Chapter 21: Further Aspects of Equilibria

Further Aspects of Equilibria on advanced equilibria, Chapter 21 of the Cambridge International AS & A Level Chemistry textbook, thoroughly explores concepts related to acids, bases, solubility, and liquid partitioning. The initial section revisits the Brønsted-Lowry theory, establishing the concept of conjugate acid and conjugate base pairs linked by the transfer of a single proton. Key mathematical definitions are introduced, including the ionic product of water (K w ), which is the product of hydrogen ion and hydroxide ion concentrations (typically 1.00×10 −14 mol 2 dm −6 at 298 K), and the pH scale, defined as the negative logarithm (to base 10) of the hydrogen ion concentration. Calculation methodologies are detailed for determining the pH of strong acids, where the hydrogen ion concentration approximates the initial acid concentration, and strong alkalis, which requires using K w​ to first determine the hydrogen ion concentration from the hydroxide concentration. For weak acids, the chapter introduces the acid dissociation constant (K a​ ) as the specific equilibrium constant quantifying the extent of ionisation, alongside pK a​ (negative logarithm of K a​ ) for comparing acid strengths; procedures are outlined for calculating K a​ from pH data or calculating pH using the K a value and acid concentration. The text then focuses on buffer solutions, defining them as mixtures (such as a weak acid and its conjugate base) that stabilize pH by possessing reserve supplies of both the acid and its conjugate base to consume any added hydrogen or hydroxide ions. Crucial applications of buffers, including the role of the HCO 3 (hydrogencarbonate ion) system in maintaining the stable pH of human blood, are described. Moving to heterogeneous equilibria, the solubility product (K sp​ ) is defined as the equilibrium expression for sparingly soluble ionic compounds in saturated solutions, allowing for the calculation of solubility and the prediction of precipitation by comparing ion concentration products to the K sp​ value. The related common ion effect explains the reduction in a compound's solubility when a solution containing an ion common to the compound is introduced, shifting the equilibrium toward the solid state. Finally, the principle of partitioning is covered through the partition coefficient (K pc​ ), which is the equilibrium constant relating the concentration of a solute between two immiscible solvents, emphasizing how the numerical value of K pc​ is determined by the relative polarities and intermolecular bonding between the solute and the two solvents.