Name: how ionic strength changes activity coefficient
Uploaded: 2024-04-04T10:51:15.000Z
Duration: 1 min 17 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.8

IPC.8.B

Factors Affecting Activity Coefficient

Il coefficiente di attività di uno ione dipende dalla forza ionica della soluzione.
Come indicato dalla teoria di Debye-Hückel, quando la forza ionica si avvicina allo zero, il coefficiente di attività si avvicina all'unità.
All'aumentare della forza ionica da 0 a 0,1 moli per litro, il valore del coefficiente di attività diminuisce.
Tuttavia, con un aumento della forza ionica superiore a 0,1 moli per litro, il coefficiente di attività può aumentare.
L'influenza della forza ionica sul coefficiente di attività è associata alla carica dello ione, che influenza direttamente anche il coefficiente di attività.
Con una maggiore carica sullo ione, la diminuzione del coefficiente di attività è ancora più pronunciata.
Per una particolare forza ionica, gli ioni con le stesse cariche hanno approssimativamente gli stessi coefficienti di attività.
Un altro fattore significativo che influenza il coefficiente di attività di uno ione è il parametro della dimensione dello ione, il diametro effettivo dello ione idrato.
Gli ioni più piccoli con una carica elevata hanno diametri effettivi più grandi e si discostano meno dall'idealità rispetto agli ioni con carica simile con diametri più piccoli.

fattori che influenzano il coefficiente di attività

L'equazione estesa di Debye-Hückel indica che il coefficiente di attività di uno ione in una soluzione acquosa a 25°C dipende da tre proprietà parzialmente interdipendenti: la forza ionica della soluzione, la carica dello ione e la dimensione dello ione.

Il valore del coefficiente di attività per uno ione è vicino a uno quando la soluzione ha una forza ionica quasi pari a zero, cioè quando la soluzione mostra un comportamento vicino a quello ideale. Quando la forza ionica della soluzione aumenta da 0 a 0,1 mol/L, si osserva una diminuzione del valore del coefficiente di attività.

Le soluzioni con una forza ionica superiore a 0,1 mol/L non sono ben rappresentate dall'equazione di Debye-Hückel. In tali soluzioni, il coefficiente di attività di uno ione può superare l'unità. La forza ionica della soluzione è strettamente correlata alla carica degli ioni presenti nella soluzione, influenzandone così il coefficiente di attività. Per una particolare forza ionica, gli ioni con carica minore hanno valori di coefficiente di attività superiori rispetto agli ioni con carica maggiore. In altre parole, la deviazione dall’ideale è più pronunciata sia per gli ioni a carica multipla che per gli ioni a carica singola.

Gli ioni con cariche simili hanno spesso coefficienti di attività simili. Il fattore dominante nelle discrepanze dei coefficienti qui è il parametro della dimensione degli ioni. La dimensione dello ione descrive il diametro effettivo di uno ione idratato, e anche gli ioni con cariche simili tendono ad avere diversi gradi di idratazione e quindi diverse dimensioni degli ioni. Gli ioni con cariche simili possono essere idratati in misura diversa, risultando in diametri effettivi diversi. Questi diametri sono noti come parametri della dimensione degli ioni. Gli ioni con parametri di dimensione ionica più piccoli si discostano maggiormente dall'ideale, risultando in un coefficiente di attività inferiore rispetto a quelli con parametri di dimensione ionica più grandi ma con la stessa carica.

Fattori che influenzano il coefficiente di attività

Il coefficiente di attività di uno ione dipende dalla forza ionica della soluzione.
Come indicato dalla teoria di Debye-Hückel, quando il ...

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how ionic strength changes activity coefficient

The activity coefficient of an ion in water depends on ionic strength, ion charge, and ion size. The extended Debye-Hückel equation describes these effects for aqueous solutions at 25°C. These three factors are partly linked, so changes in one can influence the others.

When a solution has almost zero ionic strength, the activity coefficient is close to one. That means the solution behaves almost ideally. As ionic strength rises from 0 to 0.1 mol/L, the activity coefficient usually drops. Solutions with ionic strength above 0.1 mol/L are not well described by the Debye-Hückel equation, and the activity coefficient may even become greater than one.

Ion charge also affects the activity coefficient because ionic strength depends on the ions present in the solution. At the same ionic strength, ions with lower charge usually have higher activity coefficient values than ions with higher charge. This means multiply charged ions show a larger departure from ideal behavior than singly charged ions. Ions with similar charges often have similar activity coefficients.

Ion size is the main reason that activity coefficients can still differ for ions with the same charge. Ion size refers to the effective diameter of a hydrated ion, which depends on how much water surrounds it. Ions with similar charges can be hydrated to different extents, so they can have different ion size parameters. Smaller ion size parameters lead to greater deviation from ideal behavior and a lower activity coefficient than larger ion size parameters for ions with the same charge.

How Ionic Strength Changes Activity Coefficient

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factors affecting activity coefficient

The activity coefficient of an ion in water depends on ionic strength, ion charge, and ion size. The extended Debye-Hückel equation describes these effects for aqueous solutions at 25°C. These three factors are partly linked, so changes in one can influence the others. When a solution has almost zero ionic strength, the activity coefficient is close to one. That means the solution behaves almost ideally. As ionic strength rises from 0 to 0.1 mol/L, the activity coefficient usually drops.

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key terms and concepts

Learn how ionic strength, ion charge, and ion size affect activity coefficients in aqueous solutions using the extended Debye-Hückel equation.

How Ionic Strength Changes Activity Coefficient | JoVE K12

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How Ionic Strength Changes Activity Coefficient

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