Acid-Base chemistry Acidity of blood (pH range of

Heartburn (acid-reflux) – Tums, Rolaids, Milk of Magnesia; The Purple Pill, Nexium

Acidity regulation (tropical fish / goldfish tanks)

Pepsi (most sodas); just how acidic?– Loosens rusty bolts; cleans windshields

Battery acid (H2SO4)

Acid Rain (SO2, NO2, CO2)

Drain cleaners (Drano, Liquid Plumber)

Nature of acids and bases Acids – sour, tart taste (vinegar, lemon juice) Bases – bitter taste, slippery feel between fingers (drano,

detergent)

Arrhenius definitions (1884):

Acids: produce H+ ions (protons) in solution

Bases: produce OH- (hydroxide) ions in solution

Lowry-Brønsted definitions Danish & British chemists

More general definitions than Arrhenius definition:

Acids: Proton donors Bases: proton acceptors

H2O + HCl H3O+ + Cl-

Lewis Acids / Lewis Bases LA: electron pair acceptor LB: electron pair donor

NH3(aq) + H+(aq) NH4+(aq)

LB LA

OH-(aq) + H+(aq) H2O(l)

General reaction for HA + H2O

Position of equilibrium dictated between bases in equation (competition for H+)

H2O stronger base: A- stronger base:

Keq = [H3O+][A-] Ka = Acid dissociation constant [HA][H2O] [H3O+] = [H+] in H2O

Ka = [H3O+][A-] Ka = Acid dissociation constant [HA]

HA(aq) + H2O(l) H3O+(aq) + A-(aq)Acid Base Conjugate Conjugate

Acid Base

Acid dissociation reactions Dissociation of acid (HA) most important here

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

H2O still important (required for aqueous conditions)

Ka for this equilibrium process?

Can predict dissociation reaction for any acid (no matter how complex-looking)

HCl (aq) HC2H3O2 (aq)

NH4+ (aq)

C6H5NH3+ (aq)

[Al(H2O)6]3+ (aq)

Acid Strength Defined by equilibrium position of dissociation reaction:

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

Strong acid – lies far to right side (a) Weak acid – lies far to left side (b)

Describing acid strength

Insert Figure 14.6

Insert table 14.1

strong acid weak base

Property Strong Acid Weak Acid Ka value

Equilibrium Posn. [H+]e vs [HA]o

Strength of conj.

base compared

to water

Common strong acids HCl, HClO4, HNO3 (monoprotic acids)

H2SO4 (diprotic acid)

Ka values – very large

Relative base strengths Arrange H2O, F-, Cl-, NO2

-, CN- in order of increasing base strength:

Water: Amphoteric substance Can exist as both an acid and a base Autoionization of water

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

Kw = [H3O+][OH-]; Kw = Dissociation constant for H2O

Kw = [H+][OH-]

[H+]=[OH-] = 1.0 x 10-7 M (at 25 °C, in pure water) Kw = [H+][OH-] = 1.0 x 10-14

In any aqueous solution, the product of [H+] and [OH-] must always equal 1.0 x 10-14

Kw

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

3 possible situations:

Neutral solutions; [H+] = [OH-] Acidic solutions; [H+] > [OH-] Basic solutions; [H+] < [OH-]

In all cases: Kw = [H+][OH-] = 1.0 x 10-14

Varies with T (as do all K values)

Calculating [H+],[OH-] [H+] with [OH-] = 1.0 x 10-5 M

[OH-] with [H+] = 10.0 M

[OH-] and [H+] in neutral solution at 60 °C (Kw = 1x10-13 at 60 °C)

pH scale (acidity measurement) pH = -log [H+] [H+] = 1.0 x 10-7 M; pH =

pOH = -log[OH-]; pK = -log Ka

Log scale; pH changes by 1 for every power of 10 change in [H+]

pH decreases as [H+] increases

Calculating pH / pOH 1.0 x 10-3 M OH-

1.0 M H+

pH + pOH = 14.00

pH of sample of human blood = 7.41 @ 25 °C. Calculate pOH, [H+], [OH-]

pH of strong acid solutions What species are present? 1M HCl What are the major species that can furnish H+ ions?

pH of 0.10 M HNO3

pH of 1.0 x 10-10 M HCl

pH of weak acid solutions pH of 1.00 M HF (weak acid); Ka = 7.2 x 10-4

Major species in solution? HF, H2O

Major species which furnish H+?

Which of these two is the stronger acid? Ka = [H+][F-] / [HF] = 7.2 x 10-4

In order to calculate pH, need equilibrium value of [H+]

[HF]o = 1.00 M

[H+]o = 0 M (approximation, as H+ from H20 not included here)

[F-]o = 0 M

Let x be the change required to reach equilibrium…..

H2O(l) H+(aq) + OH-(aq) Kw = 1.0 x 10-14

HF(aq) H+(aq) + F-(aq) Ka = 7.2 x 10-4

pH of weak acid solutions Equilibrium concentration of [HF] =

Equilibrium concentration of [H+] =

Equilibrium concentration of [F-] =

Substitute these into Ka = [H+][F-] / [HF] = 7.2 x 10-4 eqn, and solve for x….

Bases Cleaning solutions (ammonia, bleach) Antacids (Tums, Rolaids, Milk of Magnesia)

Arrhenius: produces OH- ions Lowry – Brønsted: H+ acceptor

Strong Bases - NaOH

pH of strongly basic solution 5.0 x 10-2 M NaOH solution (same procedure as for acidic pH

calculations); Expected pH range? Major species:

Weak bases Many types of bases don’t contain OH-, but do increase [OH-]

when dissolved in water (through reaction with water).

Base Acid

Lone pair of electrons on N picks up H+ from H2O

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

Weak bases General reaction with H2O:

Kb =

Kb always refers to reaction of a base with H2O to produce a conjugate acid and OH-

B(aq) + H2O(l) BH+(aq) + OH-(aq)

pH of weak bases - calculations Very similar to those of weak acids pH of 15.0 M solution of NH3 (Kb = 1.8 x 10-5)

Polyprotic acids More than 1 acidic H; H2SO4, H3PO4; H2CO3

Consider H3PO4

Only 1st dissociation step usually important for [H+] determination

Ka1 >>> Ka2 >>> Ka3

H3PO4(aq) H+(aq) + H2PO4-(aq) Ka1 = [H+][H2PO4

-] / [H3PO4]= 7.5 x 10-3

H2PO4-(aq) H+(aq) + HPO4

2-(aq) Ka2 = [H+][HPO42-] / [H2PO4

-] = 6.2 x 10-8

HPO42-(aq) H+(aq) + PO4

3-(aq) Ka3 = [H+][PO43-] / [HPO4

2-] = 4.8 x 10-13

Polyprotic acids pH of 5.0 M H3PO4 solution; eq. concs. of H3PO4, H2PO4

2-, HPO4

3- and PO43-.

Sulfuric acid (H2SO4) Unique acid Strong acid in 1st dissociation step Weak acid in 2nd dissociation step

pH of 1.0 M H2SO4 solution

Does HSO4- make significant contribution to [H+]? No.

Acid-base properties of salts Salts producing neutral solutions

Salts producing basic solutions

Salts producing acidic solutions

Acid-base properties of salts

Structure effects on acid/base properties

Hydrogen halides

Oxyacids