2.5
化学平衡：重新定义平衡常数
惰性盐对微溶盐中溶解度的影响称为盐效应。盐效应的程度会随着溶液的离子强度而发生变化，而离子强度又取决于溶液中物质的活性。可以将其中的活性表示为浓度与物种活性系数的乘积。
为了能够计算中等高离子强度溶液的平衡常数，则必须考虑到盐效应。这个重新定义的平衡常数也称为热力学平衡常数或标准平衡常数，因为它是用…
请记住，难溶盐的溶解度会随着惰性盐的加入而增加，这种现象称为盐效应。
盐效应的程度取决于溶液的离子强度。
为了考虑与理想状态的偏差，平衡常数必须关联各物种的活度——活度包含了偏摩尔吉布斯能，即化学势。
这个平衡常数称为热力学平衡常数或标准平衡常数，它表示该过程的吉布斯自由能变化，并包含了离子强度的影响。
在可忽略的离子强度下，活度系数近似为1，浓度平衡常数近似等于热力学平衡常数。
该近似方法适用于离子强度低于 0.01 摩尔每升的单价离子溶液或非解离物种。
然而，对于离子强度大于 0.01 摩尔每升或含有多种电荷离子的溶液，需要进行活度系数校正，以避免产生显著误差。
View the full transcript and gain access to JoVE Core videos
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.