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

Os líquidos são mantidos juntos por interações fracas entre suas moléculas. Para fazer a transição para a fase gasosa, uma molécula deve obter energia suficiente do calor ou de outras fontes para superar essas interações.
Vamos considerar um recipiente fechado de um líquido puro à temperatura ambiente. Algumas das moléculas da fase líquida na superfície ganham energia suficiente para evaporar, e parte desse vapor perde energia e se condensa de volta ao líquido. Quando as taxas de evaporação e condensação são as mesmas, o líquido e o vapor atingem o equilíbrio. Neste ponto, o espaço acima do líquido está saturado de vapor e não há alteração líquida na quantidade de líquido e vapor no recipiente. A pressão exercida pelo vapor é chamada de pressão de vapor.
Agora, considere um recipiente aberto de líquido. Aqui o vapor pode escapar, evitando que a área acima do líquido fique saturada. Assim, a evaporação pode continuar até que não haja mais líquido. À medida que a temperatura de um líquido aumenta, também aumenta a taxa de evaporação e, portanto, a pressão de vapor. Se calor suficiente for adicionado, as moléculas mais profundas no líquido começam a vaporizar, o que vemos como bolhas de vapor se formando.
Isso é chamado de ebulição e começa quando a pressão de vapor do composto é igual à pressão atmosférica. A temperatura na qual isso ocorre é o ponto de ebulição. Como cada substância pura tem diferentes tipos e intensidades de interações intermoleculares, diferentes substâncias têm diferentes pontos de ebulição.
Agora, vamos dar uma olhada em como as interações intermoleculares afetam o ponto de ebulição. Muitas interações intermoleculares envolvem áreas com densidade eletrônica desigual chamadas dipolos. Todas as moléculas têm breves flutuações em sua distribuição de elétrons, portanto, mesmo moléculas apolares podem ter dipolos temporariamente. Em resposta, dipolos opostos se formam em moléculas próximas.
As forças atrativas entre dipolos temporários e induzidos são chamadas de forças de dispersão de London e são as interações dominantes entre moléculas apolares como hidrocarbonetos. As forças de dispersão de London são mais fortes entre moléculas grandes com muitos elétrons porque essas moléculas mantêm seus elétrons fracamente, tornando mais fácil para as moléculas vizinhas afetarem sua distribuição de elétrons.
Por exemplo, um alcano não ramificado de alto peso molecular tem um ponto de ebulição mais alto do que um alcano não ramificado de menor peso molecular. Além disso, um alcano não ramificado tem um ponto de ebulição mais alto do que um alcano ramificado com peso molecular comparável devido à sua maior área de superfície.
Moléculas com ligações polares têm dipolos permanentes, permitindo interações dipolo-dipolo regulares entre elas. As interações dipolo-dipolo são mais fortes do que as forças de dispersão de Londres, por isso geralmente é necessária mais energia térmica para ferver líquidos polares do que líquidos apolares. Por exemplo, o ponto de ebulição de um cloropentano, uma molécula polar, é 108 °C. O hexano, que é semelhante em área de superfície, mas não possui dipolo, tem um ponto de ebulição mais baixo.
Moléculas com um hidrogênio ligado covalentemente a um átomo mais eletronegativo e um átomo de retirada de elétrons com um par de elétrons solitário são capazes de ligações de hidrogênio. As ligações de hidrogênio são mais fortes do que as interações dipolo-dipolo e as forças de dispersão de Londres, por isso é preciso ainda mais energia térmica para superá-las. Por exemplo, propano e etanol têm pesos moleculares e áreas de superfície semelhantes. Mas o ponto de ebulição do propano é muito menor do que o do etanol porque as moléculas de etanol podem formar ligações de hidrogênio umas com as outras, enquanto o propano não.
Neste laboratório, você usará o método capilar para determinar o ponto de ebulição de dois solventes orgânicos.

pontos de ebulição

Determinação do ponto de ebulição
Semelhante ao ponto de fusão, o ponto de ebulição é uma propriedade física. Se a amostra for um composto puro, o ponto de ebulição pode ser usado para determinar a identidade do composto. Em última análise, determinar experimentalmente o ponto de ebulição exato é um desafio. Como os pontos de fusão, os pontos de ebulição experimentais são dados como um intervalo e variam em alguns graus do valor real da literatura.
Pressão de vapor
Para entender por que um solvente ferve, que é caracterizado pelo borbulhar familiar da solução, é importante entender a dinâmica entre as fases líquida e gasosa. Considere um composto líquido puro em um recipiente lacrado. Algumas das moléculas na superfície do líquido terão energia suficiente para superar as forças intermoleculares e entrar na fase gasosa. No entanto, as moléculas na fase gasosa também podem perder energia e condensar de volta em um líquido. Portanto, existem dois processos concorrentes neste sistema: evaporação e condensação.
Quando a taxa de evaporação é igual à taxa de condensação, o sistema atingiu um estado de equilíbrio. Isso significa que para cada molécula que entra na fase gasosa, outra se condensa na fase líquida e não há ganho ou perda líquida da quantidade de líquido ou gás no recipiente. Uma vez estabelecido o equilíbrio, a pressão exercida pelo vapor acima do líquido é chamada de pressão de vapor. A tendência de um líquido vaporizar é chamada de volatilidade. Um líquido mais volátil tem uma pressão de vapor mais alta, enquanto um líquido menos volátil tem uma pressão de vapor mais baixa.
A pressão de vapor varia de acordo com a temperatura. Se você aumentar a temperatura da solução, mais moléculas terão energia suficiente para escapar da fase líquida e, assim, a pressão de vapor aumenta. Em última análise, se calor suficiente for aplicado, as moléculas que não estão na interface entre o líquido e o gás farão a transição para a fase gasosa e formarão as bolhas familiares que associamos à ebulição.
O ponto de ebulição de um líquido é atingido quando a pressão total de vapor do líquido é equivalente à pressão atmosférica. A temperatura na qual isso ocorre é chamada de ponto de ebulição. Em altitudes mais altas e, portanto, com uma pressão atmosférica mais baixa, um líquido ferverá a uma temperatura mais baixa, pois menos calor é necessário para aumentar a pressão de vapor para a pressão atmosférica. Além disso, a volatilidade, ou a capacidade de um solvente de vaporizar, também afeta a pressão de vapor. Solventes com alta volatilidade têm maior pressão de vapor do que solventes com menor volatilidade.
Fatores que afetam o ponto de ebulição
A semelhança entre os pontos de fusão e os pontos de ebulição significa que os mesmos fatores que afetam o ponto de fusão de um composto também afetarão o ponto de ebulição. Portanto, a força e os tipos de forças intermoleculares encontradas no composto líquido afetarão o ponto de ebulição. Lembre-se de que existem três tipos de forças moleculares: ligações de hidrogênio, interações dipolo-dipolo e forças de dispersão de Londres. Cada um deles tem diferentes forças de atração e requer diferentes quantidades de energia para superar. Os compostos que podem se ligar ao hidrogênio terão pontos de ebulição mais altos do que os compostos que só podem interagir por meio das forças de dispersão de London. Uma consideração adicional para os pontos de ebulição envolve a pressão de vapor e a volatilidade do composto. Normalmente, quanto mais volátil for um composto, menor será seu ponto de ebulição.
O método capilar para determinar o ponto de ebulição
Um método simples para determinar o ponto de ebulição de um composto orgânico é utilizar o método capilar. Nesta configuração, um tubo capilar de vidro vazio é invertido em um recipiente do composto puro na fase líquida. À medida que o líquido é aquecido, a pressão de vapor da amostra aumenta e o vapor gasoso começa a entrar no tubo capilar de vidro. Isso força a saída do ar preso no interior e resulta em bolhas emergindo do fundo do tubo capilar. Neste ponto, o líquido pode esfriar. Uma vez que a pressão de vapor da amostra seja igual à pressão atmosférica dentro do tubo capilar de vidro, o líquido começará a entrar no tubo. A temperatura da solução quando esse fenômeno ocorre é o ponto de ebulição do composto líquido.

Pontos de ebulição

Determinação do ponto de ebulição
Semelhante ao ponto de fusão, o ponto de ebulição é uma propriedade física. Se a amostra for um composto puro, então a ebulição ...

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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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