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

A medida que un líquido se calienta, gana energía hasta que el aumento del desorden de la fase gaseosa supera las interacciones intermoleculares en la fase líquida. Una vez que hay suficientes moléculas en la fase gaseosa, escapan del líquido en forma de burbujas. Este efecto, llamado ebullición, ocurre cuando la presión total de vapor de la sustancia es igual a la presión atmosférica.
La presión de vapor es la presión del vapor en equilibrio con su fase condensada, y varía con la temperatura. En una mezcla de líquidos, cada componente tiene su propia presión de vapor, que llamamos presión parcial. La presión total de vapor de la mezcla es igual a la suma de las presiones parciales. Si los líquidos son miscibles, es decir, siempre forman una solución homogénea, la presión parcial de cada componente es la presión de vapor del compuesto puro a esa temperatura multiplicada por su fracción molar en el líquido.
La temperatura a la que comienza a formarse la primera burbuja de vapor en una mezcla líquida se denomina punto de burbuja. Para un líquido puro, tanto el punto de burbuja como la temperatura a la que el vapor comienza a condensarse, o el punto de rocío, son los mismos que el punto de ebullición. Sin embargo, para una mezcla de dos líquidos miscibles, tanto el punto de burbuja como el punto de rocío estarán entre los puntos de ebullición de los componentes.
Cuando la mezcla hierve por primera vez, el vapor es rico con el compuesto con el punto de ebullición más bajo, o el compuesto más volátil. Esto aumenta la proporción del compuesto con el punto de ebullición más alto, o el compuesto menos volátil, en la mezcla líquida.
La destilación es una técnica de separación que aprovecha este fenómeno. En una destilación simple, se hierve una mezcla líquida homogénea. A continuación, el vapor ascendente entra en la cámara interior de un condensador refrigerado por agua. El vapor se condensa en un líquido, llamado destilado, que luego se recoge en un recipiente separado.
A medida que continúa la ebullición, las composiciones del líquido y el vapor cambian a medida que se elimina el componente más volátil. Así, si recogemos el destilado en fracciones pequeñas, veremos que cada fracción contiene los compuestos en una proporción molar diferente.
A medida que aumenta la proporción del componente menos volátil en la mezcla líquida, también lo hacen el punto de burbuja y el punto de rocío. Al trazar los puntos de burbuja y rocío de la mezcla frente a las fracciones molares de los componentes, se obtiene un diagrama de punto de ebullición. Una vez que tenemos este diagrama, podemos usar la curva del punto de rocío para determinar la composición del vapor a una temperatura dada.
En este laboratorio, configurará y realizará la destilación simple de una mezcla de ciclohexano y tolueno y registrará la temperatura del vapor durante todo el experimento. A continuación, utilizará el diagrama de punto de ebullición publicado para el ciclohexano y el tolueno para determinar la composición del vapor, lo que le permitirá estimar la composición de la mezcla líquida a lo largo de la destilación.

destilación simple

Simple Distillation

<h4>Destilación de una mezcla miscible</h4>
Algunos líquidos se evaporarán por completo con el tiempo si se dejan en un recipiente abierto a temperatura ambiente. Sin embargo, este proceso de evaporación puede acelerarse significativamente si el líquido se calienta. A medida que el líquido se calienta, las moléculas que contiene ganan la energía necesaria para escapar de la fase líquida y pasar a la fase gaseosa en forma de burbujas. Este fenómeno se llama ebullición.
<h4>presión de vapor</h4>
Considere un recipiente cerrado de líquido. Inicialmente, parte de este líquido se evapora, pero solo hasta que la tasa de vaporización es igual a la tasa de condensación. Una vez alcanzado este punto, no hay más cambios en el sistema, y el líquido y el vapor están en equilibrio. Una vez que esto se ha establecido, la presión ejercida por el vapor sobre el líquido se denomina presión de vapor. La tendencia de un líquido a vaporizarse se denomina volatilidad. Un líquido más volátil tiene una presión de vapor más alta, mientras que un líquido menos volátil tiene una presión de vapor más baja.
Cuando se calienta un recipiente abierto de líquido, se evapora una mayor cantidad de líquido. Si se agrega suficiente calor, se forman burbujas de vapor en el líquido y el líquido hierve. La temperatura a la que la presión de vapor del líquido es la misma que la presión atmosférica se conoce como punto de ebullición.
Para una sustancia pura, la presión de vapor es fácil de determinar. ¿Qué tal una mezcla de diferentes líquidos? Si los líquidos puros forman una solución homogénea miscible, cada uno contribuirá a la presión total de vapor como presiones parciales. La presión total del gas de una mezcla de gases es igual a la suma de las presiones individuales que cada gas ejercería de forma aislada. Esta regla se conoce como la ley de Dalton. Por lo tanto, para determinar la presión total de vapor de la mezcla, debe conocer las presiones de vapor de las sustancias puras y la contribución molar de cada líquido a la mezcla total, un valor conocido como fracción molar. Esta relación se conoce como la ley de Raoult:
pA = pA* xA
donde pA es la presión de vapor de un componente líquido en una mezcla, pA* es la presión de vapor del líquido puro aislado, que se puede referenciar en la literatura, y xA es la fracción molar del componente líquido en la mezcla líquida. Las fracciones molares se calculan dividiendo el número de moles del componente líquido entre el número total de moles de cada componente en la mezcla líquida.
La ley de Dalton de presiones parciales se puede aplicar una vez que se conoce la presión de vapor de cada componente individual.
P = pA + pB
La presión total (P) es la suma de la presión de vapor de ambos líquidos por encima de la mezcla, donde pA y pB son las presiones de vapor de los líquidos A y B, respectivamente, por encima de la mezcla.
<h4>punto de ebullición</h4>
La temperatura a la que una sustancia orgánica pura cambia de la fase líquida a la fase gaseosa se conoce como punto de ebullición. En una mezcla de líquidos miscibles, la solución hierve cuando la presión total de vapor de la solución es igual a la presión atmosférica. Por lo tanto, el punto de ebullición de una mezcla se produce a una temperatura entre los puntos de ebullición de los dos líquidos puros.
A medida que la mezcla se calienta hasta su punto de ebullición, algunas de las moléculas escapan de la fase líquida y entran en la fase gaseosa. La temperatura a la que comienzan a formarse las primeras burbujas en una solución miscible que se está calentando es la temperatura del punto de burbuja. En el caso de un líquido puro, el punto de burbuja es el mismo que el punto de ebullición.
La fase gaseosa es rica en moléculas del componente más volátil, o el componente con la mayor presión de vapor y el punto de ebullición más bajo. El número de moléculas que se evaporan aumenta a medida que se aplica más calor. Por lo tanto, la fase líquida es rica en moléculas del componente menos volátil, o el componente con la presión de vapor más baja y el punto de ebullición más alto. La temperatura a la que comienzan a formarse las primeras gotas de líquido durante la destilación se conoce como temperatura del punto de rocío.
<h4>Diagrama de equilibrio vapor-líquido</h4>
Un diagrama de equilibrio vapor-líquido es un gráfico de la temperatura de equilibrio de la fracción molar de los componentes de una mezcla binaria, con una curva dibujada para la fase líquida y la fase de vapor. El eje x representa la fracción molar de cada uno de los dos componentes de la mezcla, y el eje y es la temperatura. Estos gráficos están disponibles en la literatura para mezclas comunes y se pueden utilizar para identificar las temperaturas del punto de ebullición de una mezcla dada la fracción molar de cada componente. También se utilizan para determinar la composición de cada fase en un experimento de destilación.
<h4>destilación </h4>
La destilación es una técnica de separación que aprovecha las propiedades del punto de ebullición de las mezclas. Para realizar la destilación, se calienta una mezcla miscible de dos líquidos con una diferencia significativa en los puntos de ebullición, al menos 20 °C. A medida que la solución se calienta y alcanza el punto de burbuja del componente más volátil, algunas moléculas del componente más volátil pasan a la fase gaseosa y viajan hacia arriba en el condensador. El condensador es un tubo de vidrio con secciones internas y externas separadas. El vapor viaja a la sección interna del condensador, donde se condensa en líquido por el agua fría que fluye en la sección exterior del condensador. Este vapor condensado se llama destilado y se recoge en un cilindro graduado o tubo de ensayo.
A medida que avanza la destilación, la temperatura necesaria para hervir la solución aumenta a medida que el componente más volátil hierve antes. Por lo tanto, la composición del destilado cambia con el tiempo. Al principio de la destilación, el destilado es rico en el componente más volátil; En medio de la destilación, el destilado contiene una mezcla de los dos componentes; y al final de la destilación, el destilado es rico con el componente menos volátil.
El diagrama de equilibrio vapor-líquido muestra el cambio tanto en la composición del líquido en el matraz como en el destilado en el transcurso de la destilación. Hay dos curvas en la parcela; La curva inferior describe el punto de ebullición del líquido en el matraz en relación con su composición, mientras que la curva superior describe la temperatura del vapor en relación con su composición. Por extensión, la curva superior describe la composición del destilado.
Un diagrama de equilibrio vapor-líquido publicado en la literatura se puede utilizar para identificar la composición del líquido y el vapor a una temperatura dada durante el experimento. Esto puede ayudar a determinar cuándo finalizar la destilación para separar los dos componentes.
<h4>Referencias</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Química y reactividad química. Belmont, CA: Brooks/Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). Química: La naturaleza molecular de la materia y el cambio. Boston, MA: McGraw Hill.</li>
	<li style="font-size:17px">Felder, R.M., Rousseau, R.W. (2000). Principios elementales de los procesos químicos. Nueva York, NY: Wiley and Sons, Inc.</li>
</ol>

Destilación simple

Destilación de una mezcla miscible
Algunos líquidos se evaporarán por completo con el tiempo si se dejan en un recipiente abierto a temperatura ambiente. Sin embargo, esto ...

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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

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