7-3 Acid and Base Monoprotic acid: an acid that donates one H + ion per molecule. HBr → H + + Br -

Diprotic acid: an acid that donates two H+ ion per molecule H 2 SO 4 + H 2 O → H 3 O + + HSO 4 - HSO H 2 O → H 3 O + + SO 4 2-

Polyprotic acid: an acid that donates three or more H+ ion per molecule. H 3 PO 4 + H 2 O → H 3 O + + H 2 PO 4 - H 2 PO H 2 O → H 3 O + + HPO 4 2- HPO H 2 O → H 3 O + + PO 4 3-

Polyprotic can yield a higher concentration of hydronium ions than their given concentration. H 3 PO 4 (aq) + 3 H 2 O(l) PO 4 3- (aq) + 3H 3 O + (aq) M M M

Strength is defined in terms of the degree of dissociation of the acid or base into ions. For example: HX + H 2 O → H 3 O + + X - BOH → B + + OH - Strength of Acids and Bases

A strong acid or base is defined as one which completely or nearly completely dissociates into ions in solution. A weak acid or base is defined as one which only a small amount dissociates into ions in solution.

Keq = [H 3 O + ][X - ] [HX] strong acid: a very high Keq (favours the product). weak acid: a very low Keq (favours the reactant).

Note: The equilibrium expression for an acid is referred to as a Ka; the constant for a base is a Kb. Note: A polyprotic acid would have a different Ka for each step; the value of the Ka would decrease for each step.

Strength vs. Concentration: Strength is a measure of the degree of dissociation of an acid or base. Concentration is a measure how many moles of acid or base are present per litre of solution. Comparing Strength of Acids: The strength of an acid is measured by its Ka value. The higher the Ka, the stronger the acid.

If two acids are put in solution the strongest one will dominate; it will act as the acid, forcing the weaker acid to act as a base. For example: Oxalic acid and Citric acid HOOCCOOH H + + HOOCCOO - H 3 C 6 H 5 O 7 H + + H 2 C 6 H 5 O 7 -

Because oxalic acid is higher on the table it is the strongest acid; it will go in the forward direction and will force citric acid to act as the base and go in the reverse direction. HOOCCOOH H + + HOOCCOO - H + + H 2 C 6 H 5 O 7 - H 3 C 6 H 5 O 7 ___________________________________________ HOOCCOOH + H 2 C 6 H 5 O 7 - H 3 C 6 H 5 O 7 +HOOCCOO - acidbaseacid base

Ionization of Water and Kw Acids and bases act in aqueous solution. There is a relationship between the hydronium ion and the hydroxide ion in solution. The equation for the dissociation of water is: H 2 O(l) → H 3 O + (aq) + OH - (aq)

The equilibrium constant for the dissociation of water is Keq = [H 3 O + ][OH - ] The Keq for water, like that of acids and bases has a special notation; Kw.

At most temperatures the Kw of water is 1.00 x Kw =[H 3 O + ][OH - ] Kw = 1.00 x In a neutral solution the concentration of both hydronium and hydroxide ions is 1.00 x M

If the hydronium ion concentration is greater than 1.00 x M the solution is acidic; If it is less than that value, the solution will be basic (since the concentration of hydronium ions is less than 1.00 x M, the hydroxide ion concentration will be greater than 1.00 x M).