Transcript Document

Chapter 19
Ionic Equilibria in Aqueous
Systems
Ionic Equilibria in Aqueous Systems
19.1- Equilibria of Acid-Base Buffer Systems
19.3- Equilibria of Slightly Soluble Ionic Compounds
19.5- Application of Ionic Equilibria to Chemical Analysis
We will only be covering these sections in chapter 19
The effect of addition of acid or base to …
acid added
base added
an unbuffered solution
acid added
base added
or a buffered solution
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The Effect of Added Acetate Ion on the Dissociation of Acetic Acid
[CH3COOH]initial
[CH3COO-]added
% Dissociation*
pH
0.10
0.00
1.3
2.89
0.10
0.050
0.036
4.44
0.10
0.10
0.018
4.74
0.10
0.15
0.012
4.92
* % Dissociation =
[CH3COOH]dissoc
[CH3COOH]initial
x 100
How a buffer works
Buffer after addition of H3O+
Buffer with equal
concentrations of
conjugate base and acid
H3O+
H2O + CH3COOH
H3O+ + CH3COO-
Buffer after addition of OH-
OH-
CH3COOH + OH-
H2O + CH3COO-
Activity 1
PROBLEM:
Calculating the Effect of Added H3O+ and OHon Buffer pH
Calculate the pH:
(a) of a buffer solution consisting of 0.50M CH3COOH and 0.50M CH3COONa
(b) after adding 0.020mol of solid NaOH to 1.0L of the buffer solution in part (a)
(c) after adding 0.020mol of HCl to 1.0L of the buffer solution in part (a)
Ka of CH3COOH = 1.8x10-5. (Assume the additions cause negligible volume
changes.
The relation between buffer capacity and pH change
Activity 2
PROBLEM:
Preparing a Buffer
An environmental chemist needs a carbonate buffer of pH 10.00
to study the effects of the acid rain on limestone-rich soils. How
many grams of Na2CO3 must she add to 1.5L of freshly prepared
0.20M NaHCO3 to make the buffer? Ka of HCO3- is 4.7x10-11.
Solubility-Product Constants (Ksp) of Selected Ionic Compounds at 250C
Name, Formula
Ksp
Aluminum hydroxide, Al(OH)3
3 x 10-34
Cobalt (II) carbonate, CoCO3
1.0 x 10-10
Iron (II) hydroxide, Fe(OH)2
4.1 x 10-15
Lead (II) fluoride, PbF2
3.6 x 10-8
Lead (II) sulfate, PbSO4
1.6 x 10-8
Mercury (I) iodide, Hg2I2
4.7 x 10-29
Silver sulfide, Ag2S
8 x 10-48
Zinc iodate, Zn(IO3)2
3.9 x 10-6
Activity 3
Writing Ion-Product Expressions for Slightly
Soluble Ionic Compounds
PROBLEM: Write the ion-product expression for each of the following:
(a) Magnesium carbonate
(c) Calcium phosphate
(b) Iron (II) hydroxide
(d) Silver sulfide
Activity 4
Determining Ksp from Solubility
PROBLEM: (a) Lead (II) sulfate is a key component in lead-acid car batteries.
Its solubility in water at 250C is 4.25x10-3g/100mL solution. What is
the Ksp of PbSO4?
(b) When lead (II) fluoride (PbF2) is shaken with pure water at 250C,
the solubility is found to be 0.64g/L. Calculate the Ksp of PbF2.
Activity 5
Determining Solubility from Ksp
PROBLEM: Calcium hydroxide (slaked lime) is a major component of mortar,
plaster, and cement, and solutions of Ca(OH)2 are used in
industry as a cheap, strong base. Calculate the solubility of
Ca(OH)2 in water if the Ksp is 6.5x10-6.
Relationship Between Ksp and Solubility at 250C
No. of Ions
Formula
Cation:Anion
Ksp
Solubility (M)
2
MgCO3
1:1
3.5 x 10-8
1.9 x 10-4
2
PbSO4
1:1
1.6 x 10-8
1.3 x 10-4
2
BaCrO4
1:1
2.1 x 10-10
1.4 x 10-5
3
Ca(OH)2
1:2
5.5 x 10-6
1.2 x 10-2
3
BaF2
1:2
1.5 x 10-6
7.2 x 10-3
3
CaF2
1:2
3.2 x 10-11
2.0 x 10-4
3
Ag2CrO4
2:1
2.6 x 10-12
8.7 x 10-5
The effect of a common ion on solubility
CrO42- added
PbCrO4(s)
Pb2+(aq) + CrO42-(aq)
PbCrO4(s)
Pb2+(aq) + CrO42-(aq)
Activity 6
Calculating the Effect of a Common Ion on
Solubility
PROBLEM: In Activity 6, we calculated the solubility of Ca(OH)2 in water.
What is its solubility in 0.10M Ca(NO3)2? Ksp of Ca(OH)2 is
6.5x10-6.
Test for the
presence of a
carbonate
Activity 7
Predicting the Effect on Solubility of Adding
Strong Acid
PROBLEM: Write balanced equations to explain whether addition of H3O+ from a
strong acid affects the solubility of these ionic compounds:
(a) Lead (II) bromide
(b) Copper (II) hydroxide
(c) Iron (II) sulfide
Activity 8
PROBLEM:
Predicting Whether a Precipitate Will Form
A common laboratory method for preparing a precipitate is to mix
solutions of the component ions. Does a precipitate form when
0.100L of 0.30M Ca(NO3)2 is mixed with 0.200L of 0.060M NaF?
The amphoteric behavior of aluminum hydroxide
Activity 9
Separating Ions by Selective Precipitation
PROBLEM: A solution consists of 0.20M MgCl2 and 0.10M CuCl2. Calculate
the [OH-] that would separate the metal ions as their hydroxides.
Ksp of Mg(OH)2= is 6.3x10-10; Ksp of Cu(OH)2 is 2.2x10-20.
The general procedure for separating ions in qualitative analysis
Add
precipitating
ion
Centrifuge
Centrifuge
Add
precipitating
ion