Acid/Base Equilibria

Chapter 16

Models of Acids and Bases

Arrhenius Concept: Acids produce H+ in solution, bases produce OH ion.

Brønsted-Lowry: Acids are H+ donors, bases are proton acceptors.

HCl + H2O Cl + H3O+

acid base

A Brønsted–Lowry acid…

…must have a removable (acidic) proton.

A Brønsted–Lowry base…

…must have a pair of nonbonding electrons.

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What Happens When an Acid Dissolves in Water?

Water acts as a Brønsted–Lowry base and abstracts a proton (H+) from the acid.

As a result, the conjugate base of the acid and a hydronium ion are formed.

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Conjugate Acid/Base Pairs

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

acid 1 base 2 acid 2 base 1

conjugate base: everything that remains of the acid molecule after a proton is lost.

conjugate acid: formed when the proton is transferred to the base.

Acid Dissociation Constant (Ka)

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

Ka3H O A

HA

H A

HA

Acid Strength

- Its equilibrium position lies far to the right. (HNO3)

- Yields a weak conjugate base. (NO3)

Strong Acid:

Acid and Base Strength

Strong acids are completely dissociated in water. Their conjugate bases are

quite weak.

Weak acids only dissociate partially in water. Their conjugate bases are

weak bases.

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Strong Acid - Weak Acid

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HA

(a)

H+ A–

HA

(b)

H+ A–

Before dissociationAfter dissociation,at equilibrium

HA

Strong AcidsYou will recall that the seven strong acids

are HCl, HBr, HI, HNO3, H2SO4, HClO3, and HClO4.

These are, by definition, strong electrolytes and exist totally as ions in aqueous solution.

For the monoprotic strong acids,

[H3O+] = [acid].

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Strong AcidsHCl – Hydrochloric Acid

HBr – Hydrobromic Acid

HI – Hydroiodic Acid

HNO3 - Nitric Acid

H2SO4 – Sulfuric Acid

HClO3 – Chloric Acid

HClO4 – Perchloric Acid

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Acid Strength(continued)

- Its equilibrium lies far to the left. (CH3COOH)

- Yields a much stronger (it is relatively strong) conjugate base than water. (CH3COO)

Weak Acid:

Strong Acid Weak Acid14_1577

(a) (b)

H+H+

H+

H+

H+

H+

H+H+

H+

H+

H+

H+

H+

A-

A-

A-

A-

A-

A-

A-

A- A-

A-

A-

A- B-

HBHB

HBHB

HB HB

HB

HBHBHB

HB

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Relativeacid strength

Relativeconjugatebase strength

Veryweak

Weak

Strong

Verystrong

Veryweak

Weak

Strong

Verystrong

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Table 14.2 Values of Ka for Some Common Monoprotic Acids

Formula Name Value of Ka*

HSO4 Hydrogen sulfate ion 1.2 x 10 2

HClO2 Chlorous acid 1.2 x 10 2

HC2H2ClO2 Monochloracetic acid 1.35 x 10 3

HF Hydrofluoric acid 7.2 x 10 4

HNO2 Nitrous acid 4.0 x 10 4

HC2H3O2 Acetic acid 1.8 x 10 5

[Al(H2O)6]3+ Hydrated aluminum(III) ion 1.4 x 10 5

HOCl Hypochlorous acid 3.5 x 10 8

HCN Hydrocyanic acid 6.2 x 10 10

NH4 Ammonium ion 5.6 x 10 10

HOC6H5 Phenol 1.6 x 10 10

Incr

ea

sin

g a

cid

str

en

gth

*The units of Ka are mol/L but are customarily omitted.

Water as an Acid and a Base

Water is amphoteric (it can behave either as an acid or a base).

H2O + H2O H3O+ + OH

conj conj acid 1 base 2 acid 2 base 1

Kw = 1 1014 at 25°C

Autoionization of WaterAs we have seen, water is amphoteric.

In pure water, a few molecules act as bases and a few act as acids.

This is referred to as autoionization.

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Ion Product Constant

The equilibrium expression for this process is

Kc = [H3O+] [OH]

This special equilibrium constant is referred to as the ion product constant for water, Kw.

At 25C, Kw = 1.0 1014

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The pH Scale

pH log[H+]

pH in water ranges from 0 to 14.

Kw = 1.00 1014 = [H+] [OH]

pKw = 14.00 = pH + pOH

As pH rises, pOH falls (sum = 14.00).

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[H+] pH

10–14 14

10–13 13

10–12 12

10–11 11

10–10 10

10–9 9

10–8 8

10–6 6

10–5 5

10–4 4

10–3 3

10–2 2

10–1 1

1 0

Acidic

Neutral

Basic

1 M NaOH

Ammonia (Householdcleaner)

BloodPure water

Milk

VinegarLemon juice

Stomach acid

1 M HCl

10–7 7

How Do We Measure pH?

For less accurate measurements, one can use

Litmus paper

“Red” paper turns blue above ~pH = 8

“Blue” paper turns red below ~pH = 5

Or an indicator.

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How Do We Measure pH?

For more accurate measurements, one uses a pH meter, which measures the voltage in the solution.

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Solving Weak Acid Equilibrium Problems

- List major species in solution.

- Choose species that can produce H+ and write reactions.

- Based on K values, decide on dominant equilibrium.

- Write equilibrium expression for dominant equilibrium.

- List initial concentrations in dominant equilibrium.

Solving Weak Acid Equilibrium Problems (continued)

- Define change at equilibrium (as “x”).

- Write equilibrium concentrations in terms of x.

- Substitute equilibrium concentrations into equilibrium expression.

- Solve for x the “easy way.”

- Verify assumptions using 5% rule.

- Calculate [H+] and pH.

Percent Dissociation (Ionization)

% dissociationamount dissociated( )initial concentration( )

100% MM

Polyprotic Acids

. . . can furnish more than one proton (H+) to the solution.

H CO H HCO ( )

HCO H CO ( )

2 3 3 a

3 32

a

1

2

K

K

Acids and Bases

Bases

“Strong” and “weak” are used in the same sense for bases as for acids.

strong = complete dissociation (hydroxide ion supplied to solution)

NaOH(s) Na+(aq) + OH(aq)

Strong BasesLiOH

Lithium Hydroxide

NaOH Sodium

Hydroxide

KOH Potassium Hydroxide

RbOH Rubidium Hydroxide

CsOH Cesium

Hydroxide

Ba(OH)2 Barium

Hydroxide

Sr(OH)2 Strontium Hydroxide

Ca(OH)2 Calcium

Hydroxide

Bases(continued)

weak = very little dissociation (or reaction with water)

H3CNH2(aq) + H2O(l) H3CNH3+(aq) + OH(aq)

Acid-Base Properties of Salts

Cation AnionAcidic

or Basic Exampleneutral neutral neutral NaCl

neutral conj base ofweak acid

basic NaF

conj acid ofweak base

neutral acidic NH4Cl

conj acid ofweak base

conj base ofweak acid

depends onKa & Kb

values

Al2(SO4)3

Problem

An unknown salt is either NaF, NaCl or NaOCL. When 0.050 moles of the salt is dissolved in 0.500 L of solution, the pH of the solution is 8.08. What is the identity of the salt? Show Calculations.

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Table 14.2 Values of Ka for Some Common Monoprotic Acids

Formula Name Value of Ka*

HSO4 Hydrogen sulfate ion 1.2 x 10 2

HClO2 Chlorous acid 1.2 x 10 2

HC2H2ClO2 Monochloracetic acid 1.35 x 10 3

HF Hydrofluoric acid 7.2 x 10 4

HNO2 Nitrous acid 4.0 x 10 4

HC2H3O2 Acetic acid 1.8 x 10 5

[Al(H2O)6]3+ Hydrated aluminum(III) ion 1.4 x 10 5

HOCl Hypochlorous acid 3.5 x 10 8

HCN Hydrocyanic acid 6.2 x 10 10

NH4 Ammonium ion 5.6 x 10 10

HOC6H5 Phenol 1.6 x 10 10

Incr

easi

ng a

cid

stre

ngth

*The units of Ka are mol/L but are customarily omitted.

Problem

Sorbic acid (C5H7COOH) is a weak monoprotic acid with Ka = 1.7 x10-5. Its salt (potassium sorbate) is added to cheese to inhibit the formation of mold.

What is the pH of a solution containing 9.50 g of potassium sorbate in 2.00 L of solution?

Problem – 16.90

Trisodium phosphate (Na3PO4) is available in hardware stores as TSP and is used as a cleaning agent. The label on a box of TSP warns that the substance is very basic (caustic or alkaline). What is the pH of a solution containing 35.0 g of TSP in a liter of solution?

Structure and Acid-Base Properties

Two factors for acidity in binary compounds:

- Bond Polarity (high is good)

- Bond Strength (low is good)

14_326 Cl O H

Electron density

Cl O H

Electron density

O

Cl O H

Electron density

O

O

Cl O H

Electron density

O

OO

Oxides

Acidic Oxides (Acid Anhydrides):

- OX bond is strong and covalent.

SO2, NO2, CrO3

Basic Oxides (Basic Anhydrides):

- OX bond is ionic.

K2O, CaO

Lewis Acids and Bases

Lewis Acid: electron pair acceptor

Lewis Base: electron pair donor

Al3+ + 6H

OH

Al OH

H6

3+

Problem 16.86

Predict whether aqueous solutions of the following are acidic, basic, or neutral:

a. CrBr3

b. LiCl

c. K3PO4

d. (CH3NH3)Cl

e. KHSO4

END