CH. 16 ACID -- BASE 16.1Definition

16.2Bronstad-LowryConjugate PairsStrengthLeveling Effect

16.3Dissociation of waterKw

16.4pH scale (pOH)

16.5Strong AcidStrong BasepH & [ ]

16.6Weak AcidsKa & pH% IonizationPolyprotic Acids

16.7Weak BasesKb & pH Acid Base Yields

16.8Ka & Kb Relation

16.9Salt SolutionsAnions/CationsLewis

16.10Binary AcidsOxyacidsCarboxylic Acids

16.11Bronstad-Lowry

ACID BASE CHEMISTRY

TERMS, ETC.

AMPHOTERIC:Subst. acts as either an acid or a baseProton (Acidic p+): H+ ion; the acidic hydrogen(s) present in an acid

HNO3: one H+ H2SO4: two H+ H3PO4: three H+

MONOPROTIC ACID: An acid w/ one H+

DIPROTIC ACID: An acid w/ two H+

POLYPROTIC ACID: An acid w/ 3 or more H+

TERMS, ETC.

ACID: Subst. in H2O incr [H+]

BASE: Subst. in H2O incr [OH-]

HYDRONIUM ION:

H2O (l) + H+1 (aq) ---------> H3O+1 (aq)

Effects H+ Ion in H2O

BRONSTAD-LOWRY

ACID: Subst. that donate acidic proton, H+

Proton donor

BASE: Subst. that gains acidic proton, H+

Proton acceptor

HCl(g) + H2O(l) ----> H3O+(aq) + Cl-(aq)

CONJUGATE ACID-BASE PAIRS

Acid, loses H+, form conjugate baseBase, gains H+, forms conjugate acid

)(-1

42(aq)3)(-2

4)(1

4 POH NH HPO NH aqaqaq

acid1 acid2base2 base1

CONJUGATE ACID-BASE PAIRS

(aq)3)(1

4 NH NH

aq )(-1

42)(-2

4 POHHPO aqaq

Fig 16.3pg. 657

RELATIVE STRENGTH

H2O stronger base than X- X weaker base --- equilib

H2O stronger base than Cl-

+ acid ion of weak acid react w/ H2O, produces H3O+

HX(aq) + H2O(l) ----> H3O+(aq) + X-(aq)

Strong Acid + Weak Base --- Acidic

HCl(g) + H2O(l) ----> H3O+(aq) + Cl-(aq)

X- stronger base than H2O

CH3COO- stronger base than H2O

CH3COOH(aq) + H2O(l) ----> H3O+(aq) + CH3COO-(aq)

X stronger base <---- equilib

Weak Acid + Strong Base --- Basic

Proton Transfer: ability of 2 bases attract protons

AUTOIONIZATION OF WATER

Kc[H2O]2 = Kw = [H3O+][OH-] = 1.0*10-14

Solution Acid [H3O+] > [OH-]Neutral [H3O+] = [OH-] Basic [OH-] > [H3O+]

22

3c

-32

OHOHOH K

OH OH ---- OH 2 )()()(

aqaql

0 7 14

pH SCALE

STRONG ACIDIC

STRONG BASIC

MILDACIDIC

MILDBASIC

WEAK ACIDIC

WEAK BASIC

NEUTRAL ( )

Kw: constant for water, 1*10-14

pKw = -Log [H2O] = -Log [1*10-14] = -Log [10-14] -Log [1] = -(-14) - 0 = 14

Kw = [H+][OH-]1*10-14 = [H+][OH-]

pKw = pH + pOH

pH = -Log [H+] pOH = -Log [OH-]

[H+] = 1*10-pH [OH-] = 1*10-pOH

Log 1 = 0 Log 10exp = explog 106 = 6 Log 10-4 = -4

Fig. 16.5pg 663

pH -- pOH -- [H+] -- [OH-] CALCULATIONS

Formulas to Use

-w OH H K

pOH pH pK w

ConstantsKw = 1 *10 –14

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

pKw = 14

Rules for LOGSLog 1 = 0

Log 10exp = exponent[H+] = 1*10-pH

[OH-] = 1*10-pOH

Rules for LOGS

Log 1 = 0

Log 10exp = exponent

Log 103.17 = 3.17

b Loga Log baLog

b Loga Log baLog

84.6 Log5.8 Log 84.65.8Log

32.4 Log12.6 Log 32.412.6Log

Calculate pH for thefollowing solutions

1) [H+] = 1 * 10-9

2) [H+] = 0.001

pH = -log (1*10-9) = -(-9) = 9

0.001 = 1*10-3

pH = -log (1*10-3) = -(-3) = 3

3) [H+] = 3.6 * 10-2

pH = -log(3.6*10-2) = 1.44

OR-log 10-2 = 2 pH = 2 – log (3.6) = 2 – 0.56 = 1.44

Find pH & pOH

pOH pKw = pH + pOH 14 = 1.44 + pOHpOH = 14 – 1.44 = 12.56

OR

HK OH- w

214

10*6.310*1

= 2.78*10-13

pOH = 13 – log 2.78 = 13 – 0.44 =

If [H+] is 6.7*10-9 mol/L, what is the pH? ACIDIC, BASIC, NEUTRALWhat is the [OH-]? What is the pOH?

pH = -log(6.7*10-9) = 9 – log 6.7 = 9 – 0.83 = 8.17 BASIC

pOH = 14 – 8.17 = 5.83

[OH-] = 1*10-5.83

Find pH Find pOH

Find [OH-]

OR

[OH-] = 1*10-14

6.7*10-9 = 1.49*10-6

TITRATIONSStrong Acid + Strong Base ------ Neutral

Strong Acid + Weak Base --- Acidic

Weak Acid + Strong Base --- Basic

Weak Acid + Weak Base ---- ???????

+/- ions of acid not react w/ H2O

+ acid ion weak acid react w/ H2O, produces H3O+

HBr + NH4OH NH4 is a weak acid reacts w/ H2O to produce H3O+

-acid ion weak base produces OH-

HCN: CN- is weak base reacts w/ H2O to produce OH-

Both +/- ions react w/ H2O

DISSOCIATIONHA: acid A-: anion Ka: acid-dissociation constant Strong Acid -- SA HA(aq) + H2O(l) --------> H3O+(aq) + A-(aq)

SA in solution; no HA present, most all H3O+

@ equilibrium Qc = Kc >>>> 1

HA

AQc

OH 3

Weak Acid -- WAHCN(aq) + H2O(l) --------> H3O+(aq) + CN-(aq)

WA in solution; no H3O+ present, most all HA@ equilibrium Qc = Kc <<<< 1

Stronger Acid ==> higher [H3O+] ===> larger Ka

Smaller Ka ==> less % dissed ====> weaker acid

STRENGTH

Strong Acid/Base ionize completely equilibrium lies far to the right HCl ------> H+ + Cl-

NaOH -----> Na+ + OH-

Weak Acid slightly ionize equilibrium lies far to the leftHC2H3O2 + H2O <-----> H+ + C2H3O2

-

]OH[HC]OHC][OH[KO]H[K

O]H][OH[HC]OHC][OH[K

232

-

2323a2eq

2232

-

2323eq

Ka: acid dissociation constantKb: base dissociation constant

IONIZATION CONSTANTS

Chlorous HClO4

Nitrous HNO2

Hydrofluoric HFFormic HCOOH

Acetic CH3COOHPropanoic CH3CH2COOH

Hypochlorous HClO

Hydrocyanic HCN

Ka Values MONOPROTIC ACIDS

Incr

eas i

ng a

cid

str e

ngth

Acid Ka

1.1*10-2

7.1*10-4

6.8*10-4

1.8*10-4

1.8*10-5

1.3*10-5

2.9*10-8

6.2*10-10

Problem Solving A-B Equilibria

Steps1. Write eqn & Ka/b expression2. I.C.E. table3. Define “x” as ; HA4. Assume “x” very small5. 5% rule

2 Assumptions1. [H3O+] from water ignore2. WA diss little, no in concen

Ka * Kb = Kw

Ka = 6.8*10-4 for 0.75 M hydrofluoric acid solution is.Determine the concentrations of H3O+, A-, & OH-. Also, find the pH, pOH, & % acid dissociation.

I 0.75 — 0 0 C -x + x + x E 0.75 - x x x

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

[H3O+] = [F-]Kw = [H3O+] [OH-]

1st write eqn & Ka

2nd construct table3rd determine “change, x”

4th assume x negligible; 0.75 - x => 0.75

HFOH 3

FKa

pH = -Log(0.0223) = 1.65 pOH = 14 - 1.65 = 12.35 OR = -Log(4.4*10-13) = 12.36

% Diss = 0.0223/0.75 * 100 = 3.0 %

0.75

xx 10*8.6 4 -

3

4-

F & OH

75.010*6.8

x

13--14

10*4.4 0223.010*1 OH

HA % Dissociation

HA% = [HA]diss/[HA]init * 100The initial concen of an acid is 3.2*10-2

then at equilibrium the concen is now 6.4*10-5

What is the % diss?

HA% = [2.8*10-4]/[1.2*10-2] * 100 = 2.3 %

Classify Acid - Base Strengths

SA Halogen acids: HCl, HBr, HIOxo acids: when # O’s > 1 H+

HNO3, HClO4, H2SO4

WBCmpds w/ e--rich N atom NH3, amines

WA Halogen acid: HF H not bonded to O: HCN, H2S, H3POxo acids: when # O’s < 2 H+

HNO2, HClO2, H3PO3

Carboxylic Acids: - COOH SB

Contain O-2 or OH-

Group 1A active metals: MOH, M2O Li, Na, K, Rb, CsGroup 2A active metals: M(OH)2, MO Ca, Sr, Ba

Ka acetic acid = 1.8*10-5

Ka nitrous acid = 4.4*10-4nitrous acid more ionized in soln

BASE

NH3 + H2O <---> NH4+ +OH-

][NH]OH][NH[KO]H[K

O]H][[NH]OH][NH[K

3

-

4b2eq

23

-

4eq

Kb ammonia = 1.8*10-5

S.A.S.A. yields weak conjugate baseW.A.W.A. yields strong conj. base

MONOPROTIC ACIDSKa Acid

1.2*10-2 HSO4-

1.2*10-2 HClO2

7.2*10-4 HF

1.8*10-5 HC2H3O2

3.5*10-8 HOCl

5.6*10-10 NH4+

8.3*10-13 SO4-2

8.3*10-13 ClO2-

1.4*10-11 F-

5.56*10-10 C2H3O2-

2.9*10-7 OCl-

1.8*10-5 NH3

Kb Conj. Base

acid strength conj. base strengthHCl Strong Cl-

H2SO4 HSO4-

HNO3 NO3-

H3O+ H2O

HSO4- Weak SO4-2

H2SO3 HSO3-

H2PO4 H2PO4-

HF F-

CH3COOH CH3COO-

H2CO3 HCO3-

H2S HS-

HSO3- SO3

-2

H2PO4 HPO4-2 Weak

HCN CN-

NH4+ NH3

HCO3- CO3

-2

HPO4-2 PO4

-3

H2O OH- Strong

RXN DIRECTION Direction of SA & SB to form WA & WB

H2PO4- + NH3 <------> NH4

+ + HPO4-2

HS- + H2O <------>

H2S SA than H2O

Acid Base rxn goes dir: if HA reacts w/ base lower on list

H2PO4- SA than NH4

+

H2S + OH-

Diprotic & Triprotic Acids

H2CO3: 2 acidic protons Ka values for each H+

H CO HCO

H HCO COH2-

3

-

3

-

332 Ka1 = 4.3*10-7

Ka2 = 5.6*10-11

typically weak polyprotic acid Ka1 > Ka2 > Ka3

means???? -each step of dissociation is successively weaker-loss of 2nd & 3rd proton occurs less readily

Why???? should not be surprising think in terms of charges, +/- being attracted

Various Ways to Describe Acid Strength

Property S.A. W.A.Ka value Ka is large Ka is small

Position of dissociation equilibrium

Far to right Far to left

Equilibrium [H+] compared to [HA]o

[H+] [HA]o [H+]<<[HA]o

Strength of conj.base compared to H2O

A- much weaker base than H2O

A- much stronger base than H2O

The stronger the acid the better it is at donating H+

2 factors of Acid Strength-- depends how easily the H+ is lost1) Bond Polarity

More polarized bond, quicker H+ lost, greater acid strength2) Bond Strength

Larger “none H” atom is, weaker the bond, greater acid strength

Acid Strength Nonmetal HydridesAcid Strength Nonmetal Hydrides

Metal Ion Acid StrengthMetal Ion Acid Strength high charge density, small metal ions: Fe+3, Al+3, Cu+2, Pb+2, Zn+2, Ni+2

PROPERTIES OF SALTS pH of salt soln: A-B properties

of cation & anion1. SA + SB ----> Neutral2. SA + WB ----> Acidic3. WA + SB ---> Basic

1. Cat- & An-ions not react w/ water to form H3O+ or OH-

2. Anion inert; Cation WA, form H3O+

3. Cation inert; Anion WB, form OH-

4. WA + WB ----> ????

Both ions undergo proton transfer

Ka cation > Kb anion: ACIDICKb anion > Ka cation: BASIC

Ka cation = Kb anion: NEUTRAL

All Bronstad acids form H3O+ in water & bases OH-

SA completely form H3O+

SB completely form OH-

Lewis Acids - BasesMolecules w/ central atom < 8 val e-

Polar molecules w/ dbl bond

Metal ions dissolved in water

ID Lewis A & BCl- + BCl3 <----> BCl4

-

Cl- form coval bond to B; Cl- 4 e- pair BCl3 accepts e- pair, ACID

Leveling Effect