Chapter 10Acids and Bases

Review Chapter 6

Defined an Arrhenius acid and base Defined a Bronsted-Lowry acid and base

The Brønsted-Lowry Concept of Acids and Bases Brønsted-Lowry definitions

Acids are proton (H+) donors.

The Bronsted-Lowry Definition of Acid and Bases

A Brønsted–Lowry base is a proton (H+) acceptor.

The Brønsted-Lowry Definition of Acids and Bases Putting the acid and base definitions together, an acid–base

reaction is one in which a proton is transferred. The reaction need not occur in water.

Conjugate Acid-Base Pairs Result from the transfer of a proton The acid reactant loses the proton to produce a conjugate

base product The base reactant gains the proton to produce a conjugate

acid product Hence a conjugate acid-base pair are two chemicals (one

reactant and one product) that differ by a hydrogen ion

Base Acid+Acid Base+

Conjugate Pair

Conjugate Pair

Reaction 4 H2PO4- OH-+

Reaction 5 H2SO4 N2H5++

Reaction 6 HPO42- SO3

2-+

Reaction 1 HF H2O+ F- H3O++

Reaction 3 NH4+ CO3

2-+

Reaction 2 HCOOH CN-+ HCOO- HCN+

NH3 HCO3-+

HPO42- H2O+

HSO4- N2H6

2++

PO43- HSO3

-+

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Acid and Base Strength Also introduced in Chapter 6 and discussed Chapter

7 and 9 – Acid/Base strength The strength of an acid/base, described by the

equilibrium constant, describes the degree to which the compound dissociates into its ions Strong acids and bases dissociate entirely in water – large

equilibrium constants Weak acids only partially ionize and therefore an

equilibrium is established – small equilibrium constants

Strong acid: HA(g or l) + H2O(l) H3O+(aq) + A-(aq)

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Acid and Base Strength

Weak acid: HA(aq) + H2O(l) H3O+(aq) + A-(aq)

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Acid and Base Strength

Acid Dissociation Contants Ka – indicates the degree to which an acid will

ionize Stronger acid – Ka >> 1 meaning complete

dissociation Weaker acid – Ka < 1

hence reactant favored

Acids

Strong

hydrochloric acid, HCl

hydrobromic acid, HBr

hydroiodic acid, HI

nitric acid, HNO3

sulfuric acid, H2SO4

perchloric acid, HClO4

Weak

Begin with H or contains a carboxylic acid group, -COOH

Bases

Strong

Weak

sodium hydroxide, NaOH

calcium hydroxide, Ca(OH)2

potassium hydroxide, KOH

strontium hydroxide, Sr(OH)2

barium hydroxide, Ba(OH)2

lithium hydroxide, LiOH

Amine group - NHx

Acid and Base Strength

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Acid and Base Strength An acid base reaction always moves toward

the weaker acid and the weaker base

Conjugate pairs the weaker the acid, the stronger its conjugate

base

the weaker the base, the stronger its conjugate base

Acid Dissociation Constants Monoprotic acid – acids that have but one

proton to give up – HCl, HF, HNO3, CH3COOH

Polyprotic acids – acids that can give up more than one proton – H2SO4, H3PO4, oxalic acid Ionization occurs in a step wise manner

Polyprotic acids

acids with more than more ionizable proton

H3PO4(aq) + H2O(l) H2PO4-(aq) + H3O+(aq)

H2PO4-(aq) + H2O(l) HPO4

2-(aq) + H3O+(aq)

HPO42-(aq) + H2O(l) PO4

3-(aq) + H3O+(aq)

Ka1 =[H3O+][H2PO4

-]

[H3PO4]

Ka2 =[H3O+][HPO4

2-]

[H2PO4-]

Ka3 =[H3O+][PO4

3-]

[HPO42-]

Ka1 > Ka2 > Ka3

= 7.2x10-3

= 6.3x10-8

= 4.2x10-13

Acid Dissociation Constants

AC

ID S

TR

EN

GT

H

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Water as Both an Acid and a Base Acid reaction with water

HA (aq) + H2O (l) → H3O+ (aq) + A- (aq) H2O acts as a Brønsted-Lowry base

Base reaction with water B: (aq) + H2O (l) → BH+ (aq) + OH- (aq) H2O acts as a Brønsted-Lowry acid

Water is amphoteric An amphoteric species is a substance that can react as an acid or a

base

The Auto-ionization of Water Highly purified water

auto-ionization of water Conduct a small amount of electricity

H2O(l) H2O(l)

H3O+(aq) OH-(aq)

+

[H3O+][OH-]

Ion Product Constant for Water

Kw =

A change in [H3O+] causes an inverse change in [OH-].

= 1.0 x 10-14 at 250C

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

In an acidic solution, [H3O+] > [OH-]

In a basic solution, [H3O+] < [OH-]

In a neutral solution, [H3O+] = [OH-]

[H3O+] [OH-]Divide into Kw

ACIDIC SOLUTION

BASIC SOLUTION

[H3O+] > [OH-] [H3O+] = [OH-] [H3O+] < [OH-]

NEUTRAL SOLUTION

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Measuring Acidity in Aqueous Solution: pH pH scale allows the

expression of H3O+ concentration in less bulky numbers

pH is a number usually between 0 and 14 that indicates the hydronium concentration

Mathematically determined

pH = -log [H3O+]

10 -pH = [H3O+]

Laboratory Determination of Acidity Measuring pH

pH meter Indicator Indicator paper strips

Buffer Solutions Buffer: A combination of substances that act

together to prevent a drastic change in pH; usually a weak acid and its conjugate base.

Rearranging the Ka equation shows that the value of [H3O+] depends on the ratio [HA]/[A-].

[H3O+] = Ka [HA]/[A-] Most H3O+ added is removed by reaction with A- ,so

[HA] increases and [A-] decreases. As long as these changes are small, the ratio [HA]/[A-] changes only slightly, and there is little change in the pH.

When 0.010 mol of acid and 0.010 mol of base are added to 1.0 L of pure water and to 1.0 L of a 0.10 M acetic acid–0.10 M acetate ion buffer, the pH of the water varies between 12 and 2, while the pH of the buffer varies only between 4.85 and 4.68.

Optional Homework Text – 10.32, 10.33, 10.34, 10.44, 10.46,

10.48, 10.50, 10.58, 10.60, 10.62, 10.64, 10.66, 10.68, 10.70, 10.72, 10.78, 10.80, 10.86, 10.94, 10.100, 10.106, 10.108, 10.110

Chapter 10 Homework online