Chapter 20 Acids and Bases

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Chapter 20Acids and Bases

• Milbank High School

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Section 20.1Describing Acids and Bases

• OBJECTIVES
• List the properties of acids and bases.

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Section 20.1Describing Acids and Bases

• OBJECTIVES
• Name an acid or base, when given the formula.

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Properties of acids

• Taste sour (dont try this at home).
• Conduct electricity.
• Some are strong, others are weak electrolytes.
• React with metals to form hydrogen gas.
• Change indicators (blue litmus to red).
• React with hydroxides to form water and a salt.

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Properties of bases

• React with acids to form water and a salt.
• Taste bitter.
• Feel slippery (dont try this either).
• Can be strong or weak electrolytes.
• Change indicators (red litmus turns blue).

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Names and Formulas of Acids

• An acid is a chemical that produces hydrogen ions
  (H1) when dissolved in water
• Thus, general formula HX, where X is a
  monatomic or polyatomic anion
• HCl(g) named hydrogen chloride
• HCl(aq) is named as an acid
• Name focuses on the anion present

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Names and Formulas of Acids

• 1. When anion ends with -ide, the acid starts
  with hydro-, and the stem of the anion has the
  suffix -ic followed by the word acid
• 2. When anion ends with -ite, the anion has the
  suffix -ous, then acid
• 3. When anion ends with -ate, the anion suffix is
  -ic and then acid
• Table 20.1, page 578 for examples

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Names and Formulas of Bases

• A base produces hydroxide ions (OH1-) when
  dissolved in water.
• Named the same way as any other ionic compound
• name the cation, followed by anion
• To write the formula write symbols write
  charges then cross (if needed)
• Sample Problem 20-1, p. 579

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Section 20.2Hydrogen Ions and Acidity

• OBJECTIVES
• Given the hydrogen-ion or hydroxide-ion
  concentration, classify a solution as neutral,
  acidic, or basic.

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Section 20.2Hydrogen Ions and Acidity

• OBJECTIVES
• Convert hydrogen-ion concentrations into values
  of pH, and hydroxide-ion concentrations into
  values of pOH.

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Hydrogen Ions from Water

• Water ionizes, or falls apart into ions
• H2O H1 OH1-
• Called the self ionization of water
• Occurs to a very small extent
• H1 OH1- 1 x 10-7 M
• Since they are equal, a neutral solution results
  from water
• Kw H1 x OH1- 1 x 10-14 M2
• Kw is called the ion product constant

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Ion Product Constant

• H2O H OH-
• Kw is constant in every aqueous solution H
  x OH- 1 x 10-14 M2
• If H gt 10-7 then OH- lt 10-7
• If H lt 10-7 then OH- gt 10-7
• If we know one, other can be determined
• If H gt 10-7 , it is acidic and OH- lt 10-7
• If H lt 10-7 , it is basic and OH- gt 10-7
• Basic solutions also called alkaline
• Sample problem 20-2, p. 582

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Logarithms and the pH concept

• Logarithms are powers of ten.
• Review from earlier lessons, and p. 585
• definition pH -logH
• in neutral pH -log(1 x 10-7) 7
• in acidic solution H gt 10-7
• pH lt -log(10-7)
• pH lt 7 (from 0 to 7 is the acid range)
• in base, pH gt 7 (7 to 14 is base range)

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pH and pOH

• pOH -log OH-
• H x OH- 1 x 10-14 M2
• pH pOH 14
• Thus, a solution with a pOH less than 7 is basic
  with a pOH greater than 7 is an acid

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H
pH
0
1
3
5
7
9
11
13
14
Basic
Acidic
Neutral
pOH
OH-
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Examples

• Sample 20-3, p.586
• Sample 20-4, p.586
• Sample 20-5, p.587
• Sample 20-6, p.588

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Measuring pH

• Why measure pH?
• Everything from swimming pools, soil conditions
  for plants, medical diagnosis, soaps and
  shampoos, etc.
• Sometimes we can use indicators, other times we
  might need a pH meter

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Acid-Base Indicators

• An indicator is an acid or base that undergoes
  dissociation in a known pH range, and has
  different colors in solution (more later in
  chapter)
• Examples litmus, phenolphthalein, bromthymol
  blue Fig 20.8, p.590

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Acid-Base Indicators

• Although useful, there are limitations to
  indicators
• usually given for a certain temperature (25 oC),
  thus may change at different temperatures
• what if the solution already has color?
• ability of human eye to distinguish colors

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Acid-Base Indicators

• A pH meter may give more definitive results
• some are large, others portable
• works by measuring the voltage between two
  electrodes
• needs to be calibrated
• Fig. 20.10, p.591

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Section 20.3Acid-Base Theories

• OBJECTIVES
• Compare and contrast acids and bases as defined
  by the theories of Arrhenius, Brønsted-Lowry, and
  Lewis

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Section 20.3Acid-Base Theories

• OBJECTIVES
• Identify conjugate acid-base pairs in acid-base
  reactions.

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Svante Arrhenius

• Swedish chemist (1859-1927) - Nobel prize winner
  in chemistry (1903)
• one of the first chemists to explain the chemical
  theory of the behavior of acids and bases
• Dr. Hubert Alyea-last graduate student of
  Arrhenius. (link below)
• http//www.woodrow.org/teachers/ci/1992/Arrhenius.
  html

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1. Arrhenius Definition

• Acids produce hydrogen ions (H1) in aqueous
  solution.
• Bases produce hydroxide ions (OH1-) when
  dissolved in water.
• Limited to aqueous solutions.
• Only one kind of base (hydroxides)
• NH3 (ammonia) could not be an Arrhenius base.

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Polyprotic Acids

• Some compounds have more than 1 ionizable
  hydrogen.
• HNO3 nitric acid - monoprotic
• H2SO4 sulfuric acid - diprotic - 2 H
• H3PO4 phosphoric acid - triprotic - 3 H
• Having more than one ionizable hydrogen does not
  mean stronger!

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Polyprotic Acids

• However, not all compounds that have hydrogen are
  acids
• Also, not all the hydrogen in an acid may be
  released as ions
• only those that have very polar bonds are
  ionizable - this is when the hydrogen is joined
  to a very electronegative element

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Arrhenius examples...

• Consider HCl
• What about CH4 (methane)?
• CH3COOH (ethanoic acid, or acetic acid) - it has
  4 hydrogens like methane does?
• Table 20.4, p. 595 for bases

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2. Brønsted-Lowry Definitions

• Broader definition than Arrhenius
• Acid is hydrogen-ion donor (H or proton) base
  is hydrogen-ion acceptor.
• Acids and bases always come in pairs.
• HCl is an acid.
• When it dissolves in water, it gives its proton
  to water.
• HCl(g) H2O(l) H3O Cl-
• Water is a base makes hydronium ion.

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Acids and bases come in pairs...

• A conjugate base is the remainder of the original
  acid, after it donates its hydrogen ion
• A conjugate acid is the particle formed when the
  original base gains a hydrogen ion
• Indicators are weak acids or bases that have a
  different color from their original acid and base

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Acids and bases come in pairs...

• General equation is
• HA(aq) H2O(l) H3O(aq) A-(aq)
• Acid Base Conjugate acid Conjugate
  base
• NH3 H2O NH41 OH1-
• base acid c.a. c.b.
• HCl H2O H3O1 Cl1-
• acid base c.a. c.b.
• Amphoteric - acts as acid or base

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3. Lewis Acids and Bases

• Gilbert Lewis focused on the donation or
  acceptance of a pair of electrons during a
  reaction
• Lewis Acid - electron pair acceptor
• Lewis Base - electron pair donor
• Most general of all 3 definitions acids dont
  even need hydrogen!
• Sample Problem 20-7, p.599

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Section 20.4Strengths of Acids and Bases

• OBJECTIVES
• Define strong acids and weak acids.

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Section 20.4Strengths of Acids and Bases

• OBJECTIVES
• Calculate an acid dissociation constant (Ka) from
  concentration and pH measurements.

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Section 20.4Strengths of Acids and Bases

• OBJECTIVES
• Arrange acids by strength according to their acid
  dissociation constants (Ka).

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Section 20.4Strengths of Acids and Bases

• OBJECTIVES
• Arrange bases by strength according to their base
  dissociation constants (Kb).

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Strength

• Strong acids and bases are strong electrolytes
• They fall apart (ionize) completely.
• Weak acids dont completely ionize.
• Strength different from concentration
• Strong-forms many ions when dissolved
• Mg(OH)2 is a strong base- it falls completely
  apart when dissolved.
• But, not much dissolves- not concentrated

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Measuring strength

• Ionization is reversible.
• HA H A-
• This makes an equilibrium
• Acid dissociation constant Ka
• Ka H A- (water is
  constant) HA
• Stronger acid more products (ions), thus a
  larger Ka (Table 20.8, p.602)

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What about bases?

• Strong bases dissociate completely.
• B H2O BH OH-
• Base dissociation constant Kb
• Kb BH OH- B (we ignore
  the water)
• Stronger base more dissociated, thus a larger
  Kb.

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Strength vs. Concentration

• The words concentrated and dilute tell how much
  of an acid or base is dissolved in solution -
  refers to the number of moles of acid or base in
  a given volume
• The words strong and weak refer to the extent of
  ionization of an acid or base
• Is concentrated weak acid possible?

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Practice

• Write the expression for HNO2
• Write the Kb for NH3
• Sample 20-8, p. 604
• Carefully study Key Terms and equations, p. 608