GENERAL CHEMISTRY. Principles and Modern Applications Acids and Bases TENTH EDITION. Slide 1 of 51. General Chemistry: Chapter 16

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TENTH EDITION

GENERAL CHEMISTRY

Principles and Modern Applications

Acids and Bases

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16-1 The Arrhenius Theory: A Brief Review

General Chemistry: Chapter 16 Slide 2 of 51

HCl(g) → H+_{(aq) + Cl}-_(aq)

NaOH(s) → NaH2O +_{(aq) + OH}-_(aq) H₂O

Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq) → H₂O(l) + Na+(aq) + Cl-(aq) H+(aq) + OH-(aq) → H₂O(l)

Arrhenius theory did not handle non OH -bases such as ammonia very well.

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16-2 Brønsted-Lowry Theory of Acids and Bases

An acid is a proton donor.

A base is a proton acceptor.

NH₃+ H₂O NH₄+ + OH -NH₄+ + OH- NH₃+ H₂O

base acid

base

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Base Ionization Constant

General Chemistry: Chapter 16 Slide 4 of 51 NH₃+ H₂O NH₄+ _{+ OH} -K= a_NH3a_H2O (a_{NH4+) (}a_OH-) K_b= K [H₂O] = [NH₃] [NH₄+_][OH-_] = 1.810-5

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FIGURE 16-1 H H H N + H O -•• H H O •• •• •• •• ••

Base (1) Acid (2) Acid (1) Base (2) H

H N

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A hydrated hydronium ion FIGURE 16-2

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Acid Ionization Constant

CH₃CO₂H + H₂O CH₃CO₂- + H₃O+ K= a_CH3CO2H a_H2O a_CH3CO2-a_H3O+ K_a= K [H₂O] = = 1.810-5 [CH₃CO₂H] [CH₃CO₂-_][H 3O+] base acid conjugate acid conjugate base

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Brønsted-Lowry acid-base reaction: weak acid FIGURE 16-3

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16-3 The Self-Ionization of Water and the pH Scale

H₂O + H₂O H₃O+ + OH -K = [H₃O+][OH-] H H H O+ •• _O _H -•• H H O •• H H O •••• •• •• ••

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Ion Product of Water

K_c=

[H₂O][H₂O] [H₃O+][OH-]

H₂O + H₂O H₃O+ + OH

-base acid conjugate_acid conjugate _base

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pH and pOH

The

“potential of the hydrogen ion”

was defined

in 1909 as the negative of the logarithm of

[H

+

].

pH = -log[H

₃

O

+

]

pOH = -log[OH

-

]

-logK_W= -log[H₃O+]-log[OH-]= -log(1.010-14)

K_W= [H₃O+][OH-]= 1.010-14

pK_W= pH + pOH= -(-14) pK_W = pH + pOH = 14

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pH and pOH Scales

FIGURE 16-5

Relating [H₃O+_{], pH, [OH}-_{], and pOH}

Figure 16-6 The pH scale and pH values of some common materials

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16-4 Strong Acids and Bases

HCl CH₃CO₂H

Thymol Blue Indicator

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16-5 Weak Acids and Bases

Glycine

Acetic Acid

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Identifying Weak Acids and Bases

General Chemistry: Chapter 16 Slide 18 of 51 Acetic Acid K_a= = 1.810-5 [CH₃CO₂H] [CH₃CO₂-][H₃O+] pK_a= -log(1.810-5) = 4.74

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Identifying Weak Acids and Bases

K_b= = 4.310-4 [CH₃NH₂] [CH₃NH₃+_][HO-_] pK_b= -log(4.210-4_{) = 3.37} H H H N + H O -•• H H O •• •• •• •• ••

Base (1) Acid (2) Acid (1) Base (2) CH₃

H N

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Illustrative Examples

Some questions to ask yourself when solving

equilibrium problems:

• Which are the principal species in solution?

• What are the chemical reactions that produce them?

• Can some reactions (for example, the self-ionization of water) be ignored?

• Can you make any assumptions that allow you to simplify the equilibrium calculations?

• What is a reasonable answer to the problem? For instance, should the final solution be acidic (pH < 7) or basic (pH > 7).

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Percent Ionization

HA + H₂O H₃O+ + A -Degree of ionization = [H₃O+_{] from HA} [HA] originally Percent ionization = [H₃O+_{] from HA} [HA] originally  100%

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Percent Ionization

FIGURE 16-8

Percetn ionization of an acid as a function of concentration

K_a= [H3O +_][A-_] [HA] K_a= n_H 3O+ A -n HA n 1 V

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16-6 Polyprotic Acids

H₃PO₄ + H₂O H₃O+ + H₂PO₄ -H₂PO₄- _{+ H} 2O H3O+ + HPO4 2-HPO₄2- _{+ H} 2O H3O+ + PO4 3-Phosphoric acid: A triprotic acid. K_a = 7.110-3 K_a = 6.310-8 K_a = 4.210-13

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Phosphoric Acid

K

_a1

>> K

_a2

All H₃O+ is formed in the first ionization step.

H

₂

PO

₄-

essentially does not ionize further.

Assume [H₂PO₄-] = [H₃O+].

[HPO

₄2-

]  K

_a2

regardless of solution molarity.

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A Somewhat Different Case: H

₂

SO

₄

General Chemistry: Chapter 16 Slide 26 of 51 Sulfuric acid: A diprotic acid. H₂SO₄ + H₂O H₃O+ + HSO₄ -HSO₄- _{+ H} 2O H3O+ + SO4 2-K_a = very large K_a = 1.1 X 10-2

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General Approach to Solution Equilibrium

Calculations

Identify species present in any significant amounts in

solution (excluding H

₂

O).

Write equations that include these species.

Number of equations = number of unknowns.

• Equilibrium constant expressions.

• Material balance equations.

• Electroneutrality condition.

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[NH₃] [H₃O+] [OH-]

K_a=

[NH₄+_{] [OH}-_]

16-7 Ions as Acids and Bases

General Chemistry: Chapter 16 Slide 28 of 51 NH₄+ + H₂O NH₃+ H₃O+ base acid CH₃CO₂- + H₂O CH₃CO₂H + OH -base acid [NH₃] [H₃O+] [OH-] K_a= [NH₄+_{] [OH}-_] [NH₃] [H₃O+] K_a= [NH₄+] = ? = K_W K_b = 1.010-14 1.810-5 = 5.610 -10

K

_a

K

_b

= K

_w

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Hydrolysis

Water (hydro) causing cleavage (lysis) of a bond.

Na+ + H₂O → Na+ + H₂O NH₄+ + H₂O → NH₃ + H₃O+ Cl- + H₂O → Cl- + H₂O No reaction No reaction Hydrolysis

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The pH of Salt Solutions

• Salts of strong bases and strong acids (for example, NaCl) do not

hydrolyze: for the solution, pH=7

• Salts of strong bases and weak acids (for example, NaCH₃CO₂- ₎ hydrolyze: pH > 7 (The anion acts as a base.)

• Salts of weak bases and strong acids (for example, NH₄Cl)

hydrolyze: pH < 7 (The cation acts as an acid.)

• Salts of weak bases and weak acids (for example, CH₃CO₂NH₄ )

hydrolyze. (The cations are acids, and the anions are bases.

Whether the solution is acidic or basic, however, depends on the relative values of K_a and K_b for the ions.)

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16-8 Molecular Structure and Acid-Base Behavior

Why is HCl a strong acid, but HF is a weak one?

Why is CH

₃

CO

₂

H a stronger acid than CH

₃

CH

₂

OH?

There is a relationship between molecular structure and

acid strength.

Bond dissociation energies are measured in the gas

phase and not in solution.

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Strengths of Binary Acids

When comparing binary acids of elements in the same group of the periodic table, acid strength increases as the length

of the bond increases.

When comparing binary acids of elements in the same row of the periodic table, acid strength increases as the polarity of

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Strengths of Binary Acids

General Chemistry: Chapter 16 Slide 34 of 51 HI HBr HCl HF 160.9 > 141.4 > 127.4 > 91.7 pm 297 < 368 < 431 < 569 kJ/mol Bond length Bond energy 109 _> ₁₀8 _{> 1.310}6 _{>> 6.610}-4 Acid strength HF + H₂O → [F-···H₃O+] F- + H₃O+ ion pair H-bonding free ions

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Strengths of Oxoacids

Factors promoting electron withdrawal from the OH

bond to the oxygen atom:

High electronegativity (EN) of the central atom.

A large number of terminal O atoms in the

molecule.

H-O-Cl H-O-Br

EN_Cl = 3.0 EN_Br= 2.8

K_a = 2.910-8 _K

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Strengths of Oxoacids

General Chemistry: Chapter 16 Slide 36 of 51 S O O O O H ·· H ·· ·· ·· -2+ ·· ·· ·· ·· ·· -·· S O O O H ·· H ·· ·· ·· -+ ·· ·· ·· ·· S O O O O H ·· H ·· ·· ·· ·· ·· ·· ·· S O O O H ·· H ·· ·· ·· ·· ·· ·· K_a103 K_a=1.310-2

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Strengths of Organic Acids

C _O C O H ·· H ·· ·· ·· H H O C H ·· H ·· H H C H H K_a= 1.810-5 K_a=1.310-16

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Focus on the Anions Formed

General Chemistry: Chapter 16 Slide 38 of 51 O C H ·· ·· H H C H H C O C O H -H H C O C O H -H H ·· -Ethoxide ion Acetate ion

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Chain length has little effect:

C H H C O C O H -H H C H H H C O O -C H H C H H C H H C H H C H H K_a= 1.810-5 K_a= 1.310-5

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but substitution may strongly affect acid strength:

General Chemistry: Chapter 16 Slide 40 of 51 C O C O H -H H K_a= 1.810-5 K_a= 1.410-3 C O C O H -H Cl

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Strengths of Amines as Bases

N H H H ·· _Br _N H H ·· pK_b= 4.74 pK_b= 7.61 ammonia bromamine pK_a = 35

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Strengths of Amines as Bases

General Chemistry: Chapter 16 Slide 42 of 51 C H H H C H H C H H H C H H C H H H C H H pK_b= 3.38 pK_b = 3.37 pK_b = 3.25

methylamine ethylamine propylamine

NH₂ NH₂ NH₂ H N H H : pK_b = 4.74

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16-9 Lewis Acids and Bases

Lewis acid

A species (atom, ion or molecule) that is an

electron pair acceptor.

Lewis base

A species that is an electron pair donor.

base acid adduct

N H H H ·· B F F F·· ·· ·· ·· ·· ·· ·· ·· ·· B- F F F·· ·· ·· ·· ·· ·· ·· ·· ·· N H H H

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-Complex ions

Slide 46 of 51 General Chemistry: Chapter 16 FIGURE 16-11

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ionic charge ρ = charge density =

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