides avec une différence significative de points d’ébullition - au moins 20 °C - est chauffé. Au fur et à mesure que la solution se réchauffe et atteint le point de bulle du composant le plus volatil, certaines molécules du composant le plus volatil passent en phase gazeuse et remontent dans le condenseur. Le condenseur est un tube de verre avec des sections intérieures et extérieures séparées. La vapeur se déplace dans la section intérieure du condenseur, où elle est condensée en liquide par l’eau froide qui s’écoule dans la section extérieure du condenseur. Cette vapeur condensée est appelée distillat, et elle est recueillie dans un cylindre gradué ou un tube à essai.
Au fur et à mesure que la distillation progresse, la température nécessaire pour faire bouillir la solution augmente car le composant le plus volatil s’évapore plus tôt. Ainsi, la composition du distillat change avec le temps. Au début de la distillation, le distillat est riche en composants les plus volatils ; Au milieu de la distillation, le distillat contient un mélange des deux composants ; Et à la fin de la distillation, le distillat est riche en composant moins volatil.
Le diagramme d’équilibre vapeur-liquide montre l’évolution de la composition du liquide dans le ballon et du distillat au cours de la distillation. Il y a deux courbes sur le graphique ; La courbe inférieure décrit le point d’ébullition du liquide dans le ballon en fonction de sa composition, tandis que la courbe supérieure décrit la température de la vapeur en fonction de sa composition. Par extension, la courbe supérieure décrit la composition du distillat.
Un diagramme d’équilibre vapeur-liquide publié dans la littérature peut être utilisé pour identifier la composition du liquide et de la vapeur à une température donnée pendant l’expérience. Cela peut aider à déterminer quand terminer la distillation pour séparer les deux composants.
<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>
	<li style="font-size :17px">Felder, R.M., Rousseau, R.W. (2000). Principes élémentaires des procédés chimiques. New York, NY : Wiley and Sons, Inc.</li>
</ol>

Distillation d’un mélange miscible
Certains liquides s’évaporeront entièrement au fil du temps s’ils sont laissés dans un récipient ouvert à température ambiante. Cependant, ce ...

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separating liquids by boiling point

Separating liquids by boiling point is the goal of distillation. A miscible mixture contains liquids that mix evenly, and it can be heated so that the more volatile liquid evaporates first. Volatility is a liquid’s tendency to vaporize, and boiling begins when bubbles form in the liquid phase.

In a closed container, liquid and vapor can reach equilibrium, which means the rate of vaporization equals the rate of condensation. The pressure from the vapor above the liquid is called vapor pressure. A more volatile liquid has a higher vapor pressure, while a less volatile liquid has a lower vapor pressure. When an open container is heated, the liquid boils at the temperature where its vapor pressure matches atmospheric pressure.

For a mixture of miscible liquids, each component contributes to the total vapor pressure as a partial pressure. Dalton’s law says the total pressure is the sum of the individual pressures, and Raoult’s law links each component’s vapor pressure to its pure vapor pressure and mole fraction. The mole fraction is the number of moles of one liquid divided by the total moles in the mixture. For two liquids, the total vapor pressure is the sum of the partial pressures of both components.

A mixture boils at a temperature between the boiling points of the two pure liquids. The first bubbles that form define the bubble point temperature. In a pure liquid, the bubble point and boiling point are the same. During heating, the vapor becomes richer in the more volatile component, while the liquid left behind becomes richer in the less volatile component. The first drops of liquid that form during distillation occur at the dew point temperature.

Vapor-liquid equilibrium diagrams help track these changes. These plots show temperature against mole fraction for a binary mixture, with one curve for the liquid phase and one for the vapor phase. They are available in the literature for common mixtures and can be used to estimate boiling temperatures and phase composition at a given mole fraction. They also help show how the liquid in the flask and the distillate change during the experiment.

In distillation, a miscible mixture with a significant boiling point difference, usually at least 20 °C, is heated. As the more volatile component reaches its bubble point, vapor rises into a condenser. The condenser has inner and outer sections, and cold water in the outer section cools the vapor back into liquid. This liquid is the distillate, and it is collected in a graduated cylinder or test tube.

Separating Liquids by Boiling Point

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Separating liquids by boiling point is the goal of distillation. A miscible mixture contains liquids that mix evenly, and it can be heated so that the more volatile liquid evaporates first. Volatility is a liquid’s tendency to vaporize, and boiling begins when bubbles form in the liquid phase. In a closed container, liquid and vapor can reach equilibrium, which means the rate of vaporization equals the rate of condensation. The pressure from the vapor above the liquid is called vapor pressure.

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Suivez les procédures étape par étape de Simple Distillation dans cette vidéo de protocole étudiant de 7 minutes. Fait partie de la collection Chemistry sur JoVE Lab Manual.

Protocole étudiant de Simple Distillation dans Chemistry - 57K+ vues | JoVE Lab Manual

Separating Liquids by Boiling Point

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