2.5
Équilibres chimiques : redéfinir la constante d'équilibre
L’effet d’un sel inerte sur la solubilité d’un sel peu soluble est connu sous le nom d’effet sel. Le degré de l’effet salin varie en fonction de la fo…
Rappelons que la solubilité d’un sel peu soluble augmente avec l’ajout d’un sel inerte dans un phénomène connu sous le nom d’effet sel.
L’ampleur de l’effet salin dépend de la force ionique de la solution.
Pour tenir compte de l’écart par rapport à l’idéalité, la constante d’équilibre doit se rapporter à l’activité – qui incorpore l’énergie molaire partielle de Gibbs, ou potentiel chimique – de chaque espèce.
Cette constante d’équilibre, appelée constante d’équilibre thermodynamique ou standard, exprime le changement d’énergie de Gibbs du processus et intègre les effets de la force ionique.
À une force ionique négligeable, le coefficient d’activité est à peu près égal à l’unité, et la constante d’équilibre de concentration est à peu près égale à la constante d’équilibre thermodynamique.
Cette approximation est valable pour les solutions d’ions chargés individuellement ou d’espèces non dissociatives dont l’intensité ionique est inférieure à 0,01 mole par litre.
Cependant, pour les solutions dont l’intensité ionique est supérieure à 0,01 mole par litre ou dont les ions sont chargés en plusieurs fois, des corrections du coefficient d’activité sont nécessaires pour éviter des erreurs importantes.
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Q1: What is the salt effect and how does it influence solubility?
The salt effect describes how solubility of a sparingly soluble salt increases when an inert salt is added to the solution. This phenomenon occurs because the added salt increases the ionic strength of the solution, which affects the activity coefficients of the dissolved species and alters their effective concentrations in the equilibrium system.
Q2: Why is the thermodynamic equilibrium constant different from the concentration equilibrium constant?
The thermodynamic equilibrium constant relates the activity of each species, which incorporates both concentration and activity coefficient, accounting for deviations from ideal behavior. The concentration equilibrium constant uses only molar concentrations. At low ionic strengths, activity coefficients approach unity, making the two constants approximately equal, but at higher ionic strengths, activity coefficient corrections become necessary.
Q3: When can you ignore activity coefficient corrections in equilibrium calculations?
Activity coefficient corrections can be ignored for dilute solutions containing singly charged ions or non-dissociating species with ionic strengths lower than 0.01 mol/L. In these cases, the activity coefficient is approximately unity, and the concentration equilibrium constant is approximately equal to the thermodynamic equilibrium constant, simplifying experimental calculations.
Q4: What role does ionic strength play in determining whether activity corrections are needed?
Ionic strength determines the magnitude of the salt effect and the deviation from ideal solution behavior. Solutions with ionic strengths greater than 0.01 mol/L or containing multiply charged ions require activity coefficient corrections to avoid significant errors. The higher the ionic strength, the greater the deviation from ideality and the more critical these corrections become.
Q5: How does activity relate to concentration in chemical equilibrium systems?
Activity is the product of concentration and the activity coefficient of a species. It represents the effective concentration of a species in solution, accounting for interactions between ions and deviations from ideal behavior. The activity coefficient incorporates the partial molar Gibbs energy, or chemical potential, of each species in the system.
Q6: Why are multiply charged ions more affected by ionic strength changes?
Multiply charged ions experience stronger electrostatic interactions with other ions in solution, making their activity coefficients more sensitive to changes in ionic strength. These ions require activity coefficient corrections even at lower ionic strengths to prevent significant errors in equilibrium constant calculations and to accurately predict solubility behavior.
Q7: How does the thermodynamic equilibrium constant express Gibbs energy change?
The thermodynamic equilibrium constant incorporates the ionic strength of the solution and relates the activities of species, which are based on their chemical potential. By accounting for activity rather than concentration alone, this constant accurately expresses the Gibbs energy change of the process and reflects the true thermodynamic driving force of the reaction.