Lecture 24. Acids and bases
Tuesday 24 April 2024
Acids and bases: The Brønsted-Lowry definition. Conjugate acid-base pairs. Acid strength and the acid ionization constant, Ka. Autoinonization of water and the definition of Kw. Using Kw in calculations. Definition of pH and the pH scale. Calculating pH for strong and weak acids. The weak acid equilibrium.
Reading: Tro NJ. Chemistry: Structure and Properties (3rd ed.) - Ch.16 §16.1 - §16.7 (pp.713-736t)
Summary - I
To begin with, we'll review what we already have learned about acids and bases (see Acids and bases: An introduction). In so doing, we'll also cover the definition of pH and the pH scale. Next, we introduce a broader, more useful definition for acids and bases, the Brønsted-Lowry definition. Finally we'll apply the general concepts of chemical equilibrium to calculations involving weak acids and bases.
The most important equation to master at the outset is the acid dissociation equation. Written in a general form for the case of a neutral, undissociated acid as reactant,
HA = H+ + A−
which shows loss of H+ from the acid. Writing the equilibrium constant expression for the equation according to the usual rules immediately yields the acid dissociation constant, Ka. The other important feature of this equation is that the Brønsted-Lowry definition for acids and bases is embedded in it. HA here is shown acting as a donor of H+ - that is, as a generalized Brønsted-Lowry acid. Similarly, the product A− represents a generalized Brønsted-Lowry base, since it acts as an acceptor of H+ in the reverse direction of the acid dissociation equation.
A quantitative treatment of weak acid and weak base equilibria can be found here.