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Acids and Bases

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Chapter 15 Acids and Bases Objectives Describe the properties of acids and bases Recognize acids and bases by definitions Compare strong and weak acids/bases Describe ... – PowerPoint PPT presentation

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Title: Acids and Bases


1
Chapter 15
  • Acids and Bases

2
Objectives
  • Describe the properties of acids and bases
  • Recognize acids and bases by definitions
  • Compare strong and weak acids/bases
  • Describe electrolytes

3
What Do You Know About Acids and Bases?
4
Properties of Acids
  • Sour taste
  • (A VERY bad way to test in the lab!)
  • What acids have you tasted?
  • Turns blue litmus red
  • Reacts with metal to form hydrogen gas
  • pH less than 7
  • Electrolyte
  • Solutions that conduct electrical current

5
Properties of Bases
  • Tastes bitter
  • (Also not a good lab practice!)
  • Turns red litmus blue
  • pH greater than 7
  • Electrolyte

6
Acid/Base Definitions
  • Arrhenius Definition
  • Acids increase the H ion concentration in
    solution
  • Acids have H as the first element
  • Bases increase the OH- ion concentration in
    solution
  • Many bases end with OH
  • Definition limited to solutions!

7
Acid/Base Definitions
  • Bronsted/Lowry Definition
  • Acids are H donors
  • H ion is just a proton
  • (often called proton donors)
  • Bases are H acceptors
  • (Often called proton acceptors)
  • This is the definition of acids and bases we will
    use the most.

8
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9
Strong Acids
  • Strong Acids Acids that completely ionize in
    solution
  • Assume that HA can be an acid
  • HA ? H A-
  • There will be NO HA left in solution
  • Strong Acids Memorize the first 3
  • HNO3 HCl H2SO4
    HClO4 HBr HI

10
The Hydronium Ion
  • H ions can be written one of two ways
  • 1st H
  • Easy way to indicate the ion
  • 2nd H3O
  • Indicates that the H attaches to water molecules
  • Hydronium Ion
  • Either way is fine

11
Weak Acids
  • Weak Acids Acids that only partially ionize in
    solution
  • Equilibrium is established
  • Assume HA to be a weak acid
  • HA H2O ?? H3O A-
  • There will be lots of HA left in solution
  • Weak Acids Any acid that is not strong
  • H2CO3 HF H3PO4
    HNO2 HC2H3O2 H2SO3

12
Inorganic vs Organic Acids
  • All acids have H in the formula
  • Inorganic acids contain hydrogen and a halogen or
    hydrogen and a polyatomic ion
  • Organic acids have hydrogen and carbon and oxygen
    (not CO3-2)
  • Carboxyl group
  • Carboxylic acids
  • All weak

13
Strong Bases
  • Strong Bases Bases that completely ionize in
    solution
  • KOH ? K OH-
  • There will be NO KOH left in solution
  • Strong Bases
  • NaOH KOH
  • Group 2 hydroxides are also considered strong
    bases

14
Weak Bases
  • Weak Bases Bases that partially ionize in
    solution
  • NH3 H2O ? NH4 OH-
  • There will be lots of NH3 left in solution
  • Weak bases are ammonia derivates
  • They will have nitrogen in them

15
Electrolytes
  • Conduct current because charged particles are
    free to move about
  • Acids form ions when they are dissolved in
    solution
  • HCl ? H Cl-
  • Salts form ions when they are dissolved in
    solution
  • NaNO3 ? Na NO3-
  • Charged particles complete (or close) the circuit

16
Closed Circuit
17
Open Circuit
18
Another Type of Open Circuit
19
  • Closed circuit
  • Ions in solution close the circuit
  • Charges flow to the opposite pole
  • Current flows from negative to positive
  • Light bulb shines!

20
Nonelectrolytes
  • Solutions that do not conduct an electric current
  • No ions are present
  • Nothing to conduct the current
  • All molecules are nonelectrolytes
  • They dissolve into individual molecules

21
Electrolytes or Nonelectrolytes?
  • CH4
  • Nonelectrolyte
  • KBr
  • Electrolyte
  • C2H6O
  • Nonelectrolyte
  • H2SO4
  • Electrolyte

22
Strong and Weak Electrolytes
  • Strong electrolytes
  • Solutions that conduct current well
  • Lots of ions in solution
  • Strong acids and bases, salts
  • Weak electrolytes
  • Solutions that conduct current poorly
  • Few ions
  • Weak acids and bases

23
Homework
  • p. 625 42,43,51,55,61,67,70,86,97

24
Objectives
  • Recognize polyprotic acids.
  • Compare the strengths of weak acids and bases.
  • Describe a neutralization reaction
  • Calculate neutralization data
  • Determine if a solution is acidic, basic, or
    neutral
  • Explain the autodissociation of water

25
Polyprotic Acids
  • Acids that can donate more than 1 hydrogen
  • H2CO3 Carbonic Acid
  • Able to donate 2 Hs
  • Diprotic
  • H3PO4 Phosphoric Acid
  • Able to donate 3 Hs
  • Triprotic

26
Cont.
  • HNO3 Nitric Acid
  • Only 1 H to donate
  • Monoprotic
  • HC2H3O2 Acetic Acid
  • Only the first H can be donated
  • Same with most carboxylic acids

27
How Weak is Weak?Quantifying Weak Acids and Bases
  • Weak acids and bases have equilibrium
    dissociation values
  • The smaller the constant the weaker the acid or
    base
  • The larger the constant the stronger the acid or
    base

28
How Weak is Weak?Quantifying Weak Acids and Bases
  • HA(aq) ?? H A-
  • The Equilibrium Expression
  • Ka H A- / HA
  • B(aq) H2O ?? BH OH-
  • The Equilibrium Expression
  • Kb BH OH- / B

29
Sample Ka Values
  • Acid Ka
  • HClO2 1.2x10-2
  • HF 7.2x10-4
  • HC2H3O2 1.8x10-5
  • HClO 3.5x10-8
  • HCN 6.2x10-10
  • HIO 2.0x10-11
  • What is the strongest / weakest acid?

30
Sample Kb Values
  • Base Kb
  • NH3 1.8x10-5
  • (C2H5)2NH 1.3x10-3
  • (C2H5)3N 4.0x10-4
  • CH3NH2 4.4x10-4
  • C2H5NH2 5.6x10-4
  • What is the strongest / weakest base?

31
Neutralization Reactions
  • The reaction of an acid and a base to yield a
    salt and water
  • HI(aq) KOH(aq) ? HOH(l) KI(aq)
  • Net Ionic Equation
  • Net Ionic is ALWAYS the same

32
Example
  • What volume of 0.25M KOH is required to react
    with 50.0 mL of 0.20 M HBr?

33
Example
  • What volume of 1.2M NaOH is required to react
    with 30.0 mL of 0.70 M H2SO4?

34
Acidic, Basic, and Neutral Solutions
  • Relates to concentrations of H and OH-
  • Acidic solutions have more H than OH-
  • H from an acid
  • Basic solutions have more OH- than H
  • OH- from base
  • Neutral solutions have equal amounts of both ions
  • Both ions come from water!

35
Autoionization of Water
  • Water molecules dissociate by themselves
  • H2O(l) ?? H OH-
  • In pure water the concentrations are equal
  • Both 1.00x10-7M
  • Write the equilibrium expression
  • Kw is always 1.00x10-14 at 25ºC

36
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37
LeChatliers Principle (Again)
  • The concentration of H and OH- can vary in
    solution.
  • When H is added, OH- decreases
  • When OH- is added, H decreases
  • That is how solutions become acidic or basic

38
Example
  • What is the concentration of H and OH- in a
    0.10M solution of HCl? Is the solution acidic,
    basic, or neutral?

39
Example
  • A solution is prepared by dissolving
    6.00 grams of KOH in 200.0mL of water. What is
    the concentration of H and OH-? Is the solution
    acidic, basic, or neutral?

40
Homework
  • p. 626 80,87,89,92,101,117,119,124,131,134

41
Objectives
  • Use logarithms in calculations
  • Explain pH and calculate pH values

42
Logarithms
  • The logarithm of a number to a given base
    (commonly 10) is the power or exponent to which
    the base must be raised in order to produce the
    number.
  • SAY WHAT!

43
Examples
  • If your question says
  • log 100 x
  • It is saying to what power must 10 be raised to
    equal 100
  • 10x 100
  • 102 100
  • So x 2

44
Examples
  • log 1 x
  • x 0
  • log 10 x
  • x 1
  • log 1000 x
  • x 3
  • log 1x106 x
  • x 6

45
Examples
  • Logarithms can also be used for numbers smaller
    than 1
  • log 0.1 x
  • x -1
  • log 0.01 x
  • x -2
  • log 1x10-5
  • x -5

46
Examples
  • If they are not easy to calculate you can do it
    on your calculator
  • log 15 x
  • You can approximate it between . . .
  • Type log 15 on your calculator
  • x 1.18

47
Examples
  • If your question says
  • log x 7
  • It is saying 10 to the 7th power is what number
  • 107 1x107
  • log x 3
  • x 1000
  • log x -3
  • x 0.001

48
Tougher Examples
  • log 234 x
  • x 2.37
  • log x -3.3
  • x 5.0x10-4
  • -log 9.1x10-5 x
  • x 4.0
  • -log x 12.1
  • x 7.9x10-13

49
pH
  • Negative logarithm of the concentration of
    hydrogen ions in solution
  • pH -log H
  • pH means power of Hydrogen
  • Measures how acidic or basic a solution is
  • pH scale typically goes from 0 to 14
  • pH lt 7 Acidic
  • pH gt 7 Basic

50
  • Highly acidic low pH
  • Highly basic high pH

51
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52
Significant Figures and pH
  • Digits after the decimal are the only ones that
    are significant in pH values
  • pH 4.44
  • 2 Significant Figures
  • pH 10.874
  • 3 Significant Figures
  • If your H is 0.088 M your pH is 1.06

53
Determining H in solution
  • The concentration of a strong acid is equal to
    the H concentration.
  • 0.010 M HCl has an H of 0.010 M
  • To obtain the H for weak acids you must use
    equilibrium
  • Need Ka data
  • The concentration of strong bases is equal to the
    OH-
  • To get H you need to use Kw

54
Example
  • What is the pH of a 0.0022 M solution of Nitric
    Acid

55
Example
  • What is the pH of a .045 M Acetic Acid. Ka
    1.8x10-5

56
Example
  • What is the pH of 0.031M solution of Sodium
    Hydroxide

57
Other Info
  • Turn Kw into a log equation
  • pH pOH 14.00

58
Example
  • What is the H for a solution that has a pH of
    4.88?

59
Example
  • What is the pH of 2.5x10-6 M solution of
    Potassium Hydroxide

60
Homework
  • p.629 145,149,151,154,186,197,216 find the pH

61
Try These
  • p. 630 s 186,197,216 find the pH

62
Objectives
  • Describe conjugate acids and bases
  • Compare conjugate strength
  • Explain the process of titration
  • Describe the role of each component in titrations
  • Calculate unknown concentrations from titration
    data

63
Conjugates
  • An base that is made when an acid loses a
    hydrogen is called a conjugate base
  • An acid that is made when an base gains a
    hydrogen is called a conjugate acid
  • F- is the conjugate base of HF
  • NO3- is the conjugate base of HNO3
  • NH4 is the conjugate acid of NH3

64
Conjugate Strength
  • The strength of a conjugate is inversely related
    to the strength of its parent acid or base
  • Strong acids and bases have weak conjugates
  • Weaker than water
  • Weak acids and bases have strong conjugates
  • Stronger than water

65
Buffers
  • A solution that resists a change in pH
  • Composed of
  • A weak acid and its conjugate base
  • A weak base and its conjugate acid
  • HF ?? H F-
  • To prepare this buffer you add
  • Hydrofluoric Acid
  • Sodium Fluoride

66
How Buffers Work
  • Added strong acid reacts with the CB to produce a
    weak acid
  • Added strong base reacts with the CA to produce a
    weak base
  • HF ?? H F-

67
Titrations
  • A process to determine the concentration of a
    solution
  • Reaction is carried out until the mole ratio of
    the reaction has been reached
  • Equivalence Point
  • However, the equivalence point cannot detected
    visually.
  • Look for Endpoint
  • Marked by an indicator

68
Titration Vocabulary
  • Titrant Solution of known concentration
  • (Standard solution)
  • Analyte Solution of unknown concentration
  • Equilvalence Pt. - Mole ratio of reaction
  • Endpoint Pt. Visual change in a reaction
  • Indicator Chemical that marks endpoint
  • Change color with varying pH

69
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70
Which Indicator To Choose?
  • Depends on the pH at the equivalence pt.
  • What is the pH at equivalence for the reaction
  • HNO3 KOH ? H2O KNO3
  • Water is neutral pH 7
  • NO3- is the conjugate base of a strong acid thus
    it is a REALLY weak base
  • pH is 7.00 at equivalence
  • All Strong Acid / Strong Base reactions are the
    same

71
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72
Which Indicator To Choose?
  • What is the pH at equivalence for the reaction
  • HNO3 NH3 ? NH4 NO3-
  • NO3- is the conjugate base of a strong acid thus
    it is a REALLY weak base
  • NH4 is the conjugate acid of a weak base thus it
    is a fairly strong acid
  • pH lt 7.00 at equivalence
  • All Strong Acid / Weak Base reactions are the same

73
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74
Which Indicator To Choose?
  • What is the pH at equivalence for the reaction
  • HF KOH ? H2O KF
  • Water is neutral
  • F- is the conjugate base of a weak acid thus it
    is a fairly strong base
  • pH gt 7.00 at equivalence
  • All Weak Acid / Strong Base reactions are the same

75
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76
Summary of pH at Equivalence
  • Strong Acid and Strong Base
  • pH 7 at equivalence
  • Strong Acid and Weak Base
  • pH lt 7 at equivalence
  • Weak Acid and Strong Base
  • pH gt 7 at equivalence

77
Materials for Titrating
  • Buret
  • Used to deliver precise volumes of solution
  • Erlenmeyer Flask
  • Used to hold reaction mixture
  • Minimizes splashes
  • Stir Plate
  • Makes stirring simple

78
Data and Calculations
  • Data
  • Volume of each solution used
  • Molarity of standard solution
  • Calculations
  • Moles of analyte based and standard
  • Mol unknown / Liters unknown Molarity

79
Homework
  • p.629 160,161,166,172
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