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Chemistry 132 NT
A man is rich in proportion to the number of things he can let alone.
Henry David Thoreau
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Acids and Bases
Chapter 15
Module 1
Sections 15.1, 15.2, and 15.3 Acid-base indicator dye.
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Acid Base Concepts
Acids and bases were first recognized by simple properties, such as taste.
Acids taste sour.
Bases taste bitter
Acids and bases change the color of certain dyes called indicators.
According to Arrhenius, acids increase the concentration of H+ in a solution while bases increase the concentration of OH-.
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Acid Base Concepts
In the first part of this chapter we will look at several concepts of acid-base theory including:
The Arrhenius concept
The Bronsted Lowry concept
The Lewis concept
This chapter expands on what you learned in Chapter 3 about acids and bases.
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Acid Base Concepts
Antoine Lavoisier was one of the first chemists to try to explain what makes a substance acidic.
In 1777 he proposed that oxygen was an essential element in acids.
The actual cause of acidity and basicity was ultimately explained in terms of the effect these compounds have on water by Svante Arrhenius in 1884.
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Arrhenius Concept of Acids and Bases
According to the Arrhenius concept of acids and bases, an acid is a substance that, when dissolved in water, increases the concentration of hydronium ion (H3O+).
Chemists often use the notation H+(aq) for the H3O+(aq) ion, and call it the hydrogen ion.
Remember, however, that the aqueous hydrogen ion is actually chemically bonded to water, that is, H3O+.
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Arrhenius Concept of Acids and Bases
According to the Arrhenius concept of acids and bases, an acid is a substance that, when dissolved in water, increases the concentration of hydronium ion (H3O+).
The H3O+ is shown here hydrogen bonded to three water molecules.
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Arrhenius Concept of Acids and Bases
A base, in the Arrhenius concept, is a substance that when dissolved in water increases the concentration of hydroxide ion, OH-(aq).
The special role of the hydronium ion (or hydrogen ion) and the hydroxide ion in aqueous solutions arises from the following equilibrium.
H2O(l) + H2O(l) H3O+(aq) + OH-(aq)
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Arrhenius Concept of Acids and Bases
In the Arrhenius concept, a strong acid is a substance that ionizes completely in aqueous solution to give H3O+(aq) and an anion.
Other strong acids include HCl, HBr, HI, HNO3 and H2SO4.
An example is perchloric acid, HClO4.
)aq(ClO)aq(OH)l(OH)aq(HClO 4324
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Arrhenius Concept of Acids and Bases
In the Arrhenius concept, a strong base is a substance that ionizes completely in aqueous solution to give OH-(aq) and a cation.
Other strong bases include LiOH, KOH, Ca(OH)2, Sr(OH)2, and Ba(OH)2.
An example is sodium hydroxide, NaOH.
)aq(OH)aq(Na)s(NaOH OH 2
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Table 15.1 Common Strong Acids and Bases
Strong Acids Strong Bases
HCl LiOH
HBr NaOH
HI KOH
H2SO4 Ca(OH)2
HNO3 Sr(OH)2
HClO4 Ba(OH)2
RbOH
CsOH
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Arrhenius Concept of Acids and Bases
Most other acids and bases that you encounter are weak. They are not completely ionized and exist in reversible reaction with the corresponding ions.
Ammonium hydroxide, NH4OH, is a weak base.
)aq(OH)aq(NH )aq(OHNH 44
(aq)OHC(aq)OH 2323
An example is acetic acid, HC2H3O2.
)l(OH)aq(OHHC 2232
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Arrhenius Concept of Acids and Bases
The Arrhenius concept is limited in that it looks at acids and bases in aqueous solutions only.
In addition, it singles out the OH- ion as the source of base character, when other species can play a similar role.
Broader definitions of acids and bases are discussed in the next sections.
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Brønsted-Lowry Concept of Acids and Bases
A base is the species accepting the proton in a proton-transfer reaction.
In any reversible acid-base reaction, both forward and reverse reactions involve proton transfer.
According to the Brønsted-Lowry concept, an acid is the species donating the proton in a proton-transfer reaction.
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Brønsted-Lowry Concept of Acids and Bases
Consider the reaction of NH3 and H2O (see Figure 15.3).
)aq(OH )aq(NH )l(OH )aq(NH 423
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Brønsted-Lowry Concept of Acids and Bases
In the forward reaction, NH3 accepts a proton from H2O. Thus, NH3 is a base and H2O is an acid.
)aq(OH )aq(NH )l(OH )aq(NH 423
H+
base acid
Consider the reaction of NH3 and H2O (see Figure 15.3).
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Brønsted-Lowry Concept of Acids and Bases
In the reverse reaction, NH4+ donates a proton to
OH-. The NH4+ ion is the acid and OH- is the base.
)aq(OH )aq(NH )l(OH )aq(NH 423
H+
baseacid
Consider the reaction of NH3 and H2O (see Figure 15.3).
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Brønsted-Lowry Concept of Acids and Bases
Consider the reaction of NH3 and H2O.
A conjugate acid-base pair consists of two species in an acid-base reaction, one acid and one base, that differ by the loss or gain of a proton.
)aq(OH )aq(NH )l(OH )aq(NH 423
base acid
The species NH4+ and NH3 are a conjugate acid-base
pair.
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Brønsted-Lowry Concept of Acids and Bases
Consider the reaction of NH3 and H2O.
The Brønsted-Lowry concept defines a species as an acid or a base according to its function in the proton transfer reaction.
)aq(OH )aq(NH )l(OH )aq(NH 423
base acid
Here NH4+ is the conjugate acid of NH3 and NH3 is
the conjugate base of NH4+.
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Brønsted-Lowry Concept of Acids and Bases
Consider the reaction of NH3 and H2O.
They differ by one proton with H2O as the proton donor (acid) in the forward reaction and OH- as the proton acceptor (base) in the reverse reaction.
)aq(OH )aq(NH )l(OH )aq(NH 423
baseacid
We can also view H2O as the conjugate acid of OH- and OH- as the conjugate base of H2O.
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Brønsted-Lowry Concept of Acids and Bases
Some species can act as an acid or a base.
H+
An amphoteric species is a species that can act either as an acid or a base (it can gain or lose a proton).
For example, HCO3- acts as a proton donor (an acid)
in the presence of OH-
)l(OH)aq(CO )aq(OH)aq(HCO 22
33
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Brønsted-Lowry Concept of Acids and Bases
Some species can act as an acid or a base.
An amphoteric species is a species that can act either as an acid or a base (it can gain or lose a proton).
Or it can act as a proton acceptor (a base) in the presence of HF.
)aq(F)aq(COH )aq(HF)aq(HCO 323
H+
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Brønsted-Lowry Concept of Acids and Bases
The amphoteric characteristic of water is important in the acid-base properties of aqueous solutions.
H+
Water reacts as an acid with the base NH3.
)aq(OH)aq(NH )l(OH)aq(NH 423
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Brønsted-Lowry Concept of Acids and Bases
The amphoteric characteristic of water is important in the acid-base properties of aqueous solutions.
Water can also react as a base with the acid HF.
)aq(OH)aq(F)l(OH)aq(HF 32
H+
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Brønsted-Lowry Concept of Acids and Bases
In the Brønsted-Lowry concept:
2. Acids and bases can be ions as well as molecular substances.
3. Acid-base reactions are not restricted to aqueous solution.
4. Some species can act as either acids or bases depending on what the other reactant is.
1. A base is a species that accepts protons; OH- is only one example of a base.
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A Problem To Consider
In the following equation, label each species as an acid or a base. Show the conjugate acid-base pairs.
HCO3-(aq) + OH-(aq) CO3
2-(aq) + H2O(l)
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A Problem To Consider
In the following equation, label each species as an acid or a base. Show the conjugate acid-base pairs.
HCO3-(aq) + OH-(aq) CO3
2-(aq) + H2O(l)
On the left, HCO3- is the proton donor, making it
an acid.
H+
acid
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A Problem To Consider
In the following equation, label each species as an acid or a base. Show the conjugate acid-base pairs.
HCO3-(aq) + OH-(aq) CO3
2-(aq) + H2O(l)
On the right, H2O is the proton donor, making it an acid.
acid
H+
acid
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A Problem To Consider
In the following equation, label each species as an acid or a base. Show the conjugate acid-base pairs.
HCO3-(aq) + OH-(aq) CO3
2-(aq) + H2O(l)
The proton acceptors are OH- and CO32- making
them bases.
H+
acid
H+
acidbase base
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A Problem To Consider
In the following equation, label each species as an acid or a base. Show the conjugate acid-base pairs.
HCO3-(aq) + OH-(aq) CO3
2-(aq) + H2O(l)
Here, HCO3- and CO3
2- are a conjugate acid-base pair.
acid acidbase base
Conjugate acid-base pair
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A Problem To Consider
In the following equation, label each species as an acid or a base. Show the conjugate acid-base pairs.
HCO3-(aq) + OH-(aq) CO3
2-(aq) + H2O(l)
H2O and OH- are also a conjugate acid-base pair.
acid acidbase base
Conjugate acid-base pair
(see Exercise 15.1 and Problem 15.27)
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Lewis Concept of Acids and Bases
The Lewis concept defines an acid as an electron pair acceptor and a base as an electron pair donor.
This concept broadened the scope of acid-base theory to include reactions that did not involve H+.
The Lewis concept embraces many reactions that we might not think of as acid-base reactions.
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Lewis Concept of Acids and Bases
The reaction of boron trifluoride with ammonia is an example.
Boron trifluoride accepts the electron pair, so is a Lewis acid. Ammonia donates the electron pair and so is the Lewis base.
+ N
H
H
H:
::
: B
F
F
F
: :
::
::
::
: B
F
F
F
: :
::
:: N
H
H
H
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A Problem To Consider
In the following reaction, identify the Lewis acid and the Lewis base.
B(OH)3 + H2O B(OH)4- + H+
We must write the equation using Lewis structures.
+ OH
H: :::
::
: B
O
O
O
: :
::
::
H
HH
H+
:
:OH
:
::
: B
O
O
O
: :
::
::
H
HH
+
-
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A Problem To Consider
In the following reaction, identify the Lewis acid and the Lewis base.
B(OH)3 + H2O B(OH)4- + H+
Notice the new bond formed between the boron and the oxygen.
+ H+ OH
H: ::: :
:OH
:
::
: B
O
O
O
: :
::
::
H
HH
+
::
: B
O
O
O
: :
::
::
H
HH
-
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A Problem To Consider
In the following reaction, identify the Lewis acid and the Lewis base.
B(OH)3 + H2O B(OH)4- + H+
The electron pair was donated by the oxygen making water the Lewis base.
+ H+
::
: B
O
O
O
: :
::
::
H
HH
OH
H: ::: :
:OH
:
::
: B
O
O
O
: :
::
::
H
HH
+
Lewis base
-
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A Problem To Consider
In the following reaction, identify the Lewis acid and the Lewis base.
B(OH)3 + H2O B(OH)4- + H+
The electron pair was accepted by the boron making B(OH)3 the Lewis acid.
+ H+
::
: B
O
O
O
: :
::
::
H
HH
OH
H: ::: :
:OH
:
::
: B
O
O
O
: :
::
::
H
HH
+
Lewis acid
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A Problem To Consider
In the following reaction, identify the Lewis acid and the Lewis base.
B(OH)3 + H2O B(OH)4- + H+
The electron pair was accepted by the boron making B(OH)3 the Lewis acid.
+ H+
::
: B
O
O
O
: :
::
::
H
HH
OH
H: ::: :
:OH
:
::
: B
O
O
O
: :
::
::
H
HH
+
(see Exercise 15.2 and Problems 15.29 and 15.31)
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Summary
According to the Arrhenius concept of acids and bases, an acid is a H+ (proton) donor.
According to the Arrhenius concept of acids and bases, an base is a OH- (hydroxide) donor.
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Summary
According to the Bronsted-Lowry concept of acids and bases, an acid is a H+ (proton) donor.
According to the Bronsted-Lowry concept of acids and bases, a base is a H+ (proton) acceptor.
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Summary
The Lewis concept defines an acid as an electron pair acceptor and a base as an electron pair donor.
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Homework
Chapter 15 Homework: collected at the first exam.
Review Questions: 1, 4, 7.Problems: 23, 27, 29.
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Operational Skills
Identifying acid and base species
Identifying Lewis acid and base species
Time for a few review questions.
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