Download Chapter 8 Brønsted-Lowry Theory of Acids and Bases

Chapter 8
Acids and Bases and OxidationReduction
Petr Vanýsek, Instructor
14 November 2013
Brønsted-Lowry Theory of Acids
and Bases
• Acid - proton donor
• Base - proton acceptor
– Notice that acid and base are not defined
using water
– When writing the reactions, both accepting
and donation are evident
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Brønsted-Lowry Theory of Acids
and Bases
HCl(aq) + H2O(l)  Cl-(aq) + H3O+(aq)
acid
base
What donated the proton? HCl
Is it an acid or base?
Acid
What accepted the proton? H2O
Is it an acid or base?
Base
Brønsted-Lowry
Theory of Acids
.
and Bases
NH3(aq) + H2O(l)
base
acid
NH4+(aq) + OH-(aq)
Now let us look at NH3 and see why it is a
base
Did NH3 donate or accept a proton?
Is it an acid or base?
Accept
Base
What is water in this reaction? Acid
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Acid-Base Properties of Water
• Water possesses both acid and base
properties
– Amphiprotic – a substance possessing both acid
and base properties
– Water is the most commonly used solvent for
both acids and bases
– Solute-solvent interactions between water and
both acids and bases promote solubility and
dissociation
Acid and Base Strength
• Acid and base strength – degree of dissociation
– Not a measure of concentration
– Strong acids and bases – reaction with water is
virtually 100% (strong electrolytes)
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Strong Acids and Bases
• Strong Acids:
–
–
–
–
HCl, HBr, HI
HNO3
H2SO4
HClO4
hydrochloric acid, etc.
nitric acid
sulfuric acid
perchloric acid
• Strong Bases:
– NaOH, KOH, Ba(OH)2
– (All alkali metal hydroxides)
Weak Acids
• Weak acids and bases – only a small
percent dissociates (weak electrolytes)
• Weak acid examples:
– acetic acid:
CH3COOH(aq) + H2O(l)
CH3COO-(aq) + H3O+(aq)
– carbonic Acid:
H2CO3(aq) + H2O(l)
HCO3-(aq) + H3O+(aq)
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Weak Bases
• Weak base examples:
– ammonia:
NH3(aq) + H2O(l)
NH4+(aq) + OH-(aq)
– pyridine:
C5H5NH2(aq) + H2O(l)
C5H5NH3+(aq) + OH-(aq)
– aniline:
C6H5NH2(aq) + H2O(l)
C6H5NH3+(aq) + OH-(aq)
Conjugate Acids and Bases
• The acid base reaction can be written in
the general form:
HA + B
A- + HB+
acid
base
• Notice the reversible arrows
• The products are also an acid and base
called the conjugate acid and base
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HA + B
acid
base
A- + HB+
base
acid
• Conjugate Acid – what the base
becomes after it accepts a proton.
• Conjugate Base – what the acid
becomes after it donates its proton
• Conjugate Acid-Base Pair – the acid and
base on the opposite sides of the
equation
Acid-Base Dissociation
HA + B
A- + HB+
• The reversible arrow is not always written
– Some acids or bases essentially dissociate 100%
– One way arrow is used
• HCl + H2O  Cl- + H3O+
– All of the HCl is converted to Cl– HCl is called a strong acid – an acid that
dissociates 100%
• Weak acid - one which does not dissociate
100%
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Conjugate Acid-Base Pairs
• Which acid is
stronger:
HF or HCN? HF
• Which base is
stronger:
CN- or H2O? CN -
Acid-Base Practice Problems
Write the chemical reaction for the following acids
or bases in water.
Identify the conjugate acid base pairs.
1. HF (a weak acid)
2. H2S (a weak acid)
3. HNO3 (a strong acid)
4. CH3NH2 (a weak base)
Note: The degree of dissociation also defines weak
and strong bases
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The Dissociation of Water
• Pure water is virtually 100% molecular
• Very small number of molecules dissociate
– Dissociation of acids and bases is often called
ionization
H2O(l) + H2O(l)
H3O+(aq) + OH-(aq)
• Called autoionization
• Very weak electrolyte
Hydronium Ion
• H3O+ is called the hydronium ion
• In pure water at room temperature:
– [H3O+] = 1 x 10-7 M
– [OH-] = 1 x 10-7 M
• What is the equilibrium expression for:
H2O(l) + H2O(l)
H3O+(aq) + OH-(aq)
K eq  [H 3O  ][OH - ]
Remember, liquids are not included in equilibrium
expressions
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Ion Product of Water
• This constant is called the ion product for
water and has the symbol Kw
K w  [H 3O  ][OH - ]
• Since [H3O+] = [OH-] = 1.0 x 10-7 mol/l,
what is the value for Kw?
– 1.0 x 10-14
– It is unitless
pH: A Measurement Scale for
Acids and Bases
• pH scale – a scale that indicates the acidity
or basicity of a solution
– Ranges from 0 (very acidic) to 14 (very basic)
• The pH scale is rather similar to the
temperature scale assigning relative values
of hot and cold
• The pH of a solution is defined as:
pH = -log[H3O+]
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The log function
• The pH of a solution is
defined as:
pH = -log[H3O+]
A Definition of pH
• Use these observations to develop a concept of
pH
–
–
–
–
if known one concentration, can calculate the other
if added an acid, [H3O+]  and [OH-] 
if added a base, [OH-]  and [H3O+] 
[H3O+] = [OH-] when equal amounts of acid and base
are present
• In each of these cases 1 x 10-14 = [H3O+][OH-]
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Measuring pH
• pH of a solution can be:
– Calculated if the concentration of either
• [H3O+] or [OH-]
is known
– Approximated using indicator / pH paper that develops a
color related to the solution pH
– Measured using a pH meter whose sensor measures an
electrical property of the solution that is proportional to
pH
Calculating pH
• How do we calculate the pH of a solution when
either the hydronium or hydroxide ion
concentrations are known?
• How do we calculate the hydronium or
hydroxide ion concentration when the pH is
known?
• Use two facts:
pH = -log[H3O+]
1 x 10-14 = [H3O+][OH-]
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Calculating pH from Acid
Molarity
What is the pH of a 1.0 x 10-4 mol/l HCl solution?
– HCl is a strong acid and dissociates in water
– If 1 mol HCl is placed in 1 L of aqueous solution
it produces 1 mol [H3O+]
– 1.0 x 10-4 mol/l HCl solution has [H3O+]=1.0x10-4
mol/l
pH = -log[H3O+]
= -log [H3O+]
= -log [1.0x10-4]
= -[-4.00] = 4.00
Calculating [H3O+] from pH
What is the [H3O+] of a solution with pH = 4.00?
pH = -log[H3O+]
• 4.00 = -log [H3O+]
• Multiply both sides of equation by –1
• - 4.00 = log [H3O+]
• Take the antilog of both sides
• Antilog – 4.00 = [H3O+]
• Antilog is the exponent of 10 ( 10 x )
• 1.0 x 10-4 mol/l = [H3O+]
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Calculating the pH of a Base
What is the pH of a 1.0 x 10-3 mol/l KOH
solution?
• KOH is a strong base (as are any metal hydroxides)
• 1 mol KOH dissolved and dissociated in aqueous
solution produces 1 mol OH-3
• 1.0 x 10-3 mol/l KOH
-14 solution has
+ [OH-] = 1.0 x 10
mol/l
1 x 10
= [H3O ][OH ]
• Solve equation for [H3O+] = 1 x 10-14 / [OH-]
-3 = 1 x 10
+ -11
• [H3O+] = 1 x 10-14 / 1.0
pHx =10-log[H
3O ]
• pH = -log [1 x 10-11]
= 11.00
Calculating pH from Acid Molar
Concentration
What is the pH of a 2.5 x 10-4 mol/l HNO3
solution?
• We know that as a strong acid HNO3
dissociates to produce 2.5 x 10-4 mol/l [H3O+]
pH = -log[H3O+]
• pH = -log [2.5 x 10-4]
•
= 3.60
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Calculating [OH-] from pH
What is the [OH-] of a solution with pH = 4.95?
• First find [H3O+]
• 4.95 = -log [H3O+]
•
•
•
•
pH = -log[H3O+]
[H3O+] = 10-4.95
[H3O+] = 1.122 x 10-5
-14
+
Now solve for [OH-] 1 x 10 = [H3O ][OH ]
[OH-] = 1 x 10-14 / 1.122 x 10-5
= 8.93 x 10-10 mol/l
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