The Equilibrium of Weak Acids and Bases

Weak Acids Molecules HF (aq) + H 2 O (l) ↔ H 3 O + (aq) + F - (aq) Cations NH 4 + (aq) + H 2 O (l) ↔ H 3 O + + NH 3 Zn(H 2 O) H 2 O ↔ H 3 O + + Zn(H 2 O) 3 (OH) + Dissociation Constants HA + H 2 O ↔ H 3 O + + A - K a =

Demo - The Tortoise and the Hare 1. For oxyacids that have the same number of O-H groups and the same number of O atoms, acid strength increases with increasing electronegativity of the central atom Y. 2. For oxyacids that have the same central atom Y, acid strength increases as the number of oxygen atoms attached to Y increases.

For example: increasing strength of acids: HClO<HClO 2 <HClO 3 <HClO 4 Or HIO 3 <HBrO 3 <HClO 3

Ex pH of a M HC 2 H 3 O 2 solution is 2.87; what is the K a ?

Ex Find [H 3 O + ] in M HC 2 H 3 O 2. (Ka = 1.8 x )

ex Find [H + ] in M HF. (Ka = 6.9 x )

Who is the Strongest Acid? Weak Bases Molecules NH 3(aq) + H 2 O (l) ↔OH - (aq) + NH 4 + (aq) Anions derived from weak acids F - (aq) + H 2 O (l) ↔HF (aq) + OH - (aq) Dissociation constant B - (aq) + H 2 O (l) ↔ HB (aq) + OH - (aq) K b = [HB][OH - ]/[B - ]

Ka x Kb = Kw Ka Kb HF6.9 x F x HAc1.8 x Ac x NH x NH x Strength of base is inversely related to that of the acid.

Ex Calculate [OH - ] and pH of a M NaF solution.

Acid-Base Properties of Salt Solutions Cations 1.Spectator - derived from strong bases: Li +, Na +, K +, Ca 2+, Sr 2+, Ba 2+ 2.Acidic - all other cations including those of transition metals. Anions 1.Spectator - derived from strong acids: Cl -, Br -, I -, NO 3 -, ClO 4 -, SO Basic - anions derived from weak acids such as F - or NO 2 -

Overall Results SaltCatio nAnionEffect NaNO 3 Na + NO 3 - neutral KFK + F - basic FeCl 2 Fe 2+ Cl - acidic If the cation is acidic and the anion basic, Compare Ka and Kb values: NH 4 F:Ka = 5.6 x Kb = 1.4 x Ka > Kb: acidic

Lewis acid - electron pair acceptor Lewis base - electron pair donor demo = MOM to the Rescue Demo-Hydrolysis of Salts

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