Write the increasing order of concentration of H+
ion in the following: HCl, Formic
acid, water.
Answers
Acids are classified as either strong or weak, based on their ionization in water. A strong acid is an acid which is completely ionized in an aqueous solution. Hydrogen chloride (HCl) ionizes completely into hydrogen ions and chloride ions in water.
text{HCl}(g) rightarrow text{H}^+(aq) + text{Cl}^-(aq)
A weak acid is an acid that ionizes only slightly in an aqueous solution. Acetic acid (found in vinegar) is a very common weak acid. Its ionization is shown below.
text{CH}_3text{COOH}(aq) rightleftarrows text{H}^+(aq) + text{CH}_3text{COO}^-(aq)
The ionization of acetic acid is incomplete, and so the equation is shown with a double arrow. The extent of ionization of weak acids varies, but 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. Because HCl is a strong acid, its conjugate base (Cl − ) is extremely weak. The chloride ion is incapable of accepting the H + ion 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.
Relative Strengths of Acids and their Conjugate Bases
Acid Conjugate Base
Strong Acids
HCl (hydrochloric acid) (strongest) Cl − (chloride ion) (weakest)
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) (weakest) CN − (cyanide ion) (strongest)
Strong acids are 100% ionized in solution. Weak acids are only slightly ionized. Phosphoric acid is stronger than acetic acid and so is ionized to a greater extent. Acetic acid is stronger than carbonic acid, and so on.
The Acid Ionization Constant, Ka
The ionization for a general weak acid, HA, can be written as follows:
text{HA}(aq) rightarrow text{H}^+ (aq) + text{A}^-(aq)
Because the acid is weak, an equilibrium expression can be written. An acid ionization constant (Ka) is the equilibrium constant for the ionization of an acid.