Acids and Bases: Introduction

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Acids and Bases Introduction

• Section 19.1

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Objectives

• Identify the physical and chemical properties of
  acids and bases
• Classify solutions as acidic, basic, or neutral
• Compare the Arrhenius and Bronsted-Lowry models
  of acids and bases

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Key Terms

• Acidic solutions
• Basic solutions
• Arrhenius model
• Bronsted-Lowry model
• Conjugate acid
• Conjugate base
• Conjugate acid-base pair
• amphoteric

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Properties of Acids and Bases
Acids Bases
Tastes sour Tastes bitter
Turns litmus red Turns litmus blue
React with base to form salt water React with acid to form salt water
Electrolyte Electrolyte
React with metal to form hydrogen gas Feels slippery
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Self-Ionization

• Water molecules can react to form hydronium
  (H3O)and hydroxide ions(OH-)
• H2O H2O ? H3O OH-
• H2O ? H OH-
• The relative amounts of hydronium ions (also
  referred to as hydrogen ions, H) and hydroxide
  ions determine whether a solution is acid, base,
  or neutral.

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Determining acid, base, neutral

• If hydrogen ions (H)and hydroxide ions (OH-) are
  equal in concentration, the solution is neutral
• If there are more hydrogen ions (H) than
  hydroxide ions (OH-), the solution is acidic
• If there are more hydroxide ions (OH-), than
  hydrogen (H) ions the solution is basic

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Arrhenius model of acid

• Arrhenius acid a substance that contains
  hydrogen and ionizes to produce hydrogen ions in
  water.
• Example
• HCl (aq) ? H (aq) Cl- (aq)
• HCl H2O ? H3O Cl-

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Arrhenius model of base

• Arrhenius base a substance that contains a
  hydroxide group and dissociates to produce a
  hydroxide ions in aqueous solution.
• Example
• NaOH ? Na OH-

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Arrhenius

• The Arrhenius acid and base model explains many
  acids and bases.
• However, there are acids and bases that have the
  appropriate properties, but do not fit the
  Arrhenius model.
• ENTER Brønsted-Lowry . . .

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Brønsted-Lowry Acid

• a hydrogen-ion donor
• Example
• HCl H2O ? H3O Cl
• HCl is an acid because it donates a hydrogen ion
  to the water.
• Of course, it was an acid according to Arrhenius
  too. So what is different?

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Brønsted-Lowry Acid

• Example
• NH3 H2O ? NH4 OH-
• Water is the acid here, because it donates a
  hydrogen to the ammonia.

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Brønsted-Lowry Base

• a hydrogen ion acceptor
• NH3 H2O ? NH4 OH-
• Ammonia is the base here, because it accepts a
  hydrogen from the water.

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Brønsted-Lowry Base

• HCl H2O ? H3O Cl
• Water is the base here because it accepts the
  hydrogen ion from HCl.

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Amphoteric

• a substance that can act as a Brønsted-Lowry acid
  OR base.
• Example water as an acid
• NH3 H2O ? NH4 OH-
• Example water as a base
• HCl H2O ? H3O Cl

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Conjugate acid-base pairs

• Conjugate acid species produced when a bases
  accepts a hydrogen ion from an acid
• Conjugate base species that results from an
  acid donating a hydrogen ion to a base
• NH3 H2O ? NH4 OH-
• base acid conj a conj b
• More on page 599

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Monoprotic and Polyprotic Acids(all classes)

• Monoprotic acid acid that produces one hydrogen
  ion (HCl).
• Polyprotic acid acid that produces more than
  one hydrogen ion.
• Diprotic acid acid that produces 2 hydrogen
  ions (H2SO4)
• Triprotic acid acid that produces 3 hydrogen
  ions (H3PO4)

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Strengths of Acids and Bases

• Section 19.2

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Objectives

• Relate the strength of an acid or base to its
  degree of ionization
• Compare the strength of a weak acid with the
  strength of its conjugate base and the strength
  of a weak base with the strength of its conjugate
  acid
• Explain the relationship between the strengths of
  acids and bases and the values of their
  ionization constants

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Key Terms

• Strong acid
• Weak acid
• Acid ionization constant
• Strong base
• Weak base
• Base ionization constant

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Strengths of Acids

• Strong acid acid that ionizes completely.
• Common examples
• HCl, HNO3, H2SO4
• Others are listed on table 19-1
• HCl ? H Cl -
• HCl H2O ? H3O Cl

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Weak Acid

• Weak acid acid that does not ionizes
  completely.
• Examples acids other than those memorized as
  strong.
• HCN ? H CN -
• HCN H2O ? H3O CN -

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Strengths of Bases

• Strong base base that dissociates completely to
  the metal ion and hydroxide ion.
• Examples Group I II hydroxides
• NaOH(s) ? Na OH-

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Weak Bases

• weak base base that does NOT dissociates
  completely to the metal ion and hydroxide ion.
• Examples any base other than group I II
  hydroxides
• NH3 H2O ? NH4 OH-

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Weak vs Strong Electrolyte
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Concentration

• Is concentrated the same as strong?
• Is dilute the same as weak?

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• NO
• Concentrated is NOT the same as strong when you
  are discussing acids and bases.
• Dilute is NOT the same as weak when you are
  discussing acids and bases.

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• As you just learned, strong and weak have to do
  with ionization or dissociation being complete or
  incomplete.
• In an earlier chapter, you learned that
  concentrated and dilute have to do with the
  amount of water added, or molarity of a solution.

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• Hydrochloric acid is ALWAYS a strong acid because
  it ionizes completely.
• However, it may be concentrated because little to
  no water is added.
• Or- it may be dilute if a lot of water is added.
• HCl can be a concentrated strong acid or a dilute
  strong acid.

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• Ammonia is ALWAYS a weak base because it doesnt
  dissociate completely.
• However, it may be concentrated because little to
  no water is added.
• Or- it may be dilute if a lot of water is added.
• Ammonia can be a concentrated weak base or a
  dilute weak base.

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Acid ionization constant, Ka

• Just like Keq
• HCN (aq) H2O (l) ? H3O (aq) CN- (aq)
• Ka H3O CN
• HCN
• Mathematically, a large Ka means larger numerator
  ?more ions
• so the larger the Ka , the more the acid ionizes
  and the stronger it is.

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Acid ionization constant, Ka

• Table 19-2 page 605
• Polyprotic acids have a Ka for each H

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Base ionization constant, Kb

• Just like Ka
• CH3NH3 (aq) H2O (l) ? CH3NH4 (aq) OH- (aq)
• Kb CH3NH4OH-
• CH3NH3
• Mathematically, a large Kb means larger numerator
  ?more ions
• so the larger the Kb , the more the base ionizes
  and the stronger it is.