Strengths of Acids and Bases

Strong Acids and Bases The strength of an acid depends on how much it ionizes in water Strong acids completely ionize, releasing all their H + Ex. HCl and H 2 SO 4 HCl (g) + H 2 O (l) → H 3 O + (aq) + Cl - (aq) (100% ionized) Weak acids only ionize slightly in solution, only releasing a small amount of H + Ex. Acetic acid, CH 3 COOH CH 3 COOH (aq) + H 2 O (l) ↔ H 3 O + (aq) + CH 3 COO - (aq)

Acid Dissociation Constant With a weak acid like vinegar, less than 1% of the H + (aq) or H 3 O + (aq) are being ionized at any given moment The level of ionization determines the strength of the acid The more it ionizes, the greater its strength The equilibrium constant can be written as follows: K eq = [H 3 O + ]x[CH 3 COO - ]/[CH 3 COOH]x[H 2 O]

Dilute Solutions The concentration of water is constant It can be combined with K eq to give an acid dissociation constant This is the ratio of the dissociated (ionized) form of the acid to the concentration of the undissociated form of the acid K eq = K a = [H 3 O + ]x[CH 3 COO - ]/[CH 3 COOH] Strong acids have high K a values and weak acids are small

Table 19.7, p. 607 Name the monoprotic, diprotic and triprotic acid. What do you notice about the polyprotic acids with respect to their K a ? What is considered to be the strongest of the weak acids? Which ionization constant is considered to be the weakest?

What makes a strong acid – strong? In solution, the negative ion of an acid is a base In strong acids, the negative ion is a very weak base Its ability to combine with a hydrogen ion is slight Weak acids contain an anion that is considered to be a strong base This negative ion will combine with a hydrogen ion readily to form an acid again Concentration is different from acid and base strength. The equilibrium of a strong acid or base favours products. Weak acids and bases' equilibrium favours reactants.

Base Dissociation Constant Strong bases completely dissociate into metal ions and hydroxide ions in aqueous solution Even though they are not always soluble, what does dissolve, dissociates completely in water Ex. Ca(OH) 2 Weak bases – NH 3(aq) + H 2 O (l) ↔ NH 4 + (aq) + OH - (aq) The reactants are favoured at equilibrium.

Base Dissociation Constant K eq = [NH 4 + ]x[OH - ]/[NH 3 ]x[H 2 O] Again, water is a constant in dilute solutions… K eq = K b = [NH 4 + ]x[OH - ]/[NH 3 ] The general form of the equation is as follows: K b = [conjugate acid]x[OH - ]/[base]

Determining the K a of an acid K a = [H + ] x [A - ]/[HA] Sample Problem 19.5, p. 610 Questions 22, 23