Chemistry: Matter and Change

1
CHEMISTRY Matter and Change
Chapter 18 Acids and Bases
2
Table Of Contents
CHAPTER18
Section 18.1 Introduction to Acids and
Bases Section 18.2 Strengths of Acids and
Bases Section 18.3 Hydrogen Ions and pH Section
18.4 Neutralization
Click a hyperlink to view the corresponding
slides.
Exit
3
Introduction to Acids and Bases
SECTION18.1

• Identify the physical and chemical properties of
  acids and bases.

• Classify solutions as acidic, basic, or neutral.
• Compare the Arrhenius, Brønsted-Lowry, and Lewis
  models of acids and bases.

Lewis structure a model that uses electron-dot
structures to show how electrons are arranged in
molecules
4
Introduction to Acids and Bases
SECTION18.1
acidic solution basic solution Arrhenius
model Brønsted-Lowry model conjugate acid
conjugate base conjugate acid-base
pair amphoteric Lewis model
Different models help describe the behavior of
acids and bases.
5
Introduction to Acids and Bases
SECTION18.1
Properties of Acids and Bases

• Acids taste sour. Bases taste bitter and feel
  slippery.

• Acids and bases are conductors of electricity.
• Acids and bases can be identified by their

6
Introduction to Acids and Bases
SECTION18.1
Properties of Acids and Bases (cont.)

• Acids turn blue litmus red.

• Bases turn red litmus blue.
• Magnesium and zinc react with acids to produce
  hydrogen gas.
• Geologists identify limestone because it produces
  bubbles of carbon dioxide when exposed to
  hydrochloric acid.

7
Introduction to Acids and Bases
SECTION18.1
Properties of Acids and Bases (cont.)

• All water solutions contain hydrogen ions (H)
  and hydroxide ions (OH).

• An acidic solution contains more
• A basic solution contains more

8
Introduction to Acids and Bases
SECTION18.1
Properties of Acids and Bases (cont.)

• The usual solvent for acids and bases is
  waterwater produces equal numbers of hydrogen
  and hydroxide ions in a process called
  self-ionization.

• The hydronium ion is H3O.

9
Introduction to Acids and Bases
SECTION18.1
The Arrhenius Model

• The Arrhenius model states that an acid is a
  substance that contains hydrogen and ionizes to
  produce hydrogen ions in aqueous solution, and a
  base is a substance that contains a hydroxide
  group and dissociates to produce a hydroxide ion
  in solution.

10
Introduction to Acids and Bases
SECTION18.1
The Arrhenius Model (cont.)

• Arrhenius acids and bases

• HCl ionizes to produce H ions.
• HCl(g) ? H(aq) Cl(aq)
• NaOH dissociates to produce OH ions.
• NaOH(s) ? Na(aq) OH(aq)
• Some solutions produce hydroxide ions even though
  they do not contain a hydroxide group.

11
Introduction to Acids and Bases
SECTION18.1
The Brønsted-Lowry Model

• The Brønsted-Lowry Model of acids and bases
  states that

• The Brønsted-Lowry Model is a more inclusive
  model of acids and bases.

12
Introduction to Acids and Bases
SECTION18.1
The Brønsted-Lowry Model (cont.)

• A conjugate acid is the species produced when

• A conjugate base is the species produced when
• A conjugate acid-base pair consists of two
  substances related to each other by

13
Introduction to Acids and Bases
SECTION18.1
The Brønsted-Lowry Model (cont.)

• Hydrogen fluoridea Brønsted-Lowry acid

• HF(aq) H2O(l) ? H3O(aq) F(aq)
• HF acid, H2O base, H3O conjugate acid, F
  conjugate base

14
Introduction to Acids and Bases
SECTION18.1
The Brønsted-Lowry Model (cont.)

• Ammonia Brønsted-Lowry base

• NH3(aq) H2O(l) ? NH4(aq) OH(aq)
• NH3 base, H2O(l) acid, NH4 conjugate acid,
  OH conjugate base

• are called amphoteric.

15
Introduction to Acids and Bases
SECTION18.1
Monoprotic and Polyprotic Acids

• An acid that can donate only one hydrogen ion is
  a monoprotic acid.

• Only ionizable hydrogen atoms can be donated.

16
Introduction to Acids and Bases
SECTION18.1
Monoprotic and Polyprotic Acids

• Acids that can donate more than one hydrogen ion
  are polyprotic acids.

17
Introduction to Acids and Bases
SECTION18.1
The Lewis Model

• According to the Lewis model, a Lewis acid is

• The Lewis model includes all the substances
  classified as Brønsted-Lowry acids and bases and
  many more.

18
Strengths of Acids and Bases
SECTION18.2

• Relate the strength of an acid or base to its
  degree of ionization.

electrolyte an ionic compound whose aqueous
solution conducts an electric current

• Compare the strength of a weak acid with the
  strength of its conjugate base.
• Explain the relationship between the strengths of
  acids and bases and the values of their
  ionization constants.

19
Strengths of Acids and Bases
SECTION18.2
strong acid weak acid acid ionization
constant strong base weak base base ionization
constant
In solution, strong acids and bases ionize
completely, but weak acids and bases ionize only
partially.
20
Strengths of Acids and Bases
SECTION18.2
Strengths of Acids

• are strong acids.

• Because they produce the maximum number of
  hydrogen ions, strong acids are good conductors
  of electricity.

21
Strengths of Acids and Bases
SECTION18.2
Strengths of Acids (cont.)

• are called weak acids.

22
Strengths of Acids and Bases
SECTION18.2
Strengths of Acids (cont.)

• With a strong acid, the conjugate base is a weak
  base.

• Equilibrium lies almost completely to the right
  in the equation because the conjugate base has a
  weaker attraction for the H ion than does the
  base in the forward reaction.
• In a weak acid, the ionization equilibrium lies
  to the far left in the ionization equation
  because the conjugate base has a greater
  attraction for H ions than does the base in the
  forward reaction.

23
Strengths of Acids and Bases
SECTION18.2
Strengths of Acids (cont.)

• The equilibrium constant, Keq, provides a
  quantitative measure of the degree of ionization
  of an acid.

• The acid ionization constant is
• Ka indicates whether products or reactants are
  favored at equilibrium.

24
Strengths of Acids and Bases
SECTION18.2
Strengths of Acids (cont.)

• For weak acids, the products tend to be smaller
  compared to the un-ionized molecules (reactant).

• Weaker acids have a smaller Ka.

25
Strengths of Acids and Bases
SECTION18.2
Strengths of Bases

• is known as a strong base.

• A weak base

26
Strengths of Acids and Bases
SECTION18.2
Strengths of Bases (cont.)

• The base ionization constant, Kb, is the value of
  the equilibrium constant expression for the
  ionization of a base.

27
Hydrogen Ions and pH
SECTION18.3

• Explain pH and pOH.

Le Châteliers principle states that if a stress
is applied to a system at equilibrium, the system
shifts in the direction that relieves the stress

• Relate pH and pOH to the ion product constant for
  water.
• Calculate the pH and pOH of aqueous solutions.

pH and pOH are logarithmic scales that express
the concentrations of hydrogen ions and hydroxide
ions in aqueous solutions.
ion product constant for water pH pOH
28
Hydrogen Ions and pH
SECTION18.3
Ion Product Constant for Water

• Pure water contains equal concentrations of H
  and OH ions.

• The ion product constant of water,
  Kw HOH.
• The ion product constant for water is the value
  of the equilibrium constant expression for the
  self-ionization of water.

29
Hydrogen Ions and pH
SECTION18.3
Ion Product Constant for Water (cont.)

• With pure water at 298 K, both H and OH are
  equal to 1.0 107M.

• Kw at 298 K 1.0 1014
• Kw and LeChâteliers Principle proves H
  OH must equal 1.0 1014 at 298 K, and as
  H goes up, OH must go down.

30
Hydrogen Ions and pH
SECTION18.3
pH and pOH

• Concentrations of H ions are often small numbers
  expressed in scientific notation.

• pH is the pH log H

31
Hydrogen Ions and pH
SECTION18.3
pH and pOH (cont.)

• pOH of a solution is

• pOH log OH
• The sum of pH and pOH equals 14.

32
Hydrogen Ions and pH
SECTION18.3
pH and pOH (cont.)

• For all strong monoprotic acids, the
  concentration of the acid is the concentration of
  H ions.

• For all strong bases, the concentration of the
  OH ions available is the concentration of the
  base.
• Weak acids and weak bases only partially ionize
  and Ka and Kb values must be used.

33
Hydrogen Ions and pH
SECTION18.3
pH and pOH (cont.)

• Litmus paper and a pH meter with electrodes can
  determine the pH of a solution.

34
Neutralization
SECTION18.4

• Write chemical equations for neutralization
  reactions.

stoichiometry the study of quantitative
relationships between the amounts of reactants
used and products formed by a chemical reaction
is based on the law of conservation of mass

• Explain how neutralization reactions are used in
  acid-base titrations.
• Compare the properties of buffered and unbuffered
  solutions.

35
Neutralization
SECTION18.4
neutralization reaction salt titration titrant equ
ivalence point
acid-base indicator end point salt
hydrolysis buffer buffer capacity
In a neutralization reaction, an acid reacts with
a base to produce a salt and water.
36
Neutralization
SECTION18.4
Reactions Between Acids and Bases

• A neutralization reaction is a reaction in which
  an

• A salt is an
• Neutralization is a double-replacement reaction.

37
Neutralization
SECTION18.4
Reactions Between Acids and Bases (cont.)
38
Neutralization
SECTION18.4
Reactions Between Acids and Bases (cont.)

• Titration is a method for determining the
  concentration of a solution by

39
Neutralization
SECTION18.4
Reactions Between Acids and Bases (cont.)

• In a titration procedure, a measured volume of an
  acid or base of unknown concentration is placed
  in a beaker, and initial pH recorded.

• A buret is filled with the titrating solution of
  known concentration, called a titrant.

40
Neutralization
SECTION18.4
Reactions Between Acids and Bases (cont.)

• Measured volumes of the standard solution are
  added slowly and mixed into the solution in the
  beaker, and the pH is read and recorded after
  each addition. The process continues until the
  reaction reaches the equivalence point, which is
  the point at which

• An abrupt change in pH occurs at the equivalence
  point.

41
Neutralization
SECTION18.4
Reactions Between Acids and Bases (cont.)
42
Neutralization
SECTION18.4
Reactions Between Acids and Bases (cont.)

• Chemical dyes whose color are affected by acidic
  and basic solutions are called acid-base
  indicators.

43
Neutralization
SECTION18.4
Reactions Between Acids and Bases (cont.)

• An end point is the point at which an indicator
  used in a titration changes color.

• An indicator will change color at the equivalence
  point.

44
Neutralization
SECTION18.4
Salt Hydrolysis

• In salt hydrolysis, the anions of the dissociated
  salt accept hydrogen ions from water or the
  cations of the dissociated salt donate hydrogen
  ions to water.

45
Neutralization
SECTION18.4
Salt Hydrolysis (cont.)

• Salts that produce basic solutions

• KF is the salt of a strong base (KOH) and a weak
  acid (HF).
• KF(s) ? K(aq) F(aq)

46
Neutralization
SECTION18.4
Salt Hydrolysis (cont.)

• Salts that produce acidic solutions

• NH4Cl is the salt of a weak base (NH3) and strong
  acid (HCl).
• When dissolved in water, the salt dissociates
  into ammonium ions and chloride ions.
• NH4Cl(s) ? NH4(aq) Cl(aq)

47
Neutralization
SECTION18.4
Salt Hydrolysis (cont.)

• Salts that produce neutral solutions

• NaNO3 is the salt of a strong acid (HNO3) and a
  strong base (NaOH).
• Little or no salt hydrolysis occurs because
  neither Na nor NO3 react with water.

48
Neutralization
SECTION18.4
Buffered Solutions

• The pH of blood must be kept in within a narrow
  range.

• Buffers are

• A buffer is a solution made up of a weak species
  and its conjugate.

49
Neutralization
SECTION18.4
Buffered Solutions (cont.)

• When acid is added, the equilibrium shifts to the
  left.

50
Neutralization
SECTION18.4
Buffered Solutions (cont.)

• Additional H ions react with F ions to form
  undissociated HF molecules but the pH changes
  little.

• is called the buffer capacity.

51
Neutralization
SECTION18.4
Buffered Solutions (cont.)

• A buffer is most effective when the
  concentrations of the conjugate acid-base pair
  are equal or nearly equal.