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

Los líquidos se mantienen unidos por interacciones débiles entre sus moléculas. Para pasar a la fase gaseosa, una molécula debe obtener suficiente energía del calor u otras fuentes para superar estas interacciones.
Consideremos un recipiente cerrado de un líquido puro a temperatura ambiente. Algunas de las moléculas de fase líquida en la superficie ganan suficiente energía para evaporarse, y parte de ese vapor pierde energía y se condensa de nuevo en líquido. Cuando las tasas de evaporación y condensación son las mismas, el líquido y el vapor alcanzan el equilibrio. En este punto, el espacio sobre el líquido está saturado de vapor y no hay cambio neto en la cantidad de líquido y vapor en el recipiente. La presión ejercida por el vapor se denomina presión de vapor.
Ahora, considere un recipiente abierto de líquido. Aquí el vapor puede escapar, evitando que el área por encima del líquido se sature. Por lo tanto, la evaporación podría continuar hasta que no quede líquido. A medida que aumenta la temperatura de un líquido, también lo hace la tasa de evaporación y, por lo tanto, también lo hace la presión de vapor. Si se agrega suficiente calor, las moléculas más profundas en el líquido comienzan a vaporizarse, lo que vemos como burbujas de vapor formándose.
Esto se llama ebullición y comienza cuando la presión de vapor del compuesto es igual a la presión atmosférica. La temperatura a la que esto ocurre es el punto de ebullición. Dado que cada sustancia pura tiene diferentes tipos y fuerzas de interacciones intermoleculares, diferentes sustancias tienen diferentes puntos de ebullición.
Ahora, echemos un vistazo a cómo las interacciones intermoleculares afectan el punto de ebullición. Muchas interacciones intermoleculares involucran áreas con densidad de electrones desigual llamadas dipolos. Todas las moléculas tienen fluctuaciones breves en su distribución de electrones, por lo que incluso las moléculas no polares pueden tener dipolos temporalmente. En respuesta, se forman dipolos opuestos en moléculas cercanas.
Las fuerzas de atracción entre los dipolos temporales e inducidos se denominan fuerzas de dispersión de Londres, y son las interacciones dominantes entre moléculas no polares como los hidrocarburos. Las fuerzas de dispersión de London son más fuertes entre moléculas grandes con muchos electrones porque esas moléculas mantienen sus electrones débilmente, lo que facilita que las moléculas vecinas afecten su distribución de electrones.
Por ejemplo, un alcano no ramificado de alto peso molecular tiene un punto de ebullición más alto que un alcano no ramificado de menor peso molecular. Además, un alcano no ramificado tiene un punto de ebullición más alto que un alcano ramificado con un peso molecular comparable debido a su mayor área superficial.
Las moléculas con enlaces polares tienen dipolos permanentes, lo que permite interacciones dipolo-dipolo regulares entre ellas. Las interacciones dipolo-dipolo son más fuertes que las fuerzas de dispersión de Londres, por lo que generalmente se necesita más energía térmica para hervir líquidos polares que líquidos no polares. Por ejemplo, el punto de ebullición de un cloropentano, una molécula polar, es de 108 °C. El hexano, que es similar en superficie pero carece de dipolo, tiene un punto de ebullición más bajo.
Las moléculas con un hidrógeno unido covalentemente a un átomo más electronegativo y un átomo que retira electrones con un par solitario de electrones son capaces de formar enlaces de hidrógeno. Los enlaces de hidrógeno son más fuertes que las interacciones dipolo-dipolo y las fuerzas de dispersión de Londres, por lo que se necesita aún más energía térmica para superarlos. Por ejemplo, el propano y el etanol tienen pesos moleculares y áreas superficiales similares. Pero el punto de ebullición del propano es mucho más bajo que el del etanol porque las moléculas de etanol pueden formar enlaces de hidrógeno entre sí, mientras que el propano no.
En este laboratorio, utilizará el método capilar para determinar el punto de ebullición de dos solventes orgánicos.

puntos de ebullición

Determinación del punto de ebullición
Al igual que el punto de fusión, el punto de ebullición es una propiedad física. Si la muestra es un compuesto puro, entonces el punto de ebullición se puede usar para determinar la identidad del compuesto. En última instancia, determinar experimentalmente el punto de ebullición exacto es un desafío. Al igual que los puntos de fusión, los puntos de ebullición experimentales se dan como un rango y varían en unos pocos grados del valor real de la literatura.
presión de vapor
Para entender por qué hierve un disolvente, que se caracteriza por el conocido burbujeo de la solución, es importante comprender la dinámica entre las fases líquida y gaseosa. Considere un compuesto líquido puro en un recipiente sellado. Algunas de las moléculas en la superficie del líquido tendrán suficiente energía para superar las fuerzas intermoleculares y entrar en la fase gaseosa. Sin embargo, las moléculas en la fase gaseosa también pueden perder energía y condensarse de nuevo en un líquido. Por lo tanto, hay dos procesos que compiten en este sistema: evaporación y condensación.
Cuando la tasa de evaporación es igual a la tasa de condensación, el sistema ha alcanzado un estado de equilibrio. Esto significa que por cada molécula que entra en la fase gaseosa, otra se condensa en la fase líquida, y no hay ganancia o pérdida neta de la cantidad de líquido o gas en el recipiente. Una vez que se ha establecido el equilibrio, 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.
La presión de vapor varía según la temperatura. Si aumenta la temperatura de la solución, más moléculas tienen suficiente energía para escapar de la fase líquida y, por lo tanto, aumenta la presión de vapor. En última instancia, si se aplica suficiente calor, las moléculas que no están en la interfaz entre el líquido y el gas pasarán a la fase gaseosa y formarán las burbujas familiares que asociamos con la ebullición.
El punto de ebullición de un líquido se alcanza cuando la presión total de vapor del líquido es equivalente a la presión atmosférica. La temperatura a la que esto ocurre se denomina punto de ebullición. A elevaciones más altas y, por lo tanto, a una presión atmosférica más baja, un líquido hervirá a una temperatura más baja, ya que se requiere menos calor para aumentar la presión de vapor a la presión atmosférica. Además, la volatilidad, o la capacidad de un solvente para vaporizarse, también afecta la presión de vapor. Los disolventes con alta volatilidad tienen una presión de vapor más alta que los disolventes con menor volatilidad.
Factores que influyen en el punto de ebullición
La similitud entre los puntos de fusión y los puntos de ebullición significa que los mismos factores que afectan el punto de fusión de un compuesto también afectarán el punto de ebullición. Por lo tanto, la fuerza y los tipos de fuerzas intermoleculares que se encuentran dentro del compuesto líquido afectarán el punto de ebullición. Recordemos que hay tres tipos de fuerzas moleculares: enlaces de hidrógeno, interacciones dipolo-dipolo y fuerzas de dispersión de London. Cada uno de estos tiene diferentes fuerzas de atracción y requiere diferentes cantidades de energía para superarlos. Los compuestos que pueden enlazar hidrógeno tendrán puntos de ebullición más altos que los compuestos que solo pueden interactuar a través de las fuerzas de dispersión de London. Una consideración adicional para los puntos de ebullición involucra la presión de vapor y la volatilidad del compuesto. Por lo general, cuanto más volátil es un compuesto, menor es su punto de ebullición.
El método capilar para determinar el punto de ebullición
Un método simple para determinar el punto de ebullición de un compuesto orgánico es utilizar el método capilar. En esta configuración, un tubo capilar de vidrio vacío se invierte en un recipiente del compuesto puro en la fase líquida. A medida que se calienta el líquido, la presión de vapor de la muestra aumenta y el vapor gaseoso comienza a ingresar al tubo capilar de vidrio. Esto fuerza la salida del aire atrapado en el interior y da lugar a burbujas que emergen del fondo del tubo capilar. En este punto, se deja que el líquido se enfríe. Una vez que la presión de vapor de la muestra es la misma que la presión atmosférica dentro del tubo capilar de vidrio, el líquido comenzará a ingresar al tubo. La temperatura de la solución cuando se produce este fenómeno es el punto de ebullición del compuesto líquido.

Puntos de ebullición

Determinación del punto de ebullición
Al igual que el punto de fusión, el punto de ebullición es una propiedad física. Si la muestra es un compuesto puro, entonces la ebullición ...

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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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