Name: steam distillation and low-boiling separation
Uploaded: 2020-03-26T07:22:28.000Z
Duration: 3 min 54 s
Channel: JoVE

Quando una miscela ideale di due liquidi miscibili viene riscaldata fino all'ebollizione, la soluzione bolle a una temperatura compresa tra i punti di ebollizione di ciascun componente. Se questi liquidi hanno punti di ebollizione molto diversi, quando la miscela inizia a bollire, il vapore è ricco delle molecole del componente più volatile. Questo fenomeno viene spesso utilizzato per separare le miscele utilizzando la semplice distillazione, in cui una miscela di due liquidi miscibili viene riscaldata e il vapore viene quindi condensato nuovamente in liquido e raccolto.
Man mano che il vapore ricco del componente più volatile viene raccolto come distillato, la fase liquida diventa ricca delle molecole del componente meno volatile. Tuttavia, questa tecnica richiede che la soluzione venga riscaldata almeno fino al punto di ebollizione del composto più volatile e spesso oltre.
Nel caso di composti organici sensibili alla temperatura, questa temperatura elevata potrebbe portare alla decomposizione delle molecole organiche in qualcos'altro. Quindi, come possiamo separare questi tipi di composti? Innanzitutto, facciamo un passo indietro.
Ricordiamo che la pressione di un vapore in equilibrio con la sua fase condensata è chiamata tensione di vapore. I componenti di una miscela di liquidi hanno ciascuno la propria pressione di vapore, che chiamiamo la loro pressione parziale. Sappiamo che una soluzione bolle quando la pressione totale del vapore della soluzione è uguale alla pressione atmosferica. La pressione di vapore totale è uguale alla somma delle pressioni parziali dei componenti.
Per una miscela di liquidi miscibili, il che significa che qualsiasi combinazione dei liquidi forma una soluzione omogenea, le pressioni parziali sono calcolate dalle pressioni di vapore dei composti puri moltiplicate per le loro frazioni molari nella soluzione. Tuttavia, per una miscela eterogenea di liquidi immiscibili, il che significa che i liquidi sono insolubili l'uno nell'altro, le pressioni parziali sono semplicemente le pressioni di vapore dei composti puri.
Poiché ogni componente della miscela eterogenea contribuisce alla pressione totale del vapore indipendentemente dagli altri componenti, la miscela bolle quando la pressione totale del vapore, che è la somma delle pressioni parziali, è uguale alla pressione atmosferica. Ciò si verifica a una temperatura inferiore rispetto ai singoli punti di ebollizione di ciascun componente perché la pressione totale del vapore aumenta con la temperatura molto più velocemente di quanto ci si aspetterebbe anche per il componente più volatile.
Possiamo sfruttare questo fenomeno per eseguire la distillazione in corrente di vapore, che viene utilizzata per isolare un composto organico sensibile alla temperatura che si decompone a calore elevato ed è insolubile in acqua da sostanze non volatili. La configurazione della distillazione a vapore è simile a una semplice configurazione di distillazione con l'aggiunta di un serbatoio d'acqua per reintegrare l'acqua durante tutto il processo.
Mentre la miscela bolle, sia l'acqua che il composto organico di interesse vengono vaporizzati. I vapori dell'acqua e dei composti organici viaggiano nel condensatore, vengono condensati in liquido e raccolti. I liquidi immiscibili vengono successivamente separati. Nella miscela del pallone rimangono solo acqua e materiali non volatili.
In questo laboratorio, imposterai ed eseguirai un esperimento di distillazione a vapore per estrarre l'olio essenziale dai componenti non volatili di una buccia d'arancia. Utilizzerai quindi l'estrazione liquido-liquido per estrarre l'olio essenziale dall'acqua in un solvente organico.

distillazione a vapore

<h4>Distillazione a vapore </h4>
La distillazione a vapore è una tecnica di separazione che sfrutta la proprietà del basso punto di ebollizione delle miscele immiscibili. Viene utilizzato prevalentemente per separare molecole organiche sensibili alla temperatura da un contaminante non volatile. La molecola organica deve essere immiscibile in acqua.
Nella distillazione a vapore, la miscela immiscibile viene riscaldata fino all'ebollizione, provocando la distillazione sia dell'acqua che dei composti organici volatili. Ciò significa che la miscela gassosa viaggia verso l'alto fino a un condensatore, che poi condensa il vapore in liquido in modo che possa essere raccolto. A differenza della semplice distillazione, la distillazione a vapore utilizza un serbatoio d'acqua per reintegrare l'acqua nella miscela riscaldata durante tutto il processo. La componente organica immiscibile viene distillata lentamente insieme all'acqua, mentre la componente non volatile rimane nella miscela riscaldata. Una volta che la componente organica è stata distillata, può essere separata dall'acqua mediante estrazione liquido-liquido.
<h4>Tensione di vapore di una miscela</h4>
Per una miscela miscibile che forma una soluzione omogenea, la tensione di vapore di ciascun componente dipende dalla tensione di vapore del componente puro e dalla sua frazione molare nella miscela liquida secondo la legge di Raoult.
pA = pA*xA
dove pA è la tensione di vapore di un componente liquido in una miscela liquida miscibile, pA* è la tensione di vapore del liquido puro e xA è la frazione molare di quel liquido nella miscela, che è uguale a nA/nt. nA è il numero di moli del singolo liquido nella miscela, e nt è il numero totale di moli di tutti i liquidi nella miscela.
La pressione di vapore totale al di sopra della miscela di liquidi miscibili è uguale alla somma della pressione di vapore parziale di ciascun componente in essa, nota come legge di Dalton. La pressione di vapore di un liquido aumenta con la temperatura man mano che più molecole acquisiscono energia cinetica per sfuggire dalla fase liquida alla fase gassosa. In una miscela miscibile contenente due liquidi, la pressione totale può essere descritta come:
P = pA + pB
dove pA e pB sono le pressioni di vapore del liquido A liquido B, rispettivamente, al di sopra della miscela. P è la pressione di vapore totale di entrambi i liquidi sopra la miscela. La combinazione delle equazioni descrive la relazione tra la tensione di vapore totale della soluzione e la frazione molare dei singoli componenti:
P = pA*xA + pB*xB
In una miscela immiscibile, in cui i componenti formano una miscela eterogenea, le pressioni di vapore di ciascun componente contribuiscono indipendentemente alla pressione di vapore totale. Pertanto, la pressione di vapore totale è uguale alla somma delle singole pressioni di vapore puro. In una miscela immiscibile composta da due liquidi, la pressione totale è definita come la tensione di vapore del primo liquido più la tensione di vapore del secondo liquido.
P = pA* + pB*
<h4>Punto di ebollizione di una miscela immiscibile</h4>
Quando un liquido viene riscaldato, la pressione del vapore aumenta. Ogni componente di una miscela ha il proprio punto di ebollizione. In una miscela di liquidi miscibili, l'ebollizione avviene a una temperatura compresa tra i punti di ebollizione dei liquidi costituenti.
Per una miscela immiscibile, l'ebollizione avviene a una temperatura molto più bassa rispetto ai punti di ebollizione dei singoli componenti. Poiché ogni singolo componente contribuisce in modo indipendente, è necessario meno calore per aumentare la pressione totale del vapore alla pressione atmosferica.
Ad esempio, si consideri la miscela immiscibile di benzene e acqua. Il punto di ebollizione del benzene a pressione atmosferica normale è di 80,1 °C e il punto di ebollizione dell'acqua a pressione atmosferica normale è di 100 °C. La soluzione bolle quando la pressione totale del vapore raggiunge i 760 mm Hg (pressione atmosferica normale). A 69,3 °C, la pressione di vapore dell'acqua è di 227 mm Hg e la pressione di vapore del benzene è di 533 mm Hg, che in totale equivale ai 760 mm Hg necessari per l'ebollizione. Questo è ben al di sotto del punto di ebollizione di entrambi i singoli componenti.
<h4>referenze</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chimica e reattività chimica. Belmont, CA: Brooks/Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). Chimica: la natura molecolare della materia e il cambiamento. Boston, MA: McGraw Hill.</li>
</ol>

Distillazione a vapore

Distillazione a vapore 
La distillazione a vapore è una tecnica di separazione che sfrutta la proprietà del basso punto di ebollizione delle miscele immiscibili. È prevalentemente ...

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Steam Distillation

steam distillation and low-boiling separation

Steam distillation is a separation method for temperature-sensitive organic compounds. It works best when the organic compound is immiscible in water, meaning the two liquids do not mix. The method helps remove the organic molecule from a non-volatile contaminant without using the higher temperatures needed for many other separations.

In steam distillation, the immiscible mixture is heated until both water and the volatile organic compound distill together. The vapor rises to a condenser, where it cools and becomes a liquid that can be collected. Unlike simple distillation, this setup uses a water reservoir to keep replacing water in the heated mixture during the process.

The organic component is carried over slowly with the water, while the non-volatile component stays behind in the heated flask. After distillation, the organic product can be separated from the water by liquid-liquid extraction. This makes steam distillation useful for isolating sensitive organic materials from unwanted impurities.

The same low-boiling behavior can be explained by vapor pressure. For a miscible mixture, each liquid follows Raoult’s law, so its vapor pressure depends on the pure liquid vapor pressure and its mole fraction. The mole fraction is the number of moles of one liquid divided by the total number of moles in the mixture. The total vapor pressure above a miscible mixture follows Dalton’s law and equals the sum of the partial vapor pressures.

As temperature rises, vapor pressure increases because more molecules gain enough kinetic energy to escape into the gas phase. In a two-liquid miscible mixture, the total pressure can be written as P = pA + pB, or as P = pA*xA + pB*xB when Raoult’s law is included. In an immiscible mixture, the components act independently, so the total pressure is the sum of the vapor pressure of the first liquid and the vapor pressure of the second liquid.

This independent vapor-pressure behavior is why immiscible mixtures boil at lower temperatures than either pure liquid. In a miscible mixture, boiling usually happens between the boiling points of the components. In an immiscible mixture, less heat is needed to reach atmospheric pressure, so the boiling point drops well below the boiling points of the individual liquids.

A benzene-water mixture shows this effect clearly. Benzene boils at 80.1 °C, and water boils at 100 °C at normal atmospheric pressure. The mixture boils when the total vapor pressure reaches 760 mm Hg. At 69.3 °C, water contributes 227 mm Hg and benzene contributes 533 mm Hg, which adds up to 760 mm Hg. That means the mixture boils at a temperature below either pure component.

Steam Distillation and Low-Boiling Separation

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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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Learn how steam distillation separates temperature-sensitive organic compounds from water-insoluble mixtures using vapor pressure and low boiling points.

Steam Distillation and Low-Boiling Separation | JoVE K12

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Steam Distillation and Low-Boiling Separation

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