Copyright © 2001 by Harcourt, Inc. All rights reserved. Chapter 13 Acids and Bases Copyright © 2001 by Harcourt, Inc. All rights reserved. Requests for

Copyright © 2001 by Harcourt, Inc. All rights reserved.

Chapter 13

Acids and Bases

Copyright © 2001 by Harcourt, Inc.All rights reserved. Requests for permission to make copies of any part of the work should be mailed to the following address: Permissions Department, Harcourt, Inc. 6277 Sea Harbor Drive, Orlando, Florida 32887-6777

Bronsted-Lowry Acid-Base ModelThe Ion Product of WaterpH and pOHWeak Acids and Their Equilibrium ConstantsWeak Bases and Their Equilibrium ConstantsAcid-Base Properties of Salt Solutions


The Arrhenius Theory:A Brief Review

HCl(g) → H+(aq) + Cl-(aq)

NaOH(s) → Na+(aq) + OH-(aq)H2O

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.


Brønsted-Lowry Acid-Base Model

Acid : a proton donor, base : a proton acceptor

HB(aq)+A-(aq) HA(aq)+B-(aq)

amphiprotic: H2O, HCO3-….

OHOHOH HH32

acid base

conj

ugat

e ac

id

conj

ugat

e ba

se


Brønsted-Lowry Model of Acids and Bases

Acid is proton donor, base is proton acceptor

HB(aq)+A-(aq) HA(aq)+B-(aq)

HB, HA are acids; A-, B- are bases. HB – B- and HA – A- are conjugate acid-base pairs.

Amphiprotic


Acidic and Basic Water Solutions

Aqueous solution : acidic and basic properties of aqueous solutions are dependent on an equilibrium that involves

the solvent, water

pH : power of the hydrogen ion


Aqueous solution

In any aqueous solution, there is an equilibrium between H30+ (

H+) ions and OH– ions: ion product constant of waterH2O H+(aq) + OH–(aq)

KW = [H+] [OH–] = 1.0 10-14 at 25C

1. Pure water: [H+] = [OH–] = 1.0 10–7 M; neutral solution

2. Acidic solution: [H+] > 1.0 10–7 M > [OH–]

3. Basic solution: [OH–] > 1.0 10–7 M > [H+]In seawater, [H+] = 5 10-9 M; [OH–] = ?[OH–] = (1.0 10-14) / (5 10-9) = 2 10-6 M


pH

pH = –1og10[H+]

Neutral solution: pH = 7.0

Acidic solution: pH < 7.0

Basic solution: pH > 7.0Suppose [H+] = 2.4 10-6 M; calculate pH

pH = –1og10(2.4 10–6) = 5.62

Suppose pH = 8.68; calculate [H+]

[H+] = 10–8.68 = 2.1 10–9 MpOH = –1og10[OH–]; pH + pOH = 14.00 at 25C


Ionization of aqueous HCl


Relationship between pH and [H+]


Relationship between pH and [H+]


Strong AcidsHCl, HBr, HI, HClO4, HNO3, H2SO4

Strong BasesLiOH, NaOH, KOH, Ca(OH)2, Sr(OH)2, Ba(OH)2

HCl solution

Strong Acids & Bases


Weak Acid and Their Eqilibrium Consts.

Acetic Acid HC2H3O2 or CH3CO2H


Weak Acids

Molecules : molecules containing an ionizable hydrogen atom

Cations


Molecules

HF(aq) + H2O H3O+(aq) + F –(aq)

There are literally thousands of molecular weak acids, most of them organic in nature.


Cations

NH4+(aq) + H2O H3O+(aq) + NH3(aq)

Zn(H2O)42+(aq) + H2O H3O+(aq) + Zn(H2O)3(OH)+(aq)


Weak Acids

Equilibrium constantsCalculation of Ka : the weaker the acid, the smaller the value of Ka

Calculation of [H+] in solution : by known amount of the weakacid to form a given volume of solution


Weak Acids

Equilibrium constants

HB(aq) H+(aq) + B–(aq)

Ka [H+][B- ]

[HB]

pKa = -log10Ka

Weak acida : pKa HNO2 pKa = 3.22, HCN pKa=9.24


Percent Ionization

HA + H2O H3O+ + A-

Degree of ionization =[H3O+] from HA

[HA] originally

Percent ionization =[H3O+] from HA

[HA] originally X 100%


Polyprotic Weak Acid

H3PO4 H+ + H2PO4- Ka1 = 7.1 10-3

H2PO4- H+ + HPO4

2- Ka2 = 6.2 10-8

HPO42- H+ + PO4

3- Ka3 = 4.5 10-13

Ka1 >> Ka2 >> Ka3

All H3O+ is formed in the first ionization step.

H2PO4- essentially does not ionize further.

Assume [H2PO4-] = [H3O+].

ex2) H2CO3 , H2SO3, H2S


Strengths of Binary Acids

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 > 1.3x106 >> 6.6x10-4 Acid strength

HF + H2O → [F-·····H3O+] F- + H3O+

ion pairH-bonding

free ions

Tips )Structure & AcidityTips )Structure & Acidity


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

ENCl = 3.0 ENBr= 2.8

Ka = 2.9x10-8 Ka = 2.1x10-9

Tips1)Structure & AcidityTips1)Structure & Acidity


Calculation of Ka example

pH of 0.100 M HC2H3O2 solution is 2.87; Ka=?

[H+] = [C2H3O2–]=1.3 10–3 M

[HC2H3O2] = 0.100 M – 0.0013 M = 0.099 M

Ka = = 1.7 10–5(1.3 10–3)2

0.099


Calculation of [H+] in solution example

Find [H+] in 0.200 M HC2H3O2, Ka=1.8 10–5

Let [H+] = x, then [C2H3O2–] = x,

[HC2H3O2] = 0.200 – x

= 1.8 10–5

Assume 0.200 - x 0.200 ; x = 1.9 10–3 M

In this case, % ionization = 100 = 1.0%

x2

0.200 – x

1.9 10–3

0.200


Calculation of [H+] in solution example

Find [H+] in 0.100 M HF (Ka= 6.9 10–4)

= 6.9 10–4

Set 0.100 – x = 0.100, solve: x = 8.3 10–3 > 5%

Make second approximation:

= 6.9 10–4; x = 8.0 10–3

x2

0.100 – x

x2

0.100 – 0.008


Weak Bases

Molecules : there are many molecular weak bases, including

the organic compounds known as amines

Anions derived from weak acids


Molecules

NH3(aq) + H2O OH– (aq) + NH4+(aq)

The simplest weak base is ammonia.


Anions derived from weak acids

F– (aq) + H2O HF(aq) + OH– (aq)

F- is conjugate base of the weak acid HF.


Weak Bases

Equilibrium constant

•Expression for Kb : omitting the term for solvent water

•Relation between Ka and Kb : the larger the value of Ka, theSmaller the value of Kb and vise versa

•Calculation of [OH–] in solution of weak base


Expression for Kb

NH3: Kb =[NH4

+] [OH–][NH3]


Relation between Ka and Kb

Ka Kb = KW = 1.0 10–14

Ka Kb

HF 6.9 10–4 F– 1.4 10–11

HAc 1.8 10–5 Ac– 5.6 10–10

NH4+ 5.6 10–10 NH3 1.8 10–5

Strength of base is inversely related to that of conjugate weak acid.


Calculation of [OH–] in solution of weak base

= 1.4 10–11 ; = 1.4 10–11

[OH–] = 1.2 10–6 M ; pH = 8.08

[HF][OH–][F–]

[OH–]2

0.10


Acid-Base Properties of Salt Solutions

Cations : weak acids or spectator ions

Anions : weak bases or spectator ions

Overall Results


Cations

Spectator: derived from strong bases

Li+, Na+, K+; Ca2+, Sr2+, Ba2+

Acidic: all other cations, including those of the transition metals.


Anions

Spectator: derived from strong acids

C1–, Br–, I–, NO3–, SO4

2–

Basic: anions derived from weak acids, such as F–, NO2–



Overall Results

Salt Cation AnionNaNO3 Na+(Sp.) NO3

–(Sp.) neutral

KF K+(Sp.) F–(Ba.) basicFeCl2 Fe2+(Ac.) Cl–(Sp.) acidic

If cation is acidic, anion basic, compare Ka, Kb values

NH4F: Ka = 5.6 10–10, Kb = 1.4 10–11; acidic


Home work

3, 9,15,31,35,45,49,55,65,71

Documents

Copyright © 2001 by Harcourt, Inc. All rights reserved. Chapter 13 Acids and Bases Copyright © 2001 by Harcourt, Inc. All rights reserved. Requests for