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

Der Aktivitätskoeffizient eines Ions hängt von der Ionenstärke der Lösung ab.
Wie die Debye-Hückel-Theorie zeigt, nähert sich der Aktivitätskoeffizient Eins, wenn sich die Ionenstärke Null nähert.
Mit zunehmender Ionenstärke von 0 auf 0,1 mol pro Liter nimmt der Wert des Aktivitätskoeffizienten ab.
Mit einer Erhöhung der Ionenstärke über 0,1 mol pro Liter kann sich der Aktivitätskoeffizient jedoch erhöhen.
Der Einfluss der Ionenstärke auf den Aktivitätskoeffizienten ist mit der Ladung des Ions verbunden, die auch den Aktivitätskoeffizienten direkt beeinflusst.
Mit zunehmender Ladung des Ions ist die Abnahme des Aktivitätskoeffizienten noch ausgeprägter.
Für eine bestimmte Ionenstärke haben Ionen mit den gleichen Ladungen ungefähr die gleichen Aktivitätskoeffizienten.
Ein weiterer wichtiger Faktor, der den Aktivitätskoeffizienten eines Ions beeinflusst, ist der Ionengrößenparameter – der effektive Durchmesser des hydratisierten Ions.
Kleinere Ionen mit einer hohen Ladung haben größere effektive Durchmesser und weichen weniger von der Idealität ab als ähnlich geladene Ionen mit kleineren Durchmessern.

faktoren, die den aktivitätskoeffizienten beeinflussen

Die erweiterte Debye-Hückel-Gleichung zeigt, dass der Aktivitätskoeffizient eines Ions in einer wässrigen Lösung bei 25 °C von drei teilweise voneinander abhängigen Eigenschaften abhängt: der Ionenstärke der Lösung, der Ladung des Ions und der Ionengröße.

Der Aktivitätskoeffizientwert für ein Ion liegt nahe bei eins, wenn die Lösung eine Ionenstärke von nahezu Null aufweist, d. h. wenn die Lösung ein nahezu ideales Verhalten zeigt. Wenn die Ionenstärke der Lösung von 0 auf 0,1 mol/L ansteigt, wird eine Abnahme des Aktivitätskoeffizientenwerts beobachtet.

Lösungen mit einer Ionenstärke über 0,1 mol/L werden durch die Debye-Hückel-Gleichung nicht gut dargestellt. In solchen Lösungen kann der Aktivitätskoeffizient eines Ions eins überschreiten. Die Ionenstärke der Lösung hängt eng mit der Ladung der in der Lösung vorhandenen Ionen zusammen und beeinflusst dadurch deren Aktivitätskoeffizient. Bei einer bestimmten Ionenstärke weisen Ionen mit geringerer Ladung höhere Aktivitätskoeffizientenwerte auf als Ionen mit höherer Ladung. Mit anderen Worten: Die Abweichung von der Idealität ist bei mehrfach geladenen Ionen stärker ausgeprägt als bei einfach geladenen Ionen.

Ionen mit ähnlichen Ladungen haben oft ähnliche Aktivitätskoeffizienten. Der dominierende Faktor bei den Koeffizientenunterschieden ist hier der Ionengrößenparameter. Die Ionengröße beschreibt den effektiven Durchmesser eines hydratisierten Ions, und selbst Ionen mit ähnlichen Ladungen weisen tendenziell unterschiedliche Hydratationsgrade und daher unterschiedliche Ionengrößen auf. Ionen mit ähnlicher Ladung können unterschiedlich stark hydratisiert werden, was zu unterschiedlichen effektiven Durchmessern führt. Diese Durchmesser werden als Ionengrößenparameter bezeichnet. Ionen mit kleineren Ionengrößenparametern weichen stärker von der Idealität ab, was zu einem niedrigeren Aktivitätskoeffizienten führt als solche mit größeren Ionengrößenparametern, aber gleicher Ladung.

Faktoren, die den Aktivitätskoeffizienten beeinflussen

Der Aktivitätskoeffizient eines Ions hängt von der Ionenstärke der Lösung ab.
Wie die Debye-Hückel-Theorie zeigt, ...

Education

Lehre

JoVE Core

Analytical Chemistry

Analytische Chemie

Unit 1

Einheit 1

Chemical Equilibria

Chemische Gleichgewichte

KEY TERMS AND CONCEPTS

ZENTRALE BEGRIFFE UND KONZEPTE

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

ionic strength: overview

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Chemical potential and activity describe how a species behaves in a solution. Activity is the effective concentration of a species. It accounts for the electrolytes near that species and depends on the ionic strength of the solution. Activity is calculated as molar concentration multiplied by the activity coefficient. This makes it a more precise way to describe a species in solution than concentration alone. The thermodynamic equilibrium constant is also defined more accurately using activity...

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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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Chelating Ligands and Metal Complex Stability

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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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