Unit 6 - Chpt 15 - Acid/Base Equilibria

• Common Ion Effect

• Buffers / Buffer Capacity

• Titration / pH curves

• Acid / Base Indicators• HW set1: Chpt 15 - pg. 736-742 # 17, 19, 21, 23,

25, 34, 38, 40, 44 Use Appendix 5 for Ka Kb values - Due Mon. Mar 3

Common Ion Effect• Shift in equilibrium position that occurs because of the

addition of an ion already involved in the equilibrium reaction.

• An application of Le Châtelier’s principle.

HCN(aq) + H2O(l) H3O+(aq) + CN-(aq)

• Addition of NaCN will shift the equilibrium to the left because of the addition of CN-, which is already involved in the equilibrium reaction.

• A solution of HCN and NaCN is less acidic than a solution of HCN alone.

Ka Problem with common ion

Calculate the pH of a 0.50 M aqueous solution of the weak acid HF (Ka = 7.2 x 10–4) and 0.10 M NaF (a strong electrolyte).

HF(aq) + H2O H3O+(aq) + F–(aq)Initial 0.50 M ~ 0 0.10MChange –x +x +xEquilibrium 0.50–x x 0.10M + x

[H3O+]= 3.6 x 10–3

pH = 2.44 What was pH before adding NaF?

[ ]3H O A

= HA

+ −⎡ ⎤⎡ ⎤⎣ ⎦⎣ ⎦K

Buffered Solutions

• Buffered Solution – resists a change in pH.

• They are weak acids or bases containing a common ion.

• After addition of strong acid or base, deal with stoichiometry first, then the equilibrium.

Buffer Problems flow chart

Buffers - How do they work?

Buffers - How do they work? (2)

Henderson–Hasselbalch Equation

• For a particular buffering system (conjugate acid–base pair), all solutions that have the same ratio [A–] / [HA] will have the same pH.

[ ]a

ApH = p + log

HA

−⎡ ⎤⎣ ⎦K

Buffer Problem

What is the pH of a buffer solution that is 0.45 M acetic acid (HC2H3O2) and 0.85 M sodium acetate (NaC2H3O2)? The Ka for acetic acid is 1.8 × 10–5.

pH = 5.02

Buffers graphic