Strong and Weak Acids and Bases And Dissociation Constants 10.3- 10.4 Strong and Weak Acids and Bases And Dissociation Constants

10.3 The strength of an acid is determined by the extent to which it ionizes, its percent ionization, not the concentration of the acid, the concentration of its hydronium ions, or its ability to react with a metal

Strong Acid An acid that nearly completely dissociates All molecules of the acid break up to form the ions soluble in water If more than one proton is being removed, not all steps need to be complete dissociation.

Weak Acid An acid that only slightly dissociates in a water solution Only a small percent of acid molecules donate their hydrogen, and most remain the same. Example: CH3COOH

A strong acid essentially ionizes 100%. An example of a strong acid is hydrochloric acid, HCl (aq) HCl(g) + H2O(l)  H3O+(aq) + Cl-(aq) 0.10 mol 0.10 mol 0.10 mol 100% ionization few molecules many ions

An example of a weak acid is acetic acid, CH3COOH. CH3COOH(l) + H2O(l)  H3O+(aq) + CH3COO-(aq) 0.10 mol << 0.10 mol << 0.10 mol 5% ionization at 25C many molecules few ions

Strong Base A base that dissociates almost completely into its ions. All oxides and hydroxides of group 1 and 2 are strong bases. Ex: NaOH

Weak Base Most bases are weak They dissociate only slightly in a water solution Example: NH3

Strong acids are strong electrolytes and weak acids are weak electrolytes

A strong base dissociates 100%. An example of a strong base is sodium hydroxide, NaOH. NaOH(s) + H2O(l)  Na +(aq) + OH-(aq) 0.10 mol 0.10 mol 0.10 mol 100% dissociation few formula units (NaOH) many ions

A weak base ionizes to a small extent. An example of a weak base is NH3(g). NH3(g) + H2O(l)  NH4+(aq) + OH-(aq) 0.10 mol << 0.10 mol << 0.10 mol 5% ionization at 25C many molecules few ions

Strong bases are strong electrolytes and weak bases are weak electrolytes.

Examples of Strong Acids and Bases Strong Acids Strong Bases HClO4 perchloric acid LiOH lithium hydroxide HCl hydrochloric acid NaOH sodium hydroxide HNO3 nitric acid KOH potassium hydroxide H2SO4 sulfuric acid RbOH rubidium hydroxide HBr hydrobromic acid CsOH cesium hydroxide HI hydriodic acid Ca(OH)2 calcium hydroxide Sr(OH)2 strontium hydroxide Ba(OH)2 barium hydroxide

10.4

Dissociation Equation A balanced chemical equation showing all ions produced when an ionic compound dissolves Example: HSO4-(aq) + H2O(l)  SO42-(aq) + H3O+(aq)

The Acid-Dissociation Constant (Ka) Strong acids dissociate completely into ions in water: HA(g or l) + H2O(l) H3O+(aq) + A-(aq) In a dilute solution of a strong acid, almost no HA molecules exist: [H3O+] = [HA]init or [HA]eq = 0 Qc = [H3O+][A-] [HA][H2O] at equilibrium, Qc = Kc >> 1 Nitric acid is an example: HNO3 (l) + H2O(l) H3O+(aq) + NO3-(aq) Weak acids dissociate very slightly into ions in water: HA(aq) + H2O(aq) H3O+(aq) + A-(aq) In a dilute solution of a weak acid, the great majority of HA molecules are undissociated: [H3O+] << [HA]init or [HA]eq = [HA]init Qc = [H3O+][A-] [HA][H2O] at equilibrium, Qc = Kc << 1

The Meaning of Ka, the Acid Dissociation Constant For the ionization of an acid, HA: HA(aq) + H2O(l) H3O+(aq) + A-(aq) Since the concentration of water is high, and does not change significantly during the reaction, it’s value is absorbed into the constant. Kc = [H3O+] [A-] [HA] [H2O] Therefore: Kc = [H3O+] [A-] [HA] The stronger the acid, the higher the [H3O+] at equilibrium, and the larger the Ka: Stronger acid higher [H3O+] larger Ka For a weak acid with a relative high Ka (~10-2 ), a 1 M solution has ~10% of the HA molecules dissociated. For a weak acid with a moderate Ka (~10-5 ), a 1 M solution has ~ 0.3% of the HA molecules dissociated. For a weak acid with a relatively low Ka (~10-10 ), a 1 M solution has ~ 0.001% of the HA molecules dissociated.

Figure 7.1: Graphical representation of the behavior of acids of different strengths in aqueous solution. A Strong Acid A Weak Acid

The Extent of Dissociation for Strong and Weak Acids