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

Activity is the measure of effective concentration. It is unitless and can be expressed as the product of the molar concentration of the species and its activity coefficient.
The activity coefficient, a dimensionless quantity measuring the deviation from ideal behavior, depends on the ionic strength. In an ideal solution, activity equals concentration.
In solutions having very low ionic strength, the activity coefficient of an ion is close to unity, and the behavior is essentially ideal.
As the ionic strength increases, the activity coefficient becomes less than unity, indicating deviation from ideality where the activity is lower than the concentration.
However, in solutions with very high ionic strength, the activity coefficient may exceed unity.
Interestingly, the activity coefficient of a neutral species is approximately unity at ionic strengths of less than 0.1 moles per liter.
The relationship between the activity coefficient and ionic strength is expressed as the extended Debye&#8211;H&#252;ckel equation.
However, for extremely dilute solutions with very low ionic strength, the equation reduces to the Debye&#8211;H&#252;ckel limiting law.

thermodynamics: activity coefficient

Activity is the measure of the effective concentration of the species in solution. It can be expressed as the product of the molar concentration of the species and its activity coefficient. The activity coefficient is a dimensionless quantity and depends on the total ionic strength of the solution.
The activity coefficient is a measure of the deviation from ideal behavior. When the ionic strength of the solution is minimal, the activity coefficient of an ionic species is close to unity, making activity approximately equal to the molar concentration. Such a solution exhibits behaviors very close to those of an ideal solution. The activity coefficient of an uncharged molecule is approximately unity at all ionic strengths less than 0.1 mol/L.
As the ionic strength of the solution increases, the activity coefficient decreases, making the activity less than the molar concentration. This indicates the deviation of the species from ideal behavior. In solutions with ionic strength higher than 0.1 mol/L, the activity coefficient increases and exceeds unity.
A useful equation for calculating the activity coefficient from ionic strength is the extended Debye&#8211;H&#252;ckel equation, which relates the coefficient to ionic strength, the effective diameter of the ion, and the ionic charges of the solute. The Debye&#8211;H&#252;ckel equation relies on the ion size - the effective diameter of a hydrate ion - which can vary widely with charge and the geometry of the ion, thereby introducing uncertainty into the equation.
The equation works well in extremely dilute solutions and solutions with ionic strength less than 0.1 mol/L and extremely dilute solutions, where the uncertainties in ion sizes have negligible effects on the activity coefficients. In these cases, the solution behaves mostly ideally, and the Debye&#8211;H&#252;ckel equation reduces to a simpler form known as the Debye&#8211;H&#252;ckel Limiting Law.

Thermodynamics: Activity Coefficient

Activity is the measure of effective concentration. It is unitless and can be expressed as the product of the molar concentration of the species and its ...

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Chemical Equilibria

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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 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

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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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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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