A large number of definitions for acids and bases have been introduced, but the 1923 definitions of J.N. Br0nsted and T.M. Lowry are the most useful for discussions of ionic equilibria in aqueous systems. According to the Br0nsted-Lowry model, an acid is a substance capable of donating a proton to another substance, such as water:
The acidic substance (HA) that originally donated the proton becomes the conjugate base (A-) of that substance, since the conjugate base could conceivably accept a proton from an even stronger acid than the original substance.
Recalling the discussion above about water and its activity, the thermodynamic equilibrium constant expression for equation (20) would be:
For an acid capable of ionizing into a univalent anion, YH+ and YA- will be approximately equal, and YHA will be approximately equal to one. Writing the concentration-based equilibrium constant expression as:
it follows that:
For weak acids, the magnitude of KA is very small, and as a result the resulting H3O+ and A- ions will be produced in small amounts. Under those conditions, both Yh+ and YA- will be approximately equal to one, and then one can approximate the thermodynamic equilibrium constant, K, by the concentration-based ionization constant, KA.
A strong acid is defined as a substance that reacts completely with water, so that the acid ionization constant defined in equation (21) or (22) is effectively infinite. This situation can only be achieved if the conjugate base of the strong acid is very weak. A weak acid will be characterized by an acid ionization constant that is considerably less than unity, so that the position of equilibrium in the reaction represented in equation (20) favors the existence of un-ionized free acid. One can define pKA as:
A discussion of the ionic equilibria associated with basic substances exactly parallels that just developed for acidic substances. A base is a substance capable of accepting a proton donated by another substance, such as water:
The basic substance (B) that originally accepted the proton becomes the conjugate acid (BH+) of that substance, since the conjugate acid could conceivably donate a proton to an even stronger base than the original substance. The concentration-based ionization constant expression corresponding to equation (25) is:
A strong base is a substance that reacts completely with water, so that the base ionization constant defined in equation (26) is effectively infinite. This situation can only be realized if the conjugate acid of the strong base is very weak. A weak base will be characterized by a base ionization constant that is considerably less than unity, so that the position of equilibrium in the reaction represented in equation (25) favors the existence of un-ionized free base.
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