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

Le coefficient d’activité d’un ion dépend de la force ionique de la solution.
Comme l’indique la théorie de Debye-Hückel, lorsque la force ionique tend vers zéro, le coefficient d’activité tend vers l’unité.
Lorsque la force ionique augmente de 0 à 0,1 mole par litre, la valeur du coefficient d’activité diminue.
Cependant, avec une augmentation de la force ionique supérieure à 0,1 mole par litre, le coefficient d’activité peut augmenter.
L’influence de la force ionique sur le coefficient d’activité est associée à la charge de l’ion, qui influence également directement le coefficient d’activité.
Avec une charge accrue sur l’ion, la diminution du coefficient d’activité est encore plus prononcée.
Pour une force ionique particulière, les ions ayant les mêmes charges ont à peu près les mêmes coefficients d’activité.
Un autre facteur important affectant le coefficient d’activité d’un ion est le paramètre de taille de l’ion, c’est-à-dire le diamètre effectif de l’ion hydraté.
Les ions plus petits avec une charge élevée ont des diamètres effectifs plus grands et s’écartent moins de l’idéalité que les ions de charge similaire avec des diamètres plus petits.

facteurs affectant le coefficient d'activité

L'équation étendue de Debye-Hückel indique que le coefficient d'activité d'un ion dans une solution aqueuse à 25°C dépend de trois propriétés partiellement interdépendantes : la force ionique de la solution, la charge de l'ion et la taille de l'ion.

La valeur du coefficient d'activité d'un ion est proche de un lorsque la solution a une force ionique presque nulle, c'est-à-dire lorsque la solution présente un comportement proche de l'idéal. À mesure que la force ionique de la solution augmente de 0 à 0,1 mol/L, une diminution de la valeur du coefficient d’activité est observée.

Les solutions avec une force ionique supérieure à 0,1 mol/L ne sont pas bien représentées par l'équation de Debye-Hückel. Dans de telles solutions, le coefficient d’activité d’un ion peut dépasser l’unité. La force ionique de la solution est étroitement liée à la charge des ions présents dans la solution, affectant ainsi leur coefficient d'activité. Pour une force ionique particulière, les ions moins chargés ont des valeurs de coefficient d’activité supérieures à celles des ions ayant une charge plus élevée. En d’autres termes, l’écart par rapport à l’idéalité est plus prononcé pour les ions à charges multiples que pour les ions à charge unique.

Les ions ayant des charges similaires ont souvent des coefficients d'activité similaires. Le facteur dominant dans les écarts de coefficients est ici le paramètre de taille des ions. La taille des ions décrit le diamètre effectif d'un ion hydraté, et même les ions avec des charges similaires ont tendance à avoir des degrés d'hydratation différents et donc des tailles d'ions différentes. Les ions ayant des charges similaires peuvent être hydratés à des degrés différents, ce qui donne lieu à des diamètres effectifs différents. Ces diamètres sont appelés paramètres de taille des ions. Les ions avec des paramètres de taille d'ion plus petits s'écartent davantage de l'idéalité, ce qui entraîne un coefficient d'activité plus faible que ceux avec des paramètres de taille d'ion plus grands mais ayant la même charge.

Facteurs affectant le coefficient d'activité

Le coefficient d’activité d’un ion dépend de la force ionique de la solution.
Comme l’indique la théorie de Debye-Hückel, lorsque l’ionique ...

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Analytical Chemistry

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Unit 1

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Chemical Equilibria

Équilibres chimiques

KEY TERMS AND CONCEPTS

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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

how-ionic-strength-changes-activity-coefficient

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Activity Coefficient in Ionic Solutions

Activity coefficient in ionic solutions describes how much a species behaves like its measured concentration. Activity is the effective concentration of a solute in solution. It equals the molar concentration multiplied by the activity coefficient, which is a unitless number. The activity coefficient depends on the total ionic strength of the solution. It shows how far a solution is from ideal behavior. When ionic strength is very low, the activity coefficient of an ionic species is close to...

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Salt effect changes how a sparingly soluble salt dissolves when an inert salt is added. The size of this effect depends on the ionic strength of the solution. Ionic strength reflects how strongly the ions in the solution interact. In solution, the activity of a species is the product of its concentration and its activity coefficient. A redefined equilibrium constant must account for these activity effects when the ionic strength is moderately high. This form is also called the thermodynamic...

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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