Chapter 18 : Acid-Base Equilibria
18.1 Acids and Bases in Water
18.3 Proton Transfer and the Brønsted-Lowery
Acid-Base Definition
18.9 Electron-Pair Donation and the Lewis AcidBase Definition
18.2 Autoionization of Water and the pH Scale
18.4 Solving Problems Involving Weak-Acid
Equilibra
18.5 Weak Bases and Their Relation to Weak Acids
18.7 Acid-Base Properties of Salt Solutions
Acid-Base Equilibria
Acid-Base Concepts
Arrhenius
Bronsted/Lowery
Acid Nomenclature
Neutralization reactions
Amphoteric
Lewis
Arrhenius Theory
Acid
• H+ [H3O+] in water
• Sour taste
• Turns litmus red
• Naming Rules
Does the anion contain Oxygen?
No
Yes
Hydro
+ element root
+ ic acid
HBr
Hydrobromic
acid
check ending of anion
ite
ate
anion root
anion root
+ ous acid + ic acid
HNO2
HNO3
nitrous
nitric
acid
acid
Base
• OH– in water
• Bitter taste
• Turns litmus blue
• slippery
Neutralization Reaction:
Acid + Base  Salt + H2O
In acidic solutions, the protons (H+) that are
released into solution will not remain alone due
to their large positive charge density and small
size.
.. ..
O
H
H
They are attracted to the negatively charged
electrons on the oxygen atoms in water, and
form hydronium ions.
H+(aq) + H2O(l) = H3O+(l)
[H+] = [H3O+]
H atom and H+ ion atomic structure?
Use symbols H+ and H3O+ to represent the same thing
Bronsted-Lowry
Acid
• H+ donor
Base
• H+ acceptor
Practice worksheet and Unit design Problems
Summary
1. Acids and Bases can be molecules or ions
2. Not just defined in aqueous solutions
3. Conjugate pairs differ by H+
4. Some substances are both, AMPHOTERIC
depending on the reaction.
Lewis Theory
Acid
Accepts electron pair
BF3
Base
Gives electron pair
NH3
Draw Lewis dot structure of BF3 and NH3/
Then show combination according to the Lewis Theory
Key point: Lewis Acids unique and rare
Relative Acid Strengths
Competition for H+ ion
H X bond
Strong?
Weak?
Reactions favor formation of _______________.
Strong and Weak Acids
The Extent
of
Dissociation
for Strong
and Weak
Acids
Note reactions.
Relate to graphs
after dissociation.
The Acid-Dissociation Constant (Ka)
Strong acids dissociate completely into ions in water:
H3O+(aq) + A-(aq)
HA(g or l) + H2O(l)
+][A-]
[H
O
3
Qc =
[HA]
at equilibrium, Qc = Kc >> 1
Weak acids dissociate very slightly into ions in water:
HA(aq) + H2O(aq)
+][A-]
[H
O
3
Qc =
[HA]
Stronger acid
H3O+(aq) + A-(aq)
at equilibrium, Qc = Kc << 1
higher [H3O+]
larger Ka
Acid Base Solutions are aqueous.
Need to look at Water
Water is amphoteric
Self-ionization of water [autoionization]
H2O(l) + H2O(l)
Kc =
Autoionization of Water
The ion-product for water, Kw:
Kc[H2O]2 =
Kw = [H3O+][OH-] = 1.0 x 10-14
(at 25°C)
For pure water the concentration of hydroxyl and
hydronium ions must be equal:
[H3O+] = [OH-] =
1000g
The molarity of pure water is:
1L
1 mole
18.02 g
= 55.5 M
Classification of solutions
according to [H3O+]
[H3O+] > 1 x 10-7 acidic
[H3O+] = 1 x 10-7 neutral
[H3O+] < 1 x 10-7 basic
•
Examples:
1 x 10 -4
1 x 10-9
pH
•To handle the very large variations in the concentrations of
the hydrogen ion in aqueous solutions, a scale called the
pH scale is used.
• pH is defined as
the - log of the molar hydronium ion concentration:
pH = - log[H3O+]
Give pH values for [H3O+]
1 x 10 -4
1 x 10-9
Acid and Base Character and the pH Scale
pH of a neutral solution = 7.00
pH of an acidic solution < 7.00
pH of a basic solution > 7.00
Summary:
pH + pOH = 14
pH
pH = - log [H3O+]
[H3O+]
pOH
pOH = - log [OH-]
[OH-]
Kw = [H+][OH-] = 1 x 10-14 at 25oC
The pH Values of
Some Familiar
Aqueous
Solutions
Relationship of
[H3O+], pH,
[OH-] and
pOH
As [H3O+] increases
pH ________; [OH-] _______; pOH _________
Calculating [H3O+], pH, [OH-], and pOH
Problem: A chemist dilutes concentrated hydrochloric acid
to make two solutions:
(a) 3.0 M and (b) 0.0024 M.
Calculate the
[H3O+], pH, [OH-], and pOH of the two solutions at 25°C.
Plan: We know that hydrochloric acid is a strong acid, so
it dissociates completely in water; therefore [H3O+] =
[HCl]init.. We use the [H3O+] to calculate the [OH-] and pH as
well as pOH. Use chart as guide.
Solution:
Answers: 3; -0 .48; 3.33 x 10-15 ;14.5; and .0024; 2.62; 4.1 x 10-12; 11.4
Experiment 7: pH Study
Methods for Measuring the pH of an
Aqueous Solution
(a) pH paper
(b) Electrodes of a pH meter
Chart
pH + pOH = 14
pH
pOH
pH = - log [H3O+]
[H3O+]
pOH = - log [OH-]
[OH-]
Kw = [H+][OH-] = 1 x 10-14 at 25oC
What type of chemical species yields hydronium ions?
How many types are there?
How do we know which is which? Next slide
Relate hydronium ion concentration to each.
What type of chemical species yields hydroxide ions?
How many types and how do you know which is which?
Relate to hydroxide ion concentrations.
Classifying the Relative Strengths of Acids and Bases–I
Strong acids. There are two types of strong acids:
1. The hydrohalic acids HCl, HBr, and HI
2. Oxoacids in which the number of O atoms exceeds the number of
ionizable H atoms by two or more, such as HNO3, H2SO4, HClO4
Strong bases. Soluble compounds containing O2- or OH- ions are strong
bases. The cations are usually those of the most active metals:
1) M2O or MOH, where M= Group 1A(1) metals (Li, Na, K, Rb, Cs)
2) MO or M(OH)2, where M = Group 2A(2) metals (Ca, Sr, Ba)
[MgO and Mg(OH)2 are only slightly soluble, but the soluble
portion dissociates completely.]
The Dissociation Constant (Ka and Kb)
Weak acids dissociate very slightly into ions in water:
H3O+(aq) + A-(aq)
HA(aq) + H2O(l)
+][A-]
[H
O
3
Ka =
[HA]
Acid dissociation constant
Weak bases dissociate very slightly into ions in water
MOH(aq)
NH3 + H2O(l)
M+ (aq) + OH- (aq)
Kb
=
Base dissociation constant
Large Ka
? [H30+]
? Acid strength
Large Kb
? [H30+]
? Base strength
Solving Problems
Class examples
There are two general types of equilibrium problems
involving weak acids and their conjugate bases:
1. Given equilibrium concentrations, find Ka or Kb.
2. Given Ka or Kb and some concentration
information, find the other equilibrium
concentrations.
Plan: problem-solving approach.
***1.
Write the balanced equation and Ka expression;
these will help identify what to solve for.
2. Define x as the unknown concentration that changes.
3. Construct a reaction table, I.C.E. table, that incorporates
the unknown, x.
4. Make assumptions that simplify the calculation, usually
that x is very small relative to the initial concentration.
5. Substitute the values into the Ka expression and solve for x.
6. Check that the assumptions are justified, if not use the
quadratic.
The Relation Between Ka and Kb of a Conjugate
Acid-Base Pair
Acid
HA + H2O
H3O+ + A-
Base
A- + H2O
HA + OH-
2 H2O
H3O+ + OH-
[H3O+] [OH-] =
[H3O+] [A-]
Kw =
[HA]
x
[HA] [OH-]
[A-]
Ka x Kb
Ka = 4.5 x 10-4
Ka x Kb = (4.5 x 10-4)(2.2 x 10-11) = 9.9 x 10-15
For HNO2
or ~ 10 x 10-15 = 1 x 10 -14 = Kw
Kb = 2.2 x 10-11
pH of Solutions
What type of solutions are each of these?
0.1 M KCl
0.1 M KNO2
0.1 M NH4Cl
pH = 8.2
pH = 5.1
pH = 7.0
K+ no effect on pH
Cl- no effect on pH
K+
no effect on pH
NO2- makes
solution basic
Conclusion:
NH4+ makes
solution acidic
Cl- no effect on pH
Salts Solutions / Hydrolysis
Salts:
Definition
 Ionization:

Strong electrolytes - 100 % ionization
Form cations and anions
 Reaction with water: HYDROLYSIS
 Ion interaction with H2O in solution
Use to look at acidic and basic properties of
salt solutions
Possible Hydrolysis Reactions
Cation:
M+ + H2O  MOH + H+
_____?____ solution
Anion:
X- + H2O  HX + OH_____?_____ solution
Bronsted/Lowry reactions
Hydrolysis and pH
Strategy for determining pH of a salt solution
1. Ionize salt
2. Identify ion undergoing hydrolysis
(cation or anion or both)
3. Write hydrolysis reaction
4. Identify solution as acidic or basic
If needed calculate Kh value.
Hydrolysis Summary
1. Both ions are strong
2. Weak anion / anion hydrolysis
________
X- + H2O  HX + OH________
Kh = Kw / Ka of weak acid formed = Kb
3. Weak Cation / cation hydrolysis
____________
M+ + H2O  MOH + H+
Kh = Kw / Kb of weak base formed = Ka
4. Both ions weak
find Kh for each reaction
largest value of Kh determines type of solution.
Examples
Following the hydrolysis strategy steps
determine the type of solution and Kh for
each of the following.
1. NaCl
2. NaC2H3O2
3. NH4Cl
4. NH4C2H3O2
Class problems
Summary for Problem Solving
1. Read problem, identify what is asked and
what is given.
2. Decide if you are working with
Acid, Base or Salt
then determine if strong or weak.
3. Write Balanced Chemical Reaction using
Bronsted/Lowry theory.
Write K expression for reaction
4. Solve problem.