Transcript Chapter 16: Acid-Base Equilibria

Chapter 16: Acid-Base Equilibria
• 16.1. Acids and Bases: A Brief
Review
• 16.7 Weak Bases
• 16.2 Brønsted-Lowry Acids and • 16.8 Relationship Between Ka
Bases
and Kb
• 16.3 The Autoionization of
Water
• 16.9 Acid-Base Properties of
Salt Solutions
• 16.4 The pH Scale
• 16.10 Acid-Base Behavior and
Chemical Structure
• 16.5 Strong Acids and Bases
• 16.11 Lewis Acids and Bases
• 16.6 Weak Acids
• Arrhenius defined an acid as a substance that
produces H+ ions in water; he defined a base
as a substance that produces OH– ions in
water. HCl—one of the strong acids—is an
Arrhenius acid. Potassium hydroxide—one of
the strong bases—is an Arrhenius base.
16.2 Brønsted-Lowry Acids and Bases
• According to Brønsted-Lowry, an acid is a
substance that donates an H+ ion to another
substance; a base is a substance that accepts
an H+ ion
Conjugate
Bronsted
Acid
Bronsted
Base
Base
Conjugate
Acid
Classifications of Acids
• Any species that contains hydrogen can be classified as one of three
types of acids.
1. The strong acids are those that completely transfer their protons
to water, leaving no undissociated molecules in solution (see
eChapter 4.3). Their conjugate bases have a negligible tendency to
be protonated (to combine with a proton) in aqueous solution.
2. The weak acids are those that only partly dissociate in aqueous
solution and therefore exist in the solution as a mixture of acid
molecules and component ions. Their conjugate bases are weak
bases, showing a slight ability to remove protons from water.
3. The substances with negligible acidity are those such as CH4, that
contain hydrogen but do not demonstrate any acidic behavior in
water. Their conjugate bases are strong bases, reacting completely
with water, abstracting a proton to form OH– ions.
16.3 The Autoionization of Water
• Pure water has a very small tendency to ionize, acting both as an
acid (donating a proton) and as a base (accepting a proton).
• At 25°C, the Kc for this process is 1.0 x 10–14, which means that only
about one molecule per billion undergoes this autoionization. The
equilibrium expression for the autoionization of water is
• (Recall that a liquid does not appear in the equilibrium expression.)
• Because the autoionization of water is a very important
equilibrium, its equilibrium constant is given a special subscript, w.
For any aqueous solution at 25°C, the product of hydronium and
hydroxide ion concentrations is equal to Kw. In neutral water, where
the only source of either ion is the autoionization, the hydronium
and hydroxide ion concentrations are equal.
• So the concentrations of both hydronium ion and hydroxide ion in
neutral water is 1.0 x 10–7 M.
16.4 The pH Scale
• Figure 16.5. Values of pH
for some common
solutions. The pH scale is
shown to extend from 0 to
14 because nearly all
solutions commonly
encountered have pH
values in that range. In
principle, however, the pH
values for strongly acidic
solutions can be less than
0, and for strongly basic
solutions can be greater
than 14.
Useful pH Equations
16.5 Strong Acids and Bases
Strong Acid
Formula
Strong Base
Formula
Hydrochloric
HCl
Sodium hydroxide
NaOH
Hydrobromic
HBr
Potassium
hydroxide
KOH
Hydroiodic
HI
Rubidium
hydroxide
RbOH
Nitric
HNO3
Cesium hydroxide
CsOH
Chloric
HClO3
Barium hydroxide
Ba(OH)2
Perchloric
HClO4
Strontium
hydroxide
Sr(OH)2
Sulfuric
H2SO4
Calcium hydroxide
Ca(OH)2
• Strong acids are those that ionize completely in water. Strong acids are also
strong electrolytes
• Strong bases are ionic compounds that dissociate completely in water. They
include the hydroxides of group 1A metals and group 2A metals
16.6 Weak Acids
• A weak acid is one that ionizes partially in
water to produce hydronium ion and a
conjugate base
• Ex:
Weak Acid Sample Problem
What is the pH of a 1.75 10–3 M nitrous acid solution?
The ionization equilibrium expression for nitrous acid is
HNO2
H+
+
NO2-
Initial
1.75 10–3 M
0
0
Change
-X
+x
+x
Equilibrium
1.75 10–3 M – x
X
X
Sample Problem Solution
Polyprotic acids
• Polyprotic acids are those that have more than
one hydrogen that can be donated as a proton.
Each proton on a polyatomic acid has a Ka value
associated with it—except for the first proton on
sulfuric acid, because it is strongly acidic
16.7 Weak Bases
• A weak base is one that ionizes partially in water to produce
hydroxide ion and a conjugate acid. The general form for Bronsted
Base ionization is,
• B(aq) + H2O  BH+(aq) + OH-(aq)
• Being an equilibrium, the ionization of a weak base has an
equilibrium constant called the base-dissociation
constant associated with it. The equilibrium constant for a weak
base ionization has the subscript b for base. The equilibrium
expression for the above equation is
• As with all equilibria, the larger the value of K, the further the
equilibrium lies to the right. This means that the larger the value
of Kb, the stronger the base.
• Two major types of weak bases are amines (Contain N-C bond) and
conjugate base of weak acid
Weak Base examples
Weak base sample problem
Conjugate Acid
Bronsted Base
(Amine type)
Using pH to Determine the
Concentration of a Salt Solution
• A solution is made by adding solid sodium
hypochlorite (NaClO, pool chlorine) to enough
water to make 2.00 L of solution has a pH of
10.50. Calculate the grams of NaClO that were
added to the water. The Kb of ClO- is 3.3 x 10-7.
Problem solving technique
• You are given 2.00 L solution of NaClO, pH is
10.50, Kb = 3.3 x 10-7.
• NaClO is a strong electrolyte (you know that
because it is ionic [metal-nonmetal], and
contains alkali metal), so [ClO-] = [NaClO]
• pH can give you [OH-], then you can ICE box to
calculate [ClO-]
• pOH = 14.00 – pH = 14.00 – 10.50 = 3.50
• [OH-] = 10-3.50 = 3.2 x 10-4 M (at equilibrium)
ClO- + H2O  HClO +
OHInitial
X
0
0
Change
- 3.2 x 10-4
+ 3.2 x 10-4
+ 3.2 x 10-4
Equilibrium
X - 3.2 x 10-4
3.2 x 10-4
3.2 x 10-4
𝐾𝑏 =
𝐻𝐶𝑙𝑂 [𝑂𝐻 − ]
[𝐶𝑙𝑂− ]
𝑋 = 0.31 𝑀
3.3 x 10-7 =
2.00 𝐿 𝑥
(3.2 𝑥 10−4 )2
(𝑋 −3.2 𝑥 10−4 )
0.31 𝑚𝑜𝑙 𝑁𝑎𝐶𝑙𝑂
𝐿
𝑥
74.43 𝑔
𝑚𝑜𝑙 𝑁𝑎𝐶𝑙𝑂
= 46 g NaClO
Relationship between Ka and Kb
𝐾𝑎 𝑥 𝐾𝑏 = 𝐾𝑤 = 10−14
𝑝𝐾𝑎 + 𝑝𝐾𝑏 = 𝑝𝐾𝑤 = 14
16.9 Acid-Base Properties of Salts
• Characteristics to Determine Acid-Base
– Anion’s ability to react with water
• Anion that is conjugate base of strong acid is neutral
• Anion that is conjugate base of weak acid is basic
– Cation’s ability to react with water
• Cation of strong base is neutral
• Cation that is conjugate acid of a weak base is acidic
• Metal cations are acidic
– Combined effect
• Depends on K values, larger K wins
16.10 Acid-Base behavior and chemical
structure
• Factors that affect acid strength
– Strength of H-X bond, weaker bonds mean
stronger acid
16.10 Acid-Base behavior and chemical
structure
• Oxyacids (R-O-H)
– If R is a metal, substance is a base
– If R is a nonmetal
• Strength increases as electronegativity of R increases
• Strength increases as number of oxygen atoms increases
• Carboxylic acids
– Could have electron-withdrawing group (like element
of high electronegativity)
– Could have electron-donating group (like benzene
ring)
16.11 Lewis acids and bases
• Lewis acid is electron-pair acceptor
• Lewis base is an electron-pair donor
• Classic example: