Ionic Equilibrium Class 11 Notes for IIT JEE & NEET

The Henderson-Hasselbalch equation is pH = pKa + log([Salt]/[Acid]). It is used to calculate the pH of a buffer solution quickly without solving a full equilibrium expression. It is valid when the ratio [Salt]/[Acid] is between 0.1 and 10 — that is, within one pH unit of pKa.

Chemical equilibrium covers reversible reactions in general (including gas-phase reactions). Ionic equilibrium is a specific subset that deals only with the dissociation of weak electrolytes in solution and the equilibria involving ions. All ionic equilibria follow the same Le Chatelier's principle but use concentration-based expressions since solutions are involved.

Ionic equilibrium is the dynamic equilibrium between ions and unionised molecules formed when a weak electrolyte partially dissociates in solution. It covers acid-base theories, pH, Ka, Kb, Kw, buffer solutions, salt hydrolysis, and solubility product. This chapter appears in NCERT Class 11 Chemistry Part 1, Chapter 7

The common ion effect is the decrease in solubility (or degree of dissociation) of a weak electrolyte when a strong electrolyte sharing a common ion is added to the solution. For example, adding NaCl to a saturated AgCl solution increases [Cl⁻], shifting the equilibrium left and reducing AgCl solubility. It is a direct application of Le Chatelier's principle.

Ksp (solubility product) is the equilibrium constant for the dissolution of a sparingly soluble salt. For a salt AxBy dissolving into x A^(y+) and y B^(x-) ions, Ksp = [A]^x [B]^y. If the molar solubility is s, then for AgCl: Ksp = s². For PbCl₂: Ksp = 4s³. Knowing the salt formula directly gives you the Ksp–s relationship.

Ionic Equilibrium, combined with Chemical Equilibrium, typically contributes 4–8 marks in JEE Main (1–2 questions per paper). According to NTA's subject-wise paper analysis, this topic has appeared in every JEE Main session from 2019 to 2024 without exception, making it one of the most reliable scoring areas in Physical Chemistry.