Which framework is used to account for non-ideality in activity coefficients at moderate ionic strengths?

• A. Arrhenius
• B. Van't Hoff
• C. Nernst
• D. Debye-Hückel or extended Debye-Hückel

Answer: (D)

Non-ideality in electrolyte solutions comes from electrostatic interactions between ions, so activity coefficients deviate from unity as ions influence each other. The Debye-Hückel framework captures these effects by modeling the ionic atmosphere around each ion and how it perturbs the ion’s chemical potential. At very low ionic strength, you get the limiting law, which shows how the deviation scales with the square root of ionic strength. To handle moderate ionic strengths, the extended Debye-Hückel equation adds a size parameter for each ion and uses a more practical form that remains accurate as I increases, typically written with a log of the activity coefficient that depends on the ion’s charge, its size parameter, and the ionic strength. This approach embodies how real solutions deviate from ideal behavior due to electrostatic interactions, and it smoothly reduces to the limiting law in dilute conditions. The other options describe temperature effects or electrode potentials rather than a framework for correcting non-ideality in activity coefficients across moderate ionic strengths.