Common Ion Effect, Buffers and Indicators

Common Ion Effect

• Shift in equilibrium caused by the addition of a compound having an ion in common with the dissolved substance.

• Special case of the Le Châtelier’s principle

Common Ion Effect

• CH3COONa and CH3COOH in solution

CH3COONa(s) → CH3COO-(aq) + Na+

(aq)

CH3COOH(aq) + H2O ↔ CH3COO-(aq) + H3O+

(aq)

Addition of CH3COO- will suppress the ionization of CH3COOH

Henderson-Hasselbalch equation

For a weak acid HA:(derive please)

pH = pKa + log

NOTE: we use [ ]i and not [ ]eq as long as [ ]i≥0.1M

Calculate….

• What is the pH of a 0.30M HCOOH solution? pKa= 3.77

• What will be the pH in the presence of 0.52M HCOOK?

Henderson-Hasselbalch equation

• For a weak base and a salt with a common ion

• pOH = pKb + log

Buffer Solutions

• Solution of (1) a weak acid/base and (2) its salt.

• Both components must be present

• The solution has the ability to resist pH changes upon the addition of either an acid or a base.

Buffer Solutions

• For a simple acetic acid/acetate buffer

CH3COONa(s) → CH3COO-(aq) + Na+

(aq)

CH3COO-(aq) + H+

(aq) → CH3COOH(aq)

CH3COOH(aq) + OH-(aq) → CH3COO-

(aq) + H2O

Buffer Solutions

• See animation

Analyze….

• Which of the following are buffer systems?

• KH2PO4/H3PO4

• NaClO4/HClO4

• C5H5N(weak base)/C5H5NHCl (C5H5N+H is a weak acid)

• KF/HF• KBr/HBr

Buffering Capacity

• Effectiveness of a buffer• Ability to resist pH changes• Depends on the amount of acid and conjugate

base • Amount of acid/base needed to

decrease/increase the pH by 1.0• pH = pKa (maximum buffering capacity)

Buffer range/pH range

• pH range over which a buffer is effective

pH = pKa ± 1

Calculate….

• What is the pH of a buffer system containing 1.0 M CH3COOH and 1.0 M CH3COONa?

• What is the pH after adding 0.10 mole of HCl to 1.0 L of the solution?

• What is the pH after adding 0.10 mole of NaOH to 1.0 L of the solution?

• NOTE: Assume no volume changes

Preparing a buffer solution

• If [acid] ≈ [conj. base], then log ≈ 0• pH ≈ pKa

Calculate….

• How would you prepare a phosphate buffer with a pH of 7.40?

• Ka1= 7.5 x 10-3

• Ka2= 6.2 x 10-8

• Ka3= 4.8 x 10-13

Calculate….

• How will you prepare a carbonate buffer at pH 10.10? You are provided with carbonic acid, sodium hydrogen carbonate and sodium carbonate.

• Ka1 = 4.2 x 10-7

• Ka2 = 4.8 x 10-11

Acid-Base Indicators

• Weak acids/bases that have distinctly different colors in their HIn and In- forms.

• HIn(aq) ↔ H+(aq) + In-

(aq)

• Acidic: HIn color predominates• Basic: In- color predominates

Acid-Base Indicators

• color of HIn predominates

• color of In- predominates

• If [Hin] ≈ [In-], combination of 2 colors

Acid-Base Indicators

¿listo? Teneís un QUIZ

• What is the pH of a solution that is 0.20 M in NH3 and 0.30 M in NH4Cl? Compare its pH in a 0.20 M NH3 solution. Kb = 1.8 x 10-5

• Mr. Eugenio was asked to prepare a buffer at pH = 8.60, using one of the ff. weak acids, HA (Ka = 2.7 x 10-3), HB (Ka = 4.4 x 10-6), HC (Ka = 2.6 x 10-9) Which acid did Mr. Eugenio choose (well, if he performed it correctly hehehe)?