Chapter 20 Acids and Bases - PowerPoint PPT Presentation

1 / 44
About This Presentation
Title:

Chapter 20 Acids and Bases

Description:

Some are strong, others are weak electrolytes. React with metals to form hydrogen gas. ... Amphoteric - acts as acid or base. 3. Lewis Acids and Bases ... – PowerPoint PPT presentation

Number of Views:130
Avg rating:3.0/5.0
Slides: 45
Provided by: drrel
Category:

less

Transcript and Presenter's Notes

Title: Chapter 20 Acids and Bases


1
Chapter 20Acids and Bases
2
Section 20.1Describing Acids and Bases
  • OBJECTIVES
  • List the properties of acids and bases.

3
Section 20.1Describing Acids and Bases
  • OBJECTIVES
  • Name an acid or base, when given the formula.

4
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.

5
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).

6
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

7
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

8
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

9
Section 20.2Hydrogen Ions and Acidity
  • OBJECTIVES
  • Given the hydrogen-ion or hydroxide-ion
    concentration, classify a solution as neutral,
    acidic, or basic.

10
Section 20.2Hydrogen Ions and Acidity
  • OBJECTIVES
  • Convert hydrogen-ion concentrations into values
    of pH, and hydroxide-ion concentrations into
    values of pOH.

11
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

12
Ion Product Constant
  • H2O H OH-
  • Kw is constant in every aqueous solution H
    x OH- 1 x 10-14
  • 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

13
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)

14
pH and pOH
  • pH -logH
  • 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

15
H
pH
0
1
3
5
7
9
11
13
14
Basic
Acidic
Neutral
pOH
OH-
16
Examples
  • Sample 20-3, p.586
  • Sample 20-4, p.586
  • Sample 20-5, p.587
  • Sample 20-6, p.588

17
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

18
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

19
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

20
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

21
Section 20.3Acid-Base Theories
  • OBJECTIVES
  • Compare and contrast acids and bases as defined
    by the theories of Arrhenius, Brønsted-Lowry, and
    Lewis

22
Section 20.3Acid-Base Theories
  • OBJECTIVES
  • Identify conjugate acid-base pairs in acid-base
    reactions.

23
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

24
Svante Arrhenius (1859-1927)
25
Hubert N. Alyea (1903-1996)
26
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.

27
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!

28
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

29
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

30
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.

31
Johannes Bronsted / Thomas Lowry (1879-1947)
(1874-1936)
32
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

33
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

34
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

35
Gilbert Lewis (1875-1946)
36
Section 20.4Strengths of Acids and Bases
  • OBJECTIVES
  • Define strong acids and weak acids.

37
Section 20.4Strengths of Acids and Bases
  • OBJECTIVES
  • Calculate an acid dissociation constant (Ka) from
    concentration and pH measurements.

38
Section 20.4Strengths of Acids and Bases
  • OBJECTIVES
  • Arrange acids by strength according to their acid
    dissociation constants (Ka).

39
Section 20.4Strengths of Acids and Bases
  • OBJECTIVES
  • Arrange bases by strength according to their base
    dissociation constants (Kb).

40
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

41
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)

42
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.

43
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?

44
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!
Write a Comment
User Comments (0)
About PowerShow.com