Acids and Bases: Introduction

Section 19.1

Objectives

Identify the physical and chemical properties of acids and bases

Classify solutions as acidic, basic, or neutral Compare the Arrhenius and Bronsted-

Lowry models of acids and bases

Key Terms

Acidic solutions Basic solutions Arrhenius model Bronsted-Lowry model Conjugate acid Conjugate base Conjugate acid-base pair amphoteric

Properties of Acids and Bases

Acids Bases

Tastes sour Tastes bitter

Turns litmus red Turns litmus blue

React with base to form salt & water

React with acid to form salt & water

Electrolyte Electrolyte

React with metal to form hydrogen gas

Feels slippery

Self-Ionization Water molecules can react to form hydronium

(H3O+)and hydroxide ions(OH-) H2O + H2O ↔ H3O+ + OH-

H2O ↔ H+ + OH-

The relative amounts of hydronium ions (also referred to as hydrogen ions, H+) and hydroxide ions determine whether a solution is acid, base, or neutral.

Determining acid, base, neutral

If hydrogen ions (H+)and hydroxide ions (OH-) are equal in concentration, the solution is neutral

If there are more hydrogen ions (H+) than hydroxide ions (OH-), the solution is acidic

If there are more hydroxide ions (OH-), than hydrogen (H+) ions the solution is basic

Arrhenius model of acid

Arrhenius acid = a substance that contains hydrogen and ionizes to produce hydrogen ions in water.

Example:

• HCl (aq) H+ (aq) + Cl- (aq)

• HCl + H2O H3O+ + Cl-

Arrhenius model of base

Arrhenius base = a substance that contains a hydroxide group and dissociates to produce a hydroxide ions in aqueous solution.

Example:

• NaOH Na+ + OH-

Arrhenius

The Arrhenius acid and base model explains many acids and bases.

However, there are acids and bases that have the appropriate properties, but do not fit the Arrhenius model.

ENTER Brønsted-Lowry . . .

Brønsted-Lowry Acid

a hydrogen-ion donor Example:

HCl + H2O → H3O+ + Cl –

HCl is an acid because it donates a hydrogen ion to the water.

Of course, it was an acid according to Arrhenius too. So what is different?

Brønsted-Lowry Acid

Example:

NH3 + H2O ↔ NH4+ + OH-

Water is the acid here, because it donates a hydrogen to the ammonia.

Brønsted-Lowry Base

a hydrogen ion acceptor

NH3 + H2O ↔ NH4+ + OH-

Ammonia is the base here, because it accepts a hydrogen from the water.

Brønsted-Lowry Base

HCl + H2O H3O+ + Cl –

Water is the base here because it accepts the hydrogen ion from HCl.

Amphoteric

a substance that can act as a Brønsted-Lowry acid OR base.

Example: water as an acid

NH3 + H2O ↔ NH4+ + OH-

Example: water as a base

HCl + H2O H3O+ + Cl –

Conjugate acid-base pairs Conjugate acid = species produced when a

bases accepts a hydrogen ion from an acid Conjugate base = species that results from

an acid donating a hydrogen ion to a base NH3 + H2O ↔ NH4

+ + OH-

base acid conj a conj b More on page 599

Monoprotic and Polyprotic Acids

(all classes) Monoprotic acid = acid that produces one

hydrogen ion (HCl). Polyprotic acid = acid that produces more

than one hydrogen ion. Diprotic acid = acid that produces 2

hydrogen ions (H2SO4) Triprotic acid = acid that produces 3

hydrogen ions (H3PO4)

Strengths of Acids and Bases

Section 19.2

Objectives

Relate the strength of an acid or base to its degree of ionization

Compare the strength of a weak acid with the strength of its conjugate base and the strength of a weak base with the strength of its conjugate acid

Explain the relationship between the strengths of acids and bases and the values of their ionization constants

Key Terms

Strong acid Weak acid Acid ionization constant Strong base Weak base Base ionization constant

Strengths of Acids

Strong acid = acid that ionizes completely. Common examples:

HCl, HNO3, H2SO4

Others are listed on table 19-1

HCl H + + Cl -

HCl + H2O H3O+ + Cl –

Weak Acid

Weak acid = acid that does not ionizes completely.

Examples: acids other than those memorized as strong.

HCN ↔ H+ + CN -

HCN + H2O ↔ H3O+ + CN -

Strengths of Bases

Strong base = base that dissociates completely to the metal ion and hydroxide ion.

Examples: Group I & II hydroxides

NaOH(s) Na+ + OH-

Weak Bases

weak base = base that does NOT dissociates completely to the metal ion and hydroxide ion.

Examples: any base other than group I & II hydroxides

NH3 + H2O ↔ NH4+ + OH-

Weak vs Strong Electrolyte

Concentration

Is concentrated the same as strong?

Is dilute the same as weak?

NO Concentrated is NOT the same as strong

when you are discussing acids and bases. Dilute is NOT the same as weak when you

are discussing acids and bases.

As you just learned, strong and weak have to do with ionization or dissociation being complete or incomplete.

In an earlier chapter, you learned that concentrated and dilute have to do with the amount of water added, or molarity of a solution.

Hydrochloric acid is ALWAYS a strong acid because it ionizes completely.

However, it may be concentrated because little to no water is added.

Or- it may be dilute if a lot of water is added.

HCl can be a concentrated strong acid or a dilute strong acid.

Ammonia is ALWAYS a weak base because it doesn’t dissociate completely.

However, it may be concentrated because little to no water is added.

Or- it may be dilute if a lot of water is added.

Ammonia can be a concentrated weak base or a dilute weak base.

Acid ionization constant, Ka

Just like Keq

HCN (aq) + H2O (l) ↔ H3O+ (aq) + CN- (aq)

Ka = [H3O] [CN]

[HCN] Mathematically, a large Ka means larger

numerator more ions; so the larger the Ka , the more the acid ionizes and

the stronger it is.

Acid ionization constant, Ka

Table 19-2 page 605 Polyprotic acids have a Ka for each H

Base ionization constant, Kb

Just like Ka

CH3NH3 (aq) + H2O (l) ↔ CH3NH4 +(aq) + OH- (aq)

Kb = [CH3NH4+][OH-]

[CH3NH3]

Mathematically, a large Kb means larger numerator more ions;

so the larger the Kb , the more the base ionizes and the stronger it is.