Aqueous Ionic Solutions and Equilibrium Chapter 19

Common Ion Effect Shift in equilibrium that occurs because of the addition of an ion already involved in the equilibrium reaction. What happens to the equilibrium if 0.10 M NaF is added? What happens to the equilibrium if 0.10 M NaCl is added?

Buffers Resist a change in pH when H + or OH - is added Components:conjugate acid-base pairs acidic component reacts with OH - basic component reacts with H + Example:1.00 L of M CH 3 COOH M CH 3 COONa CH 3 COOH/CH 3 COO -

Reactions with H + or OH -

Key Points on Buffers 1.Weak acids and bases containing common ion 2.Problems involve: stoichiometry first equilibrium second

Buffer Characteristics - Contain relatively large amounts of weak acid and corresponding base. - Added H + reacts to completion with weak base. - Added OH  reacts to completion with weak acid. - pH is determined by ratio of concentrations of weak acid and weak base.

Buffer Capacity Amount of H + or OH - it can absorb without a significant change in pH. For HA/A - system, buffer capacity depends on: — [HA] and [A - ](higher = higher) — [HA] ratio(closer to 1 = higher) [A - ] [A - ]

Calculations with K a Calculate the pH of a buffer consisting of 0.50 M HF and 0.45 M F - (a)before and (b)after addition of 0.40 g NaOH to 1.0 L of the buffer. K a of HF = 6.8 x 10 -4

Calculations Find pH of a buffer Buffer preparation Find equilibrium concentrations Helpful:Hendeson-Hasselbach equation

Calculations with K a Calculate the pH of a buffer consisting of 0.50 M HF and 0.45 M F - using the Henderson-Hasselbach equation.

Preparing a buffer 1.Choose and acid-conjugate base pair 2.Calculate the ratio of the buffer component pairs 3.Determine the buffer concentration How would you prepare a benzoic acid/benzoate buffer with pH = 4.25, starting with 5.0 L of M sodium benzoate (C 6 H 5 COONa) solution and adding the acidic component? K a of benzoic acid (C 6 H 5 COOH) = 6.3 x 10 -5

Titration (pH) Curves Plot pH of solution vs. amount of titrant added Equivalence point: Enough titrant added to react exactly with solution being analyzed.

Titration of Strong Acid with Strong Base

Weak Acid-Strong Base Titration 1.Stoichiometry Reaction assumed to run to completion 2.Equilibrium Use weak acid equilibrium to find pH

Weak Acid-Strong Base Titration

Differences

Differences and K a

Titration Calculations 1.Solution of HA 2.Solution of HA and added base 3.Equivalent amounts of HA and added base 4.Excess base A chemist titrates mL of M HBrO (K a = 2.3 x ) with M NaOH. Find the pH: (a)before any base is added (b)when mL of NaOH is added (c)at the equivalence point (d)when the moles of OH - added are twice the moles of HBrO originally present?

Titration of Strong Base with Strong Acid

Weak Base-Strong Acid Titration

Acid-Base Indicator Indicates endpoint of a titration Endpoint is not necessarily the equivalence point

Polyprotic Acid, H 2 SO 3

Solubility Constant, K sp For solids dissolving to form aqueous solutions. For slightly soluble salts: equilibrium between solid and component ions

K sp Write the expression for K sp for (a)CaSO 4 (b)Cr 2 CO 3 (c)Mg(OH) 2 (d)As 2 S 3

Solubility Product Solubility s: Amount of PbCl 2 that dissolves For: [PbCl 2 ] dissolved = [Pb 2+ ] = ½[Cl - ] s:varies, especially if common ion is present

Calculations 1.5 x g of CaF 2 dissolves in mL solution at 18°C. Write the expression for K sp. Find the molar solubility of CaF 2. Find the [Ca 2+ ] and [F - ]. Calculate K sp. What is the molar solubility of Mg(OH) 2 if the value of K sp is 6.3 x ?

Complex Ions Complex Ion: Charged species - metal ion surrounded by ligands (Lewis bases). Coordination Number: No. of ligands attached to a metal ion. (Common: 6 and 4.) Formation (Stability) Constants, K f : Equilibrium constants for stepwise addition of ligands to metal ions.

Overall Formation Constant

Complex Ions Write the stepwise formation constants for Cr(NH 3 ) 6 3+, starting from Cr(H 2 O) 6 3+ and NH 3(aq) What is the coordination number of Cr 3+ ?

Applications Selective precipitation Exploit differences in K sp Qualitative analysis 1.Insoluble chlorides (Ag +, Hg 2 2+, Pb 2+ ) 2.Acid-insoluble sulfides (Cu 2+, Cd 2+, Hg 2+, As 3+, Sb 3+, Bi 3+, Sn 2+, Sn 4+, Pb 2+ ) 3.Base-insoluble sulfides and hydroxides (Zn 2+,Mn 2 +, Ni 2+, Fe 2+, Co 2+ as sulfides; Al 3+, Cr 3+ as hydroxides) 4.Insoluble phosphates (Mg 2+, Ca 2+, Ba 2+ ) 5.Alkali metal and ammonium ions