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Chapter 20 Acids and Bases. Section 20.1 Describing Acids and Bases. OBJECTIVES: List the properties of acids and bases. Section 20.1 Describing Acids and Bases. OBJECTIVES: Name an acid or base, when given the formula. Properties of acids. Taste sour (don’t try this at home).

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section 20 1 describing acids and bases
Section 20.1Describing Acids and Bases
  • OBJECTIVES:
    • List the properties of acids and bases.
section 20 1 describing acids and bases3
Section 20.1Describing Acids and Bases
  • OBJECTIVES:
    • Name an acid or base, when given the formula.
properties of acids
Properties of acids
  • Taste sour (don’t 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.
properties of bases
Properties of bases
  • React with acids to form water and a salt.
  • Taste bitter.
  • Feel slippery (don’t try this either).
  • Can be strong or weak electrolytes.
  • Change indicators (red litmus turns blue).
names and formulas of acids
Names and Formulas of Acids
  • An acid is a chemical that produces hydrogen ions (H1+) when dissolved in water
  • Thus, general formula = HA, where A is a monatomic or polyatomic anion
  • HCl(g) is hydrogen monochloride
  • HCl(aq) is named as an acid
  • Name focuses on the anion present
names and formulas of acids7
Names and Formulas of Acids

1. BINARY - 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. TERNARY - When anion ends with -ite, the anion has the suffix -ous, then acid

3. TERNARY - When anion ends with -ate, the anion suffix is -ic and then acid

names and formulas of bases
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
section 20 2 hydrogen ions and acidity
Section 20.2Hydrogen Ions and Acidity
  • OBJECTIVES:
    • Given the hydrogen-ion or hydroxide-ion concentration, classify a solution as neutral, acidic, or basic.
section 20 2 hydrogen ions and acidity10
Section 20.2Hydrogen Ions and Acidity
  • OBJECTIVES:
    • Convert hydrogen-ion concentrations into values of pH, and hydroxide-ion concentrations into values of pOH.
hydrogen ions from water
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
  • Kw is called the “ion product constant”
ion product constant
Ion Product Constant
  • H2O H+ + OH-
  • Kw is constant in every aqueous solution: [H+] x [OH-] = 1 x 10-14
  • If [H+] > 10-7 then [OH-] < 10-7
  • If [H+] < 10-7 then [OH-] > 10-7
  • If we know one, other can be determined
  • If [H+] > 10-7, it is acidic and [OH-] < 10-7
  • If [H+] < 10-7, it is basic and [OH-] > 10-7
  • Basic solutions also called “alkaline”
  • Sample problem 20-2, p. 582
logarithms and the ph concept
Logarithms and the pH concept
  • Logarithms are powers of ten.
    • Review from earlier lessons, and p. 585
  • definition: pH = -log[H+]
  • in neutral pH = -log(1 x 10-7) = 7
  • in acidic solution [H+] > 10-7
  • pH < -log(10-7)
  • pH < 7 (from 0 to 7 is the acid range)
  • in base, pH > 7 (7 to 14 is base range)
ph and poh
pH and pOH
  • pH = -log[H+]
  • pOH = -log [OH-]
  • Kw = [H+] x [OH-] = 1 x 10-14
  • pH + pOH = 14
  • Thus, a solution with a pH less than 7 is an acid; a pH greater than 7 is a base; 7 is neutral
slide15

100

10-1

10-3

10-5

10-7

10-9

10-11

10-13

10-14

14

13

11

9

7

5

3

1

0

10-14

10-13

10-11

10-9

Basic

10-7

10-5

10-3

10-1

100

[H+]

pH

0

1

3

5

7

9

11

13

14

Acidic

Neutral

Basic

pOH

[OH-]

examples
Examples:
  • Sample 20-3, p.586
  • Sample 20-4, p.586
  • Sample 20-5, p.587
  • Sample 20-6, p.588
measuring ph
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
acid base indicators
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
acid base indicators19
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
acid base indicators20
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
section 20 3 acid base theories
Section 20.3Acid-Base Theories
  • OBJECTIVES:
    • Compare and contrast acids and bases as defined by the theories of Arrhenius, Brønsted-Lowry, and Lewis
section 20 3 acid base theories22
Section 20.3Acid-Base Theories
  • OBJECTIVES:
    • Identify conjugate acid-base pairs in acid-base reactions.
svante arrhenius
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

1 arrhenius definition
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.
polyprotic acids
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!
polyprotic acids28
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
arrhenius examples
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
2 br nsted lowry definitions
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 it’s proton to water.
  • HCl(g) + H2O(l) H3O+ + Cl-
  • Water is a base; makes hydronium ion.
acids and bases come in pairs
Acids and bases come in pairs...
  • A conjugate base is the remainder of the original acid, after it donates it’s 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
acids and bases come in pairs33
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
3 lewis acids and bases
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 don’t even need hydrogen!
  • Sample Problem 20-7, p.599
section 20 4 strengths of acids and bases
Section 20.4Strengths of Acids and Bases
  • OBJECTIVES:
    • Define strong acids and weak acids.
section 20 4 strengths of acids and bases37
Section 20.4Strengths of Acids and Bases
  • OBJECTIVES:
    • Calculate an acid dissociation constant (Ka) from concentration and pH measurements.
section 20 4 strengths of acids and bases38
Section 20.4Strengths of Acids and Bases
  • OBJECTIVES:
    • Arrange acids by strength according to their acid dissociation constants (Ka).
section 20 4 strengths of acids and bases39
Section 20.4Strengths of Acids and Bases
  • OBJECTIVES:
    • Arrange bases by strength according to their base dissociation constants (Kb).
strength
Strength
  • Strong acids and bases are strong electrolytes
    • They fall apart (ionize) completely.
    • Weak acids don’t 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
measuring strength
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)
what about bases
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.
strength vs concentration
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?
practice
Practice
  • Write the expression for HNO2
  • Write the Kb for NH3
  • Sample 20-8, p. 604
  • Carefully study Key Terms and equations, p. 608
  • Be sure to do the ChemASAP programs, and take all the self-tests that are available!
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