Chapter 16: Acids and Bases

1
Chapter 16 Acids and Bases

• Understand the Arrhenius and Bronsted-Lowry
  models for acids and bases.
• Understand acid strength
• Understand the relationship between acid strength
  and strength of the conjugate base
• Learn about the ionization of water
• Understand and uses the terms pH and pOH
• Learn methods for measuring the pH of a solution
• Learn to calculate the pH of solutions of strong
  acids.
• Learn about acid-base titration.
• Understand the general characteristics of
  buffered solutions.

2
Chapter 16.1 Acids and Bases

• Objective To learn about 2 models of acids
• and bases and the relationship of conjugate
  acid-base pairs.

3
Chapter 16 Acids and Bases

• Why do we study them?
• Lemons (citric acid)
• Digest food (HCl)
• Sulfuric acid is the chemical produced in the
  greatest amount in the U.S.
• 80 billion pounds of acids are used each year
• To make fertilizers, detergents, plastics,
  pharmaceuticals, storage batteries and metals.

4
16-1 Properties of Acids and Bases

• Were first recognized as substances that taste
  sour.
• Vinegar (acetic acid)
• Lemon (citric acid)
• Bases alkalis
• Bitter taste, slippery feel
• Most hand soaps and commercial preparations for
  unclogging drains are highly basic.

5
16-1 Arrhenius Model

• Arrhenius postulated that acids produce H ions
  in aqueous solution
• HCl H Cl-
• strong acids
• Bases produce OH- ions
• NaOH Na OH-

6
16-1 Bronsted-Lowry Model

• Acid is a proton donor
• Base is a proton acceptor
• HA(aq) H2O(l) H3O(aq) A-(aq)
• Acid Base conj. Conj.
• acid base
• Conjugate acid-base pair consists of 2 substances
  related to each other by the donating and
  accepting of a single proton.

7
16-1 Writing Conjugate Bases

• HClO4
• H3PO4
• CH3NH3

H ClO4-
Acid
conjugate base
H H2PO4-
Acid
conjugate base
H CH3NH2
Acid
conjugate base
8
16.2 Acid Strength

• Objectives To understand what acid strength
  means.
• To understand the relationship between acid
  strength and that strength of the conjugate base.

9
16.2 Acid Strength

• Strong acid completely ionized or completely
  dissociated.
• HA(aq) H2O H3O(aq) A-(aq)
• Weak acidHA(aq) H2O H3O(aq)
  A-(aq)

10
Figure 16.1 Representation of the behavior of
acids of different strengths in aqueous solution.
11
Figure 16.2 Relationship of acid strength and
conjugate base strength for the dissociation
reaction.
Strong acids are sulfuric acid,
hydrochloric acid, nitric acid, perchloric acid,
12
Strong Acids/weak acids

• Oxyacids acidic hydrogen is attached to
• An oxygen atom.
• Phosphoric acid, acetic acid, hypochlorous,
• Nitrous acid.
• Organic acids those with a carbon-atom
• Backbone, commonly contain the carboxyl group.
  
• CO
• OH

Weak acids
13
16.3 Water as an Acid and a Base

• Objective To learn about the ionization of
  water.
• Amphoteric behave as either an acid or
• as a base.
• Ionization of water
• H2O(l) H2O(l) ? H3O(aq) OH-(aq)

14
16.3 Water as an Acid and a Base

• H3O OH- 1.0x10-7 M
• At 25o C, the product of the ions is constant.
• Kw H3O OH-1.0 x 10-14M
• Kw is called the ion-product constant for water.

15
16.3 Water as an Acid and a Base

• H3O OH- 1.0x10-7 M
• A neutral solution, where H OH-
• An acidic solution, where H gt OH-
• A basic solution, where OH-gt H
• Kw H3O OH-1.0 x 10-14M

16
16.3 Water as an Acid and a Base

• Calculate H or OH- as required for each of
  the following solutions at 25oC.
• 1x10-4M H
• 1x10-8M H
• 1x10-9M OH-
• 1x10-3M OH-

1x10-10 M OH-
1x10-6M OH-
1x10-5M H
1x10-11M H
17
16.4 The pH scale

• Objective To understand pH and pOH
• To learn to find pOH and pH for various
  solutions.
• To learn to use a calculator in these
  calculations.

18
16.4 The pH scale

• Logarithm scale that helps to represent
• exponents.
• LOGS ARE EXPONENTS!! Using log10 ("log to the
  base 10") log10100 2 is equivalent to 102
  100
• where 10 is the base, 2 is the logarithm (i.e.,
  the exponent or power) and 100 is the number
• 10 to the x power 1000

19
The pH scale provides a convenient way to
represent solution acidity.
pH -log H
H1x10-3M OH-1x10-5M
pOH -logOH-
pOH pH 14.00
20
16.5 Measuring pH

• Objective To learn methods for measuring the pH
  of a solution.
• INDICATORS substances that exhibit different
  colors in acidic and basic solutions.

21
Figure 16.4 Useful pH ranges for several common
indicators.
22
16.5 Measuring pH

• Indicator paper (litmus paper) paper coated with
  a combination of indicators. Indicator paper
  turns a specific color for each pH value.
• Electronically, pH meter contains a probe that is
  sensitive to the H. H produces a voltage
  that appears as a reading on the pH meter.

23
16.7 Acid-Base Titrations

• Neutralization reaction equal amounts of H and
  OH-. Result solution with pH7.
• Titration test that chemists perform to analyze
  the acid/base content of a solution.
• Deliver a measured volume of a solution (w/ known
  concentration) the titrant into the solution
  being analyzed analyte.

24
16.7 Acid-Base Titrations

• Titration
• Titrant standard solution (solution w/known
  concentration).
• Titrant is loaded into a buret.

25
Figure 16.8 Microscopic picture of the
solutions in the titration of 0.200 M HNO3 with
0.100 M NaOH.
Buret
26
Figure 16.9 The pH curve for the titration of
50.0 mL of 0.200 M HNO3 with 0.100 M NaOH.
Titration curve (pH Curve)
27
16.8 Buffered Solutions

• Objectives To understand the general
  characteristics of buffered solutions.
• Buffered solution is one that resists a change in
  its pH even when a strong acid or base is added
  to it.

28
16.8 Buffered solutions

• Why are they important?
• Many organisms can only exist in a narrow range
  of conditions.
• Blood (pH 7.35-7.45)

29
16.8 Buffered solutions

• Characteristics of a Buffer
• Solution contains a weak acid HA and its
  conjugate base A-
• The buffer resists changes in pH by reacting with
  any added H or OH- so that these ions dont
  accumulate
• Any added H reacts with the base A-.
• Any added OH- reacts with the weak acid HA.

30
16.8 Buffered solutions

• Examples
• HC2H3O2, NaC2H3O2
• In solution.
• HCl H Cl-
• H(aq) C2H3O2- HC2H3O2
• NaOH
• OH- HC2H3O2 H2O HC2H3O2

31
Dissociation of acid in water

• Ka H A-
• HA
• Kb NH4 OH-
• NH3
• To deal w/ buffers
• Henderson-Hasselbalch
• pHpKa log(base/acid)