Name: activity coefficient in ionic solutions
Uploaded: 2024-04-04T10:51:15.000Z
Duration: 1 min 24 s
Channel: JoVE

L’activité est la mesure de la concentration efficace. Elle est sans unité et peut être exprimée comme le produit de la concentration molaire de l’espèce et de son coefficient d’activité.
Le coefficient d’activité, une grandeur sans dimension mesurant l’écart par rapport au comportement idéal, dépend de la force ionique. Dans une solution idéale, l’activité est égale à la concentration.
Dans les solutions ayant une très faible force ionique, le coefficient d’activité d’un ion est proche de l’unité, et le comportement est essentiellement idéal.
À mesure que la force ionique augmente, le coefficient d’activité devient inférieur à l’unité, indiquant un écart par rapport à l’idéal lorsque l’activité est inférieure à la concentration.
Cependant, dans les solutions à très forte force ionique, le coefficient d’activité peut dépasser l’unité.
Il est intéressant de noter que le coefficient d’activité d’une espèce neutre est approximativement unitaire à des forces ioniques inférieures à 0,1 mole par litre.
La relation entre le coefficient d’activité et la force ionique est exprimée par l’équation de Debye-Hückel étendue.
Cependant, pour des solutions extrêmement diluées avec une très faible force ionique, l’équation se réduit à la loi limite de Debye-Hückel.

thermodynamique : coefficient d'activité

L'activité est la mesure de la concentration efficace de l'espèce en solution. Elle peut être exprimée comme le produit de la concentration molaire de l’espèce et de son coefficient d’activité. Le coefficient d'activité est une quantité sans dimension et dépend de la force ionique totale de la solution.

Le coefficient d'activité est une mesure de l'écart par rapport au comportement idéal. Lorsque la force ionique de la solution est minime, le coefficient d’activité d’une espèce ionique est proche de l’unité, rendant l’activité approximativement égale à la concentration molaire. Une telle solution présente des comportements très proches de ceux d’une solution idéale. Le coefficient d'activité d'une molécule non chargée est d'environ l'unité pour toutes les forces ioniques inférieures à 0,1 mol/L.

À mesure que la force ionique de la solution augmente, le coefficient d’activité diminue, rendant l’activité inférieure à la concentration molaire. Cela indique l'écart de l'espèce par rapport au comportement idéal. Dans les solutions dont la force ionique est supérieure à 0,1 mol/L, le coefficient d’activité augmente et dépasse l’unité.

Une équation utile pour calculer le coefficient d'activité à partir de la force ionique est l'équation étendue de Debye – Hückel, qui relie le coefficient à la force ionique, au diamètre effectif de l'ion et aux charges ioniques du soluté. L'équation de Debye – Hückel repose sur la taille de l'ion - le diamètre effectif d'un ion hydraté - qui peut varier considérablement en fonction de la charge et de la géométrie de l'ion, introduisant ainsi une incertitude dans l'équation.

L'équation fonctionne bien dans les solutions extrêmement diluées et les solutions dont la force ionique est inférieure à 0,1 mol/L et dans les solutions extrêmement diluées, où les incertitudes sur la taille des ions ont des effets négligeables sur les coefficients d'activité. Dans ces cas, la solution se comporte généralement de manière idéale et l’équation de Debye – Hückel se réduit à une forme plus simple connue sous le nom de loi limite de Debye – Hückel.

Thermodynamique : coefficient d'activité

L’activité est la mesure de la concentration efficace. Il est sans unité et peut être exprimé comme le produit de la concentration molaire de l’espèce et de son ...

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Chemical Equilibria

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Thermodynamics: Activity Coefficient

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 1, so activity is nearly the same as molar concentration. In that case, the solution behaves much like an ideal solution.

An uncharged molecule has an activity coefficient of about 1 at ionic strengths below 0.1 mol/L. As ionic strength rises, the activity coefficient of a species can decrease, and activity becomes less than the molar concentration. This shift shows greater deviation from ideal behavior. For ionic strengths above 0.1 mol/L, the activity coefficient increases and can exceed 1.

The extended Debye-Hückel equation is used to estimate activity coefficients from ionic strength. It relates the coefficient to ionic strength, the effective diameter of the ion, and the ionic charges of the solute. The equation depends on ion size, which is the effective diameter of a hydrated ion. Ion size can vary with charge and ion geometry, so it adds uncertainty to the calculation.

The Debye-Hückel equation works best in extremely dilute solutions and in solutions with ionic strength below 0.1 mol/L. In these cases, uncertainty in ion size has little effect on the activity coefficient. The solution behaves mostly ideally, and the equation reduces to the simpler Debye-Hückel Limiting Law.

Activity Coefficient in Ionic Solutions

ionic strength: overview

Ionic Strength in Salt and Acid Solutions

Ionic strength describes the total electrolyte concentration in a solution. It is a quantitative measure of how many ions are present and how strongly they contribute to the solution. The concept was introduced in 1921 by Gilbert N. Lewis and Merle Randall while they were describing the activity coefficient of strong electrolytes. To calculate ionic strength, written as I or μ, both cations and anions are included. The concentration of each ion matters, but the charge number z matters even...

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thermodynamics: chemical potential and activity

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Chemical Potential in Solution Activity

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...

thermodynamics: activity coefficient

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 in Equilibrium Constant Calculations

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

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.

chemical equilibria: systematic approach to equilibrium calculations

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Equilibrium Calculations with Mass and Charge

Chemical equilibrium calculations become easier when they are organized around mass balance and charge balance. This approach is useful for systems with multiple equilibria, where several reactions must be considered together. It is especially helpful when finding the solubility of a sparingly soluble salt in water with a common ion present. The first step is to identify every chemical reaction in the system. Next, write the equilibrium constant expression for each reaction. These expressions...

acid&#8211;base equilibria: activity-based definition of ph

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How Ionic Strength Changes pH

pH depends on hydrogen ion activity, not just hydrogen ion concentration. In an ideal solution, pH is the negative logarithm of the hydrogen ion concentration. In a non-ideal solution, a more accurate pH uses hydrogen ion activity, which combines concentration with an activity coefficient. This difference matters most in very low ionic strength solutions, such as pure water. At first, the hydrogen ion activity coefficient is close to one. When an electrolyte that does not donate or accept a...

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Using Ladder Diagrams to Predict pH

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Redox Potential Shifts in Ladder Diagrams

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How Solubility Reaches Equilibrium

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Water Ionization and Kw in Equilibrium

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complexation equilibria: overview

Stepwise Metal-Ligand Complexation

Stepwise metal-ligand complexation forms coordination compounds when metal ions and ligands share electrons through dative, or coordinate covalent, bonds. In these reactions, the metal ion binds to one or more ligands until its coordination number is reached. This number tells how many bonds form between the metal ion and the ligands. In aqueous solution, many metal ions are already surrounded by water molecules. These species are called aqua complexes. Complexation reactions are described by...

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Chelating Ligands and Metal Complex Stability

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complexation equilibria: factors influencing stability of complexes

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Complex Ions: Metal and Ligand Stability

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redox equilibria: overview

Redox Electron Transfer and Half-Reactions

Redox reactions involve the transfer of electrons from one species to another. In this type of reaction, electrons are not shared between atoms. They move from the species that loses them to the species that gains them. The species that loses electrons is the reducing agent, also called the reductant. It is oxidized during the reaction. The species that gains electrons is the oxidizing agent, or oxidant, and it is reduced. Redox reactions are often written as two half-reactions. One...

titrimetric methods: types and commonly used strategies

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Direct, Back, and Displacement Titrations

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Four Reaction Types in Titrimetric Analysis

Titrimetric analysis measures solution volume and is also called volumetric analysis. A standard solution with a known concentration is placed in the burette and is called the titrant. The solution with an unknown concentration in the flask is the analyte, or titrand. Titrimetric analysis is grouped into four types based on the reaction between the titrant and the analyte. Acid-base titrations involve a neutralization reaction between an acid and a base. This reaction forms water molecules.

key terms and concepts

Learn how activity coefficient changes with ionic strength, why activity differs from concentration, and when the Debye-Hückel Limiting Law applies.

Activity Coefficient in Ionic Solutions | JoVE K12

analytical chemistry

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Activity Coefficient in Ionic Solutions

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