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

I liquidi sono tenuti insieme da deboli interazioni tra le loro molecole. Per passare alla fase gassosa, una molecola deve ottenere energia sufficiente dal calore o da altre fonti per superare queste interazioni.
Consideriamo un contenitore chiuso di un liquido puro a temperatura ambiente. Alcune delle molecole in fase liquida sulla superficie guadagnano energia sufficiente per evaporare e parte di quel vapore perde energia e si condensa di nuovo in liquido. Quando i tassi di evaporazione e condensazione sono gli stessi, il liquido e il vapore raggiungono l'equilibrio. A questo punto, lo spazio sopra il liquido è saturo di vapore e non vi è alcun cambiamento netto nella quantità di liquido e vapore nel contenitore. La pressione esercitata dal vapore è chiamata pressione di vapore.
Ora, considera un contenitore aperto di liquido. Qui il vapore può fuoriuscire, impedendo alla zona sopra il liquido di saturarsi. Pertanto, l'evaporazione potrebbe continuare fino a quando non rimane più liquido. All'aumentare della temperatura di un liquido, aumenta anche la velocità di evaporazione e, quindi, anche la pressione del vapore. Se viene aggiunto abbastanza calore, le molecole più profonde nel liquido iniziano a vaporizzare, che vediamo come la formazione di bolle di vapore.
Questo è chiamato ebollizione e inizia quando la pressione di vapore del composto è uguale alla pressione atmosferica. La temperatura alla quale ciò avviene è il punto di ebollizione. Poiché ogni sostanza pura ha diversi tipi e intensità di interazioni intermolecolari, sostanze diverse hanno punti di ebollizione diversi.
Ora, diamo un'occhiata a come le interazioni intermolecolari influiscono sul punto di ebollizione. Molte interazioni intermolecolari coinvolgono aree con densità elettronica irregolare chiamate dipoli. Tutte le molecole hanno brevi fluttuazioni nella loro distribuzione elettronica, quindi anche le molecole non polari possono avere temporaneamente dipoli. In risposta, si formano dipoli opposti nelle molecole vicine.
Le forze attrattive tra i dipoli temporanei e indotti sono chiamate forze di dispersione di Londra e sono le interazioni dominanti tra molecole non polari come gli idrocarburi. Le forze di dispersione di London sono più forti tra molecole grandi con molti elettroni perché quelle molecole trattengono debolmente i loro elettroni, rendendo più facile per le molecole vicine influenzare la loro distribuzione elettronica.
Ad esempio, un alcano non ramificato ad alto peso molecolare ha un punto di ebollizione più alto rispetto a un alcano non ramificato a peso molecolare inferiore. Inoltre, un alcano non ramificato ha un punto di ebollizione più alto di un alcano ramificato con un peso molecolare comparabile a causa della sua maggiore superficie.
Le molecole con legami polari hanno dipoli permanenti, che consentono interazioni regolari dipolo-dipolo tra di loro. Le interazioni dipolo-dipolo sono più forti delle forze di dispersione di London, quindi di solito ci vuole più energia termica per far bollire i liquidi polari rispetto ai liquidi non polari. Ad esempio, il punto di ebollizione di un cloropentano, una molecola polare, è di 108 °C. L'esano, che è simile nella superficie ma manca di un dipolo, ha un punto di ebollizione più basso.
Le molecole con un idrogeno legato covalentemente a un atomo più elettronegativo e un atomo che ritira elettroni con una coppia solitaria di elettroni sono in grado di legare idrogeno. I legami idrogeno sono più forti delle interazioni dipolo-dipolo e delle forze di dispersione di London, quindi ci vuole ancora più energia termica per superarli. Ad esempio, il propano e l'etanolo hanno pesi molecolari e superfici simili. Ma il punto di ebollizione del propano è molto più basso di quello dell'etanolo perché le molecole di etanolo possono formare legami idrogeno tra loro, mentre il propano no.
In questo laboratorio, utilizzerai il metodo capillare per determinare il punto di ebollizione di due solventi organici.

punti di ebollizione

Determinazione del punto di ebollizione
Simile al punto di fusione, il punto di ebollizione è una proprietà fisica. Se il campione è un composto puro, il punto di ebollizione può essere utilizzato per determinare l'identità del composto. In definitiva, determinare sperimentalmente l'esatto punto di ebollizione è una sfida. Come i punti di fusione, i punti di ebollizione sperimentali sono indicati come intervallo e variano di alcuni gradi rispetto al valore effettivo della letteratura.
Pressione di vapore
Per capire perché un solvente bolle, che è caratterizzato dal familiare gorgogliamento della soluzione, è importante comprendere la dinamica tra la fase liquida e quella gassosa. Considera un composto liquido puro in un contenitore sigillato. Alcune delle molecole sulla superficie del liquido avranno energia sufficiente per superare le forze intermolecolari ed entrare nella fase gassosa. Tuttavia, le molecole in fase gassosa possono anche perdere energia e condensarsi nuovamente in un liquido. Pertanto, ci sono due processi concorrenti in questo sistema: evaporazione e condensazione.
Quando la velocità di evaporazione è uguale alla velocità di condensazione, il sistema ha raggiunto uno stato di equilibrio. Ciò significa che per ogni molecola che entra nella fase gassosa, un'altra si condensa nella fase liquida e non c'è guadagno o perdita netta della quantità di liquido o gas nel contenitore. Una volta stabilito l'equilibrio, la pressione esercitata dal vapore sopra il liquido è chiamata pressione di vapore. La tendenza di un liquido a vaporizzare è chiamata volatilità. Un liquido più volatile ha una pressione di vapore più elevata, mentre un liquido meno volatile ha una pressione di vapore inferiore.
La pressione del vapore varia in base alla temperatura. Se si aumenta la temperatura della soluzione, più molecole hanno abbastanza energia per sfuggire alla fase liquida, e quindi la pressione del vapore aumenta. Alla fine, se viene applicato abbastanza calore, le molecole che non si trovano all'interfaccia tra il liquido e il gas passeranno alla fase gassosa e formeranno le bolle familiari che associamo all'ebollizione.
Il punto di ebollizione di un liquido viene raggiunto quando la pressione totale di vapore del liquido è equivalente alla pressione atmosferica. La temperatura alla quale ciò si verifica è chiamata punto di ebollizione. Ad altitudini più elevate, e quindi a una pressione atmosferica più bassa, un liquido bolle a una temperatura più bassa, poiché è necessario meno calore per aumentare la pressione del vapore alla pressione atmosferica. Inoltre, anche la volatilità, o la capacità di un solvente di vaporizzare, influisce sulla pressione del vapore. I solventi con elevata volatilità hanno una pressione di vapore più elevata rispetto ai solventi con una volatilità inferiore.
Fattori che influiscono sul punto di ebollizione
La somiglianza tra i punti di fusione e i punti di ebollizione significa che gli stessi fattori che influiscono sul punto di fusione di un composto influenzeranno anche il punto di ebollizione. Pertanto, la forza e i tipi di forze intermolecolari che si trovano all'interno del composto liquido influenzeranno il punto di ebollizione. Ricordiamo che ci sono tre tipi di forze molecolari: legame idrogeno, interazioni dipolo-dipolo e forze di dispersione di Londra. Ognuno di questi ha diversi punti di forza di attrazione e richiede diverse quantità di energia per essere superato. I composti che possono legarsi all'idrogeno avranno punti di ebollizione più elevati rispetto ai composti che possono interagire solo attraverso le forze di dispersione di Londra. Un'ulteriore considerazione per i punti di ebollizione riguarda la tensione di vapore e la volatilità del composto. In genere, più un composto è volatile, più basso è il suo punto di ebollizione.
Il metodo capillare per determinare il punto di ebollizione
Un metodo semplice per determinare il punto di ebollizione di un composto organico consiste nell'utilizzare il metodo capillare. In questa configurazione, un tubo capillare di vetro vuoto viene capovolto in un contenitore del composto puro in fase liquida. Quando il liquido viene riscaldato, la pressione del vapore del campione aumenta e il vapore gassoso inizia a entrare nel tubo capillare di vetro. Questo spinge fuori l'aria intrappolata all'interno e provoca bolle che fuoriescono dal fondo del tubo capillare. A questo punto, il liquido viene lasciato raffreddare. Una volta che la pressione di vapore del campione è uguale alla pressione atmosferica all'interno del tubo capillare di vetro, il liquido inizierà a entrare nel tubo. La temperatura della soluzione quando si verifica questo fenomeno è il punto di ebollizione del composto liquido.

Punti di ebollizione

Determinazione del punto di ebollizione
Simile al punto di fusione, il punto di ebollizione è una proprietà fisica. Se il campione è un composto puro, l'ebollizione ...

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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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Video completi e incentrati sul curriculum sia per gli istruttori che per gli studenti nei corsi di laboratorio di Chimica introduttiva.

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Explore how vapor pressure, volatility, and intermolecular forces affect boiling point, and learn how the capillary method measures it in the lab.

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