21.3. Acid and Base Strength http://www.ck12.org
The directional arrow in the equation above indicates that the HCl reactant is converted completely into the product
ions. In other words, there is 100% ionization of HCl.
Aweak acidis an acid that ionizes only slightly in aqueous solution. Acetic acid, the acid found in vinegar, is a
very common weak acid. Its ionization is shown below.
CH 3 COOH(aq)⇀↽H+(aq)+CH 3 COO−(aq)The ionization of acetic acid is incomplete, and so the equation is shown with a double arrow, indicating equilibrium
between the reactant and products. The extent of ionization varies for different weak acids, but it is generally less
than 10%. A 0.10 M solution of acetic acid is only about 1.3% ionized, meaning that the equilibrium strongly favors
the reactants.
Weak acids, like strong acids, ionize to yield the H+ion and a conjugate base. In the examples so far, the conjugate
bases are the chloride ion (Cl−) and the acetate ion (CH 3 COO−). Because HCl is a strong acid, its conjugate base
(Cl−) is extremely weak. The chloride ion is essentially incapable of taking the H+ion off of H 3 O+and becoming
HCl again. In general, the stronger the acid, the weaker its conjugate base. Likewise, the weaker the acid, the
stronger its conjugate base. TheTable21.4 shows a listing of some common acids and their conjugate bases. The
acids are listed from strongest at the top to weakest at the bottom, while the order for the conjugate bases is reversed.
TABLE21.4: Relative Strengths of Acids and their Conjugate Bases
Acid Conjugate Base
Strong Acids
HCl (hydrochloric acid) Cl−(chloride ion)
H 2 SO 4 (sulfuric acid) HSO 4 −(hydrogen sulfate ion)
HNO 3 (nitric acid) NO 3 −(nitrate ion)
Weak Acids
H 3 PO 4 (phosphoric acid) H 2 PO 4 −(dihydrogen phosphate ion)
CH 3 COOH (acetic acid) CH 3 COO−(acetate ion)
H 2 CO 3 (carbonic acid) HCO 3 −(hydrogen carbonate ion)
HCN (hydrocyanic acid) CN−(cyanide ion)The top three acids in the table above (Table21.4) are strong acids and are thus all 100% ionized in solution. The
others are weak acids and are only slightly ionized. Phosphoric acid is stronger than acetic acid, so it is ionized to a
greater extent. Acetic acid is stronger than carbonic acid, and so on.
The Acid Ionization Constant, K
The ionization for a generic weak acid, HA, can be written in one of two ways.
HA(aq)+H 2 O(l)⇀↽H 3 O+(aq)+A−(aq)
HA(aq)→H+(aq)+A−(aq)The water molecule is omitted for simplicity in the second case. This will be the way that acid ionization equations
will be written for the remainder of the chapter. Because the acid is weak, an equilibrium expression can be written.
Anacid ionization constant (Ka)is the equilibrium constant for the ionization of an acid.
Ka=[H+][A−]
[HA]
The acid ionization represents the fraction of the original acid that has been ionized in solution. Therefore, the
numerical value of Kais a reflection of the strength of the acid. Weak acids with relatively higher Kavalues