Acids and Bases [PDF]

2010/11/4

Acids Have a sour taste. Vinegar owes its taste to acetic acid. Citrus fruits contain citric acid. React with certain metals to produce hydrogen gas. React with carbonates and bicarbonates to produce carbon dioxide gas

Acids and Bases

Bases

Chapter 15 Have a bitter taste. Feel slippery. Many soaps contain bases. 1

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Acid-Base Properties of Water

A Brønsted acid is a proton donor A Brønsted base is a proton acceptor

H+ (aq) + OH- (aq)

H2O (l)

autoionization of water H

O H

base base

acid

acid conjugate acid

acid

+ H

[H

O H

O

base

The Ion Product of Water Kc =

[H2O]

O

-

conjugate acid H3O+ + OHconjugate base

3

H+ (aq) + OH- (aq)

+ H

base conjugate base

H2O + H 2O

H2O (l)

+

H

acid

[H+][OH-]

]

H

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What is the concentration of OH- ions in a HCl solution whose hydrogen ion concentration is 1.3 M? [H2O] = constant

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

Kc[H2O] = Kw = [H+][OH-] [H+] = 1.3 M The ion-product constant (Kw) is the product of the molar concentrations of H+ and OH- ions at a particular temperature. Solution Is At 250C Kw = [H+][OH-] = 1.0 x 10-14

[H+] = [OH-]

neutral

[H+] > [OH-]

acidic

[H+] < [OH-]

basic 5

[OH-] =

Kw 1 x 10-14 = = 7.7 x 10-15 M [H+] 1.3

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pH – A Measure of Acidity

Other important relationships pOH = -log [OH-]

pH = -log [H+]

[H+][OH-] = Kw = 1.0 x 10-14 -log [H+] – log [OH-] = 14.00

At 250C

Solution Is neutral

[H+] = [OH-]

[H+] = 1 x 10-7

pH = 7

acidic

[H+]

[OH-]

[H+] > 1 x 10-7

pH < 7

basic

[H+] < [OH-]

[H+] < 1 x 10-7

pH > 7

>

pH + pOH = 14.00

[H+]

pH

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Strong Electrolyte – 100% dissociation

The pH of rainwater collected in a certain region of the northeastern United States on a particular day was 4.82. What is the H+ ion concentration of the rainwater?

NaCl (s)

pH = -log [H+] [H+]

H2O

CH3COO- (aq) + H+ (aq)

CH3COOH The ion concentration of a blood sample is 2.5 x What is the pH of the blood?

10-7

M. Strong Acids are strong electrolytes

pH + pOH = 14.00

HCl (aq) + H2O (l)

pOH = -log [OH-] = -log (2.5 x 10-7) = 6.60 pH = 14.00 – pOH = 14.00 – 6.60 = 7.40 9

Weak Acids are weak electrolytes HF (aq) + H2O (l)

H3

HNO2 (aq) + H2O (l) -

HSO4 (aq) + H2O (l) H2O (l) + H2O (l)

O+

(aq) +

H3

KOH (s)

H2O H2O

(aq) + SO4

2-

Ba(OH)2 (s)

H3O+ (aq) + NO3- (aq)

HClO4 (aq) + H2O (l)

H3O+ (aq) + ClO4- (aq)

H2SO4 (aq) + H2O (l)

H3O+ (aq) + HSO4- (aq)

NO2- (aq) + H2O (l)

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OH- (aq) + HF (aq) OH- (aq) + HNO2 (aq)

(aq)

Conjugate acid-base pairs:

H3O+ (aq) + OH- (aq)

Na+ (aq) + OH- (aq)

K+ (aq) + OH- (aq)

H2O

HNO3 (aq) + H2O (l)

F- (aq) + H2O (l)

(aq)

•

The conjugate base of a strong acid has no measurable strength.

•

H3O+ is the strongest acid that can exist in aqueous solution.

•

The OH- ion is the strongest base that can exist in aqeous solution.

Strong Bases are strong electrolytes NaOH (s)

H3O+ (aq) + Cl- (aq)

Weak Bases are weak electrolytes F-

H3O+ (aq) + NO2- (aq) O+

Na+ (aq) + Cl- (aq)

Weak Electrolyte – not completely dissociated

= 10-pH = 10-4.82 = 1.5 x 10-5 M

OH-

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pH Meter

Ba2+ (aq) + 2OH- (aq) 11

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Strong Acid (HCl)

Weak Acid (HF)

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Weak Acids (HA) and Acid Ionization Constants

What is the pH of a 2 x 10-3 M HNO3 solution?

Start 0.002 M HNO3 (aq) + H2O (l) End 0.0 M

H3O+ (aq) + A- (aq)

HA (aq) + H2O (l)

HNO3 is a strong acid – 100% dissociation. 0.0 M 0.0 M H3O+ (aq) + NO3- (aq) 0.002 M 0.002 M

H+ (aq) + A- (aq)

HA (aq) Ka =

pH = -log [H+] = -log [H3O+] = -log(0.002) = 2.7

[H+][A-] [HA]

Ka is the acid ionization constant

What is the pH of a 1.8 x 10-2 M Ba(OH)2 solution? Ba(OH)2 is a strong base – 100% dissociation. Start 0.018 M Ba(OH)2 (s) End 0.0 M

0.0 M 0.0 M Ba2+ (aq) + 2OH- (aq) 0.018 M 0.036 M

pH = 14.00 – pOH = 14.00 + log(0.036) = 12.6

weak acid strength

Ka

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What is the pH of a 0.5 M HF solution (at 250C)? HF (aq)

H+ (aq) + F- (aq) HF (aq)

Initial (M) Change (M) Equilibrium (M)

H+ (aq) + F- (aq) 0.00

0.00

-x

+x

+x

0.50 - x

x

x

x2 = 7.1 x 10-4 0.50

[H+] = [F-] = 0.019 M 17

[H+][F-] = 7.1 x 10-4 [HF]

0.50

x2 = 7.1 x 10-4 Ka = 0.50 - x Ka 

Ka =

[HF] = 0.50 – x = 0.48 M

Ka HClO

NaCH3COOH (s)

H2O

CH3COO- (aq) + H2O (l)

Na+ (aq) + CH3COO- (aq) CH3COOH (aq) + OH- (aq)

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Acid-Base Properties of Salts

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Acid Hydrolysis of Al3+

Acid Solutions: Salts derived from a strong acid and a weak base. NH4Cl (s) NH4+ (aq)

H2O

NH4+ (aq) + Cl- (aq) NH3 (aq) + H+ (aq)

Salts with small, highly charged metal cations (e.g. Al3+, Cr3+, and Be2+) and the conjugate base of a strong acid. 3+

Al(H2O)6 (aq)

2+

Al(OH)(H2O)5 (aq) + H+ (aq)

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Acid-Base Properties of Salts

Solutions in which both the cation and the anion hydrolyze: •

Kb for the anion > Ka for the cation, solution will be basic

•

Kb for the anion < Ka for the cation, solution will be acidic

•

Kb for the anion  Ka for the cation, solution will be neutral

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Definition of An Acid

Oxides of the Representative Elements In Their Highest Oxidation States

Arrhenius acid is a substance that produces H+ (H3O+) in water A Brønsted acid is a proton donor A Lewis acid is a substance that can accept a pair of electrons A Lewis base is a substance that can donate a pair of electrons

Na2O (s) + H2O (l)

2NaOH (aq)

CO2 (g) + H2O (l)

H2CO3 (aq)

N2O5 (g) + H2O (l)

2HNO3 (aq)

•• • H+ + OH • •• acid base

•• H O H ••

H N•• H

H + H N H

H+ + 39

acid

H base

H 40

Chemistry In Action: Antacids and the Stomach pH Balance

Lewis Acids and Bases

NaHCO3 (aq) + HCl (aq)

F +

F B

H N•• H

F

H

acid

base

F F B F

H

NaCl (aq) + H2O (l) + CO2 (g)

N H H

No protons donated or accepted! Mg(OH)2 (s) + 2HCl (aq) MgCl2 (aq) + 2H2O (l) 41

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