Chapter 7- Acid Base Theories - Weebly · 2020. 10. 18. · 2. Acid and Base theories define...

Chapter 7- Acid Base Theories

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Review of the Previous Lecture

1. We discussed different type of imperfections, both intrinsic and extrinsic, in solid-state lattices of ionic compounds and how they make an entropic contribution▪ These imperfections allow for conductivity

2. Band Theory was defined and used to distinguish between conductors (metals), insulators (nonmetals), and semiconductors.

3. An emphasis was placed on the Band gap of semiconductors▪ Intrinsic▪ Extrinsic

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Acid-Base theories are a unifiying theme for differenttypes of chemical species interactions.

1. Solubility

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A. “Like dissolves like”

I. Nonpolar solute dissolves in nonpolar solvent▪ Solvent interactions overcome weak London dispersion forces of the

nonpolar solute.

Instantaneous dipole

Induced dipole

E 1

r6

Extremely short range and weak

[A (s)]

1. Solubility

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Simple dissolution

A (s) A (soln)

Solubility constant:Ks =

[A (soln)]

Do not consider because the activityof a solid is unity.

Ks

Ks > 1, Thermodynamically favors dissolution

Δ G = -RT ln Keq = -RT ln Ks ; Δ G < 0

Therefore Ks is a measure of solubility.

1. Solubility

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II. Polar and ionic compound solutes dissolve in polar solvents. The solvents

▪ Overcome dipole-dipole interactions of polar solutes

▪ Overcome electrostatic interactions of ionic compounds

- Relatively insensitive to r and omnidirectional.- Bond strength (250-400 kJ/mol)

E 1

r3

E 1

r

H2O as the polar solvent

1. Solubility

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Can have:

a. Simple dissolution- With polar solutes

b. Dissolution with dissociation- With ionic compounds

PbI2 (s) Pb2+ (aq) + 2I- (aq)

Ksp = [Pb2+][I-]2 = 8.49 x 10-9 at 298 K

What is the maximum solubility of Pb2+?Consider: [I-] = 2 [Pb2+]

Ksp = [Pb2+](2 [Pb2+])2 = 4 [Pb2+]3 = 8.49 x 10-9

[Pb2+] = (2.12 x 10-9)1/3 = 1.28 mM

2. Acid and Base theories define affinity concepts in solution.

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A. Historical definition- Alchemists used taste to identify acids/bases

▪ Acids- Sour▪ Bases- Bitter

B. Arrhenius (1884)

▪ Acids: Yield H+ in aqueous solution▪ Bases: Yield OH- in aqueous solution

HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)

Definition limited to aqueous solutions.

Salt Water

2. Acid and Base theories define affinity concepts in solution.

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C. Brønsted-Lowry (1923)

▪ Acids: H+ donors▪ Bases: H+ acceptors

I. Reaction

HA + B- A- + HB

Conjugate Pair

Conjugate Pair

Conjugate Acid

Conjugate Base

Definition not limited to aqueous solutions.


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II. Some solvents are amphoteric. They are both acids and bases.

Ex.) Water

NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

HF (aq) + H2O (l) H3O+ (aq) + F- (aq)

Acid

Base


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III. Thermodynamics

a. Let’s consider the autoionization of water. What is the [H3O+] or [H+] at 25 ºC?

2 H2O (l) H3O+ (aq) + OH- (aq)

1.0 0 0-x +x +x

1.0 – x x x

Initial

Change

Equilibrium

[H2O]2Kw =

[H3O+][OH-]

=(1-x)2

(x) (x)

Kw

C. III. Thermodynamics

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[H2O]2Kw =

[H3O+][OH-]

=(1-x)2

(x) (x)

~1 because water is the solvent and will be virtually unchanged

= x2 = 10-14.0

[H2O]Kw =

[H+][OH-]

Kw is very low, which indicates that the autoionization is very low.

pKw = - log10 Kw = 14.0


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Determine the value of x:

= x2 = 10-14= [H+][OH-]

(x2)1/2 = (10-14)1/2

x = (10-7.0) = [H3O+] = [H+]

p [H+] = pH = -log10 (10-7.0) = 7.0

Kw = [H3O+][OH-]


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b. Measuring the strength of an acid and a base in water

i) Acid:

HA (aq) A- (aq) + H+ (aq)

[HA]i 0 0-x +x +x

[HA]i – x x x

Initial

Change

Equilibrium

[HA]Ka =

[H+][A-]=

[HA]i – x

x2

Ka


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[HA]Ka =

[H+][A-]=

[HA]i – x

x2pKa = -log10 Ka

Ka is a measure of the strength to donate a H+

Ka , pKa , the stronger the acid

▪ Very big Ka indicates that an acid will undergo full dissociation


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b. Measuring the strength of an acid and a base in water

ii) Base:

OCl- (aq) + H2O (l) HOCl (aq) + OH- (aq)

[OCl-]Kb =

[HOCl][OH-]

Kb

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ii) Base:


[OCl-]Kb =

[HOCl][OH-]

If given Ka = 3.5 x 10-8, determine Kb :

[HOCl]Ka =

[H+][OCl-]

Ka x Kb = [H+][OH-] = Kw = 1.0 x 10-14

Kb = Kw

Ka

=1.0 x 10-14

3.5 x 10-8= 2.86 x 10-7

[OCl-]

[HOCl][OH-]

[HOCl]

[H+][OCl-]x =

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ii) Base:

Given 0.028 M NaOCl, determine the %OH- that forms. Recall that Kb = 2.86 x 10-7


0.028 M 0 0-x +x +x

0.028 M – x x x

Assume full dissociation of NaOCl.

Initial

Change

Equilibrium

[OCl-]Kb =

[HOCl][OH-]

0.028 M - x

x2

= = 2.86 x 10-7

Assume ~0.028 given the very low value of Kb

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ii) Base:


[OCl-]Kb =

[HOCl][OH-]

0.028 - x

x2

= = 2.86 x 10-7

Assume ~0.028 given the very low value of Kb

0.028

x2

= 2.86 x 10-7

x2 = 2.86 x 10-7 x 0.028

(x2)1/2 = 8.01 x 10-9

x = 8.9 x 10-5 M

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ii) Base:


x = 8.9 x 10-5 M = [OH-]


0.028 M 0 0-x +x +x

0.028 M – x x x

Initial

Change

Equilibrium

[NaOCl]ini

[OH-]= X 100%

0.028 M

8.9 x 10-5 MX 100% = 0.32%

An expected low % dissociation given the very low Kb = 2.86 x 10-7

The assumption that 0.028 M – x ~ 0.028 M is valid given the % dissociation is less than 5%.

IV. Le Chatlier’s Principle

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If a chemical system @ equilibrium experiences a change in concentration, theequilibrium shifts to counteract the imposed change and a new equilibrium isestablished.

▪ [reactants] or [products]; equilibrium shifts to the left

▪ [reactants] or [products]; equilibrium shifts to the right

HA + B- A- + HB

IV. Le Chatlier’s Principle

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Let’s consider the dissociation of acetic acid.

CH3COOH (aq) CH3COO- (aq) + H+ (aq)

[Acetic Acid]initial (M) [H+] (M) pH %H+ dissociated

1.00 4.23 x 10-3 2.37 0.423

0.05 9.49 x 10-4 3.02 1.90

0.005 2.91 x 10-4 3.54 5.82

5.0 x 10-6 4.87 x 10-7 6.31 9.74

%H

+d

isso

ciat

ion

[Acetic Acid]ini (M)

V. The Brønsted-Lowry concept is applicable to different solvents.

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Reconceptualize acid and base strength:

▪ The H3O+/H2O/OH- reference for quantifying relative acid and base strengths is only

useful when the examined acid is inherently weaker than H3O+ or the examined base is

weaker than OH-

▪ In H2O, H3O+ is the strongest acid

OH- is the strongest base

V. The Brønsted-Lowry concept is applicable to different solvents.

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▪ Acids inherently stronger than H3O+ cannot be differentiated by their aqueous ionization;

this is called the Leveling Effect

▪ The strength of a strong acid is limited (“leveled”) by the basicity of the solvent

A key perspective on leveling is that the classification of a substance as “weak” or “strong”and “acid” or “base” is tied to the solvent identity.

HA + H2O H3O+ + A-

The Leveling Effect

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In H2SO4

HClO4 + H2SO4 (l)

HCl + H2SO4 (l)

HOCl + H2SO4 (l)

Weak Base

Weak Acid

Cl-

- H+

HOCl OCl-

Strong Acid

Strong Base

HCl

The Leveling Effect

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In H2SO4

HClO4 + H2SO4 (l) ClO4- + H3SO4

+

HCl + H2SO4 (l) H2Cl+ + HSO4-

HOCl + H2SO4 (l) H2OCl+ + HSO4-

HClO4 is a weak acid in H2SO4

HCl is a weak base in H2SO4

HOCl is a strong base in H2SO4

HCl Cl-

- H+

HOCl OCl-

The Leveling Effect

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In Acetic Acid (CH3COOH)

HClO4 + HAc (l)

HCl + HAc (l)

HOCl + HAc (l)

HCl Cl-

- H+

HOCl OCl-

Weak Base

Weak Acid

Strong Acid

Strong Base

The Leveling Effect

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In Acetic Acid (CH3COOH)

HClO4 + HAc (l) ClO4- + H2Ac+

HCl + HAc (l) Cl- + H2Ac+

HOCl + HAc (l) H2OCl+ + Ac-

HClO4 is a strong acid in HAc

HCl is a weak acid in HAc

HOCl is a weak base in HAc

HCl Cl-

- H+

HOCl OCl-

The Leveling Effect

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In water

HClO4 + H2O (l)

HCl + H2O (l)

HOCl + H2O (l)

HCl Cl-

HOCl OCl-

Weak Base

Weak Acid

Strong Acid

Strong Base

The Leveling Effect

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In water

HClO4 + H2O (l) ClO4- + H3O

+

HCl + H2O (l) Cl- + H3O+

HOCl + H2O (l) OCl- + H3O+

HClO4 is a strong acid in H2O

HCl is a strong acid in H2O

HOCl is a weak acid in H2O

Both HClO4 and HCl are leveled in water and have the same strength.

HCl Cl-

HOCl OCl-

The Leveling Effect

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In NH3

HClO4 + NH3 (l)

HCl + NH3 (l)

HOCl + NH3 (l)

HCl Cl-

HOCl OCl-

Strong Acid

Weak Acid

The Leveling Effect

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In NH3

HClO4 + NH3 (l) ClO4- + NH4

+

HCl + NH3 (l) Cl- + NH4+

HOCl + NH3 (l) OCl- + NH4+

HClO4 is a strong acid in NH3

HCl is a strong acid in NH3

HOCl is a strong acid in NH3

All are leveled in NH3 and have the same strength.

HCl Cl-

HOCl OCl-

Chapter 7- Acid Base Theories - Weebly · 2020. 10. 18. · 2. Acid and Base theories define...

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