Acid-Base Equilibria

BLB 10th Chapter 16

Examples of acids & bases

Acids Bases

Sour (like vinegar) Bitter and slippery (like soap)

React with bases to neutralize them and form salts

React with acids to neutralize them and form salts

Change indicator colors in opposite direction from base (e.g. litmus blue to red)

Change indicator colors in opposite direction from acid (e.g. litmus red to blue)

Aqueous solutions conduct electricity

Aqueous solutions conduct electricity

Liberate hydrogen in reactions with active metals

React in aqueous solution with salts of heavy metals to form insoluble hydroxides or oxides

16.1 Acids & Bases: A Brief Review

Arrhenius Definitions Acid – a substance that produces

hydrogen ions (H+) in water

HA → H+ + A-

Base – a substance that produces hydroxide ions (OH-) in water

BOH → B+ + OH-

16.2 Brønsted-Lowry Acids & Bases

H+ (proton) in water:

H+ + H2O → H3O+

hydronium ionHydronium ion can hydrogen bond with

more water molecules to form large clusters of hydrated hydronium ions.

H+ and H3O+ are used interchangeably.

16.2 Brønsted-Lowry Acids & Bases

Brønsted-Lowry definitions

acid – proton donor Neutral (HNO3), anionic (HCO3

-), cationic (NH4+)

Must have a removable (acidic) proton

base – proton acceptor Neutral (NH3), anionic (CO3

2-) Must have a lone pair of electrons

Acid-Base Reactions

HCl(aq) + H2O(l) → H3O+ (aq) + Cl-(aq)

NH3(aq) + H2O(l) NH⇌ 4+(aq) + OH-(aq)

HCl(aq) + NH3(aq) → NH4+(aq) + Cl-(aq)

Acid-base reaction in non-aqueous media:HCl(g) + NH3(g) → NH4Cl(s)

amphiprotic – capable of behaving as a Brønsted acid and Brønsted base

amphoteric – capable of behaving as a Lewis acid and Brønsted base (17.5)

neutralization: acid + base → salt + water

Conjugate acid/base pairs – differ by a single proton

HA(aq) + H2O(l) → H3O+(aq) + A-(aq)

acid + base conj. acid + conj. base

Relative Acid/Base Strength Strength is a measure of the ability of an

acid (or base) to donate (or accepts) a H+. Stronger acids donate H+ more readily.

Completely dissociate in water Conjugate bases have negligible tendency to

accept protons.

Weaker acids donate H+ less readily. Partially dissociate and establish equilibrium Conjugate bases have some tendency to accept

protons.

The stronger an acid, the weaker its conjugate base and vice versa.

p. 672

Acid/base reactions proceed from the stronger acid-base pair to the weaker acid-base pair.

Common strong acids (p. 679):

HClO4, HClO3, H2SO4, HI, HBr, HCl, HNO3

Monoprotic acid – capable of donating only one H+

Polyprotic acid – capable of donating more than one H+

Common strong bases (p. 680):

M(OH)n, where M = group I (n=1) & II (n=2) metals, except Be

Acid/Base Reactions

16.3 The Autoionization of Water

H2O(l) + H2O(l) H⇌ 3O+(aq) + OH-(aq) Kw = [H3O+][OH-] = [H+][OH-] = 1.0 x 10-14 (@ 25°C)

Kw – ion-product constant (or dissociation constant)

Pure water is neutral. Thus,

[H3O+] = [OH-] = 1.0 x 10-7 M @ 25°C

For an aqueous solution:

[H3O+] > [OH-] acidic

[H3O+] = [OH-] neutral

[H3O+] < [OH-] basic

Working with Kw

16.4 The pH Scale pH represents a solution’s acidity (@

25°C. 0 ← 7 → 14acid neutral base

See Table 16.1, p. 678 for summary. See Figure 16.5, p. 679 for examples. pH = −log[H3O+] = −log[H+]

[H3O+] = 10-pH

pOH = −log[OH-] pH + pOH = 14[OH-] = 10-pOH

p. 676

More common chemicals

Chemical pH

BasicWindex 10.57

Bleach 9.58

Neutral Tap water* 7.46

Acidic

Alka Seltzer (in tap water) 6.43

Distilled water** 6.37

Flat Coke 2.60

Toilet bowl cleaner 1.04

6.0 M HCl −0.29

*CaCO3 CO3- + H2O ⇌ HCO3

- + OH-

**CO2 + H2O → H2CO3

pH calculations

More about pH

pH does not necessarily indicate strength.

Measuring pH pH meters Acid-base indicators

p. 679

16.5 Strong Acids and Bases

Strong acids & bases completely ionize.

[HA]0 = [H3O+] → pH

[MOH]0 = [OH-] → pOH → pH

2[M(OH)2]0 = [OH-] → pOH → pH

H3O+ is the strongest acid that can exist in water. (produced by all acids in water)

OH- is the strongest base that can exist in water. (produced by all bases in water)

pH problems

End Test #1 material

16.6 Weak Acids & 16.7 Weak Bases

Weak acids & bases do not completely ionize.

Weak acids establish an equilibrium in aqueous solution.

HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)

HA(aq) ⇌ H+(aq) + A-(aq) They do not readily donate or accept H+’s. [HA]0 ≠ [H3O+]

[MOH]0 ≠ [OH-]

HA(aq) + H2O(l) H⇌ 3O+(aq) + A-(aq)HA(aq) H⇌ +(aq) + A-(aq)

Ka ↑ acid strength ↑For polyprotic acids: Ka1 >> Ka2 >> Ka3

pKa = −log[Ka] pKa ↑ acid strength↓

Weak Acids & Acid-dissociation Constant

][

]][[

][

]][[ 3

HA

AH

HA

AOHKa

From p. 682 + more in Appendix D, p. 1115-1116

Weak Bases & Base-dissociation Constant

Weak bases establish an equilibrium in aqueous solution.

B(aq) + H2O(l) ⇌ BH+(aq) + OH-(aq)

][

]][[

B

OHBHKb

From p. 691 + more in Appendix D, p. 1115-1116

% Dissociation (or ionization)

% dissociation decreases as concentration increases (p. 686)

%100][

%100%

0

xHA

x

xionconcentratinitial

ddissociateamountondissociati

Weak acid/base Problems1) Ka (or Kb) from equilibrium pH2) pH from Ka (or Kb)

1. Identify as weak acid or base.

2. Write the chemical equilibrium.

3. Write the equilibrium constant expression.

4. Set up concentration table. (Ch. 15.5)

5. Solve for x.

6. Check with 5% rule. If greater than 5%, use quadratic equation. (type 2 only)

7. Complete problem.

The pH of a 0.10 M solution of propanoic acid (CH3CH2CO2H) is 2.94. Calculate the Ka for propanoic acid.

Calculate the pH of a 1.0 M HF solution.

Calculate the pH of a 0.0010 M HF solution.

Calculate the pH of a 0.20 M solution of triethylamine N(CH2CH3)3.

16.8 Relationship between Ka and Kb

For a conjugate acid/base pair:

Ka x Kb = Kw (derivation p. 693)

pKa + pKb = pKw = 14.00

16.9 Acid-Base Properties of Salt Solutions

Salt – ionic compound Salts dissolve in water to produce ions. Ions can also affect the pH. Hydrolysis – reaction between an ion

and water to produce H3O+ or OH-

F-(aq) + H2O(l) ⇌ HF(aq) + OH-(aq)

NH4+(aq) + H2O(l) H⇌ 3O+(aq) + NH3(aq)

Which ions will undergo hydrolysis, i.e. react with water and affect the pH of the solution?

Anion: Conjugate base of a weak acid ► basic Conjugate base of a monoprotic strong

acid ► neutral Cation:

Conjugate acid of a weak base ► acidic Group I & II metal ions ► neutral

(exceptions Be2+ and Mg2+ ► acidic) Other metal ions ► acidic

Cation + Anion ►Acidic, basic, or neutral?

16.10 Acid-Base Behavior and Chemical Structure

Binary Acids (HX) As bond strength increases, acid strength

decreases.

Group: size of X ↑ acid strength ↓ Period: electronegativity of X ↑ acid strength↑

Oxyacids – acidic H attached to an oxygen atom Same # of OH groups and O atoms: central

atom electronegativity ↑ acid strength ↑

HClO > HBrO > HIO

Same central atom, Y: # O atoms ↑ acid strength ↑

HClO4 > HClO3 > HClO2 > HClO

Carboxylic acids – contain −COOH or CO2H # electronegative atoms ↑ acid strength ↑

Oxyacids

16.11 Lewis Acids and Bases

Lewis acid – electron-pair acceptor e--poor compounds Metal ions

Lewis base – electron-pair donor Amines, NR3

Ligands (see chapter 24.1)

Every Brønsted base is a Lewis base, but not vice versa.

Lewis acid & base examples