CHAPTER 19

ACIDS, BASES, A

ND

SALTS

19.1 ACID-BASE THEORIES

ACIDS AND BASESAcids:Taste sourElectrolytes Change indicator

colorReact with metals →

H2 gas

Bases:Taste bitterElectrolytes Change indicator

color

ARRHENIUS ACIDS AND BASESAcids: hydrogen-containing compounds that ionize to

yield hydrogen ions (H+) in aqueous solutionBases: ionize to yield hydroxide ions (OH-) in aqueous

solution

Monoprotic acid: 1 H+ i.e. HNO3, HCl,Diprotic acid: 2 H+ i.e. H2SO4, H2CO3Triprotic acid: 3 H+ i.e. H3PO4

In water, H+ joins water molecules to form hydronium ions (H3O+)

BRØNSTED-LOWRY ACIDS AND BASESAcid: Hydrogen donorBase: Hydrogen acceptor (broader than Arrhenius’

definition) includes bases such as ammonia (NH3) and sodium

carbonate (Na2CO3)

Water can be an acid (donate H+) or base (accept H+)

BRØNSTED-LOWRY ACIDS AND BASESConjugate acid: formed when the base gains a hydrogen

ionConjugate base: remains after an acid loses a hydrogen

ion

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

base acid conj. acid conj. baseConjugate acid-base pair: two molecules related by the

gain/loss of one hydrogen ion

HCl + H2O ⇌ H3O+ + Cl-

acid base conj. acid conj. base

AMPHOTERIC SUBSTANCES

Can act as either an acid or a basei.e. H2O, HCO3

-, or HSO4-

LEWIS ACIDS AND BASESAcid: accepts a pair of electrons during a reactionBase: donate a pair of electrons during a reaction

19.2 HYDROGEN IONS AND

ACIDITY

HYDROGEN IONS FROM WATER

Self-ionization of water

Neutral solution: [H+] = [OH-]Water is neutral with 1 x 10-7M H+ and 1 x 10-7M OH-

HYDROGEN IONS FROM WATERAcidic solution: [H+] > [OH-] [H+] > 1 x 10-7M

Basic solution: [H+] < [OH-] [H+] < 1 x 10-7M

HYDROGEN IONS FROM WATER

For aqueous solutions[H+] x [OH-] = 1.0 x 10-14 M

pH is the negative logarithm of the hydrogen-ion concentration pH= –log[H+]

pH <7.0 is acidicpH > 7.0 is basic

THE pH CONCEPT

• Acid-Base Indicators• Used for initial measurement, small samples• Indicator is a weak acid or base that dissociates in a

known pH range and changes color• pH range may change with temperature• Colored substances may be difficult to read

MEASURING pH

• pH Meters• Accurate within 0.01 pH units• Used in hospitals and waste treatment• Results not affected by color or cloudiness of the

sample

MEASURING pH

19.3 STRENGTHS OF ACIDS

AND BASES

STRONG AND WEAK ACIDS AND BASESClassified by the degree to which they ionize in waterStrong acids/bases: ionize completely in waterWeak acids/bases ionize only slightly in waterAcid dissociation constant (Ka)Base dissociation constant (Kb)

 

WEAK ACID/BASE DISSOCIATION CONSTANTS

WEAK ACID/BASE DISSOCIATION CONSTANTS

 

19.4 NEUTRALIZATION

REACTIONS

ACID-BASE REACTIONSAcids and bases react to form a salt and water

This is called a neutralization reaction

TITRATIONTitration: process of adding a measured amount of a

solution of known concentration to a solution of unknown concentration1. Measured volume of unknown concentration acid is

added to a flask.2. Add several drops of indicator to flask, swirl.3. Measured volumes of base of known concentration

are mixed in until indicator just barely changes color.

TITRATIONNeutralization occurs when moles H+ = moles OH-

This is called the equivalence pointIndicator chosen for titration must change color at

or near the equivalence pointEnd point: point at which the indicator has changed

color

19.5 SALTS IN SOLUTION

Salt Hydrolysis

Ion of a salt in solution either donates or removes a hydrogen ion from water

Acidic salts: cation releases H+ to waterBasic salts: anion removes H+ from water

BUFFERSBuffer: a solution in which the pH remains fairly

constant with addition of some acid or basemade of a weak acid and one of its salts or a weak

base and one of its salts

Buffer Capacity

Amount of acid or base that can be added to a buffer system without changing pH significant

The more weak acid/base and salt in solution, the greater the buffer capacity