ACIDS AND BASES

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Section 18.1 Introduction to Acids and Bases

• Identify the physical and chemical properties of acids and bases.

• Classify solutions as acidic, basic, or neutral.

• Compare the Arrhenius and Brønsted-Lowry models of acids and bases.

Different models help describe the behavior of acids and bases.

Some common acids:

• Sulfuric acid, H2SO4

• Hydrochloric acid, HCl• Nitric acid, HNO3

• Carbonic acid, H2CO3

• Phosphoric acid, H3PO4

• Acetic acid, HC2H3O2

Some common bases:• Sodium hydroxide, NaOH• Calcium hydroxide, Ca(OH)2

• Potassium hydroxide, KOH• Magnesium hydroxide,Mg(OH)2 • Aluminum hydroxide, Al(OH)3

• Ammonia, NH3

The Arrhenius Model

• The Arrhenius model states that an acid is a substance that contains hydrogen and ionizes to produce hydrogen ions in aqueous solution, and a base is a substance that contains a hydroxide group and dissociates to produce a hydroxide ion in solution.

The Arrhenius Model (cont.)

• Arrhenius acids and bases

– HCl ionizes to produce H+ ions.

– HCl(g) → H+(aq) + Cl–(aq)

– NaOH dissociates to produce OH– ions.

– NaOH(s) → Na+(aq) + OH–(aq)

– Some solutions produce hydroxide ions even though they do not contain a hydroxide group.

Properties of Acids and Bases (cont.)

• The usual solvent for acids and bases is water—water produces equal numbers of hydrogen and hydroxide ions in a process called self-ionization.

H2O(l) + H2O(l) ↔ H3O+(aq) + OH–(aq)

• The hydronium ion is H3O+.

Properties of Acids and Bases (cont.)

• All water solutions contain hydrogen ions (H+) and hydroxide ions (OH–).

• An acidic solution contains more hydrogen ions than hydroxide ions.

• A basic solution contains more hydroxide ions than hydrogen ions.

The Brønsted-Lowry Model

• The Brønsted-Lowry Model of acids and bases states that an acid is a hydrogen ion donor, and a base is a hydrogen ion acceptor.

• The Brønsted-Lowry Model is a more inclusive model of acids and bases.

The Brønsted-Lowry Model (cont.)

• A conjugate acid is the species produced when a base accepts a hydrogen ion.

• A conjugate base is the species produced when an acid donates a hydrogen ion.

• A conjugate acid-base pair consists of two substances related to each other by donating and accepting a single hydrogen ion.

The Brønsted-Lowry Model (cont.)

• Hydrogen fluoride—a Brønsted-Lowry acid

– HF(aq) + H2O(l) ↔ H3O+(aq) + F–(aq)

– HF = acid, H2O = base, H3O+ = conjugate acid, F– = conjugate base

The Brønsted-Lowry Model (cont.)

• Ammonia— Brønsted-Lowry base

– NH3(aq) + H2O(l) ↔ NH4+(aq) + OH–(aq)

– NH3 = base, H2O(l) = acid, NH4+ = conjugate

acid, OH– = conjugate base

• Substances that can act as acids or bases are called amphoteric.

HCl + H2O H3O+ + Cl-1

NH3 + H2O NH4+1 + OH-

Water is amphoteric:

Proton acceptor

Proton donor

Acts as both an acid and a base

HCl + SO3-2 HSO3

-1 + Cl-

ACID CONJUGATE BASEBASE

CONJUGATE ACID

NH3 + HNO2 NO2-1 + NH4

+

ACIDCONJUGATE

BASEBASE CONJUGATE

ACID

Monoprotic and Polyprotic Acids

• An acid that can donate only one hydrogen ion is a monoprotic acid.

• Only ionizable hydrogen atoms can be donated.

Monoprotic and Polyprotic Acids (cont.)

• Acids that can donate more than one hydrogen ion are polyprotic acids.

Properties of Acids

Acids are proton (hydrogen ion, H+) donors Acids have a pH lower than 7 Acids taste sour Acids effect indicators

Blue litmus turns red Methyl orange turns red

Acids react with active metals, producing H2

Acids react with carbonates Acids neutralize bases

Acids Effect

Indicators

Blue litmus paper turns red in contact with an acid.

Methyl orange turns red with addition of an acid

Acids React with Active Metals

Acids react with active metals to form salts and hydrogen gas.

Mg + 2HCl MgCl2 + H2(g)

Zn + 2HCl ZnCl2 + H2(g)

Mg + H2SO4 MgSO4 + H2(g)

Acids React with Carbonates

2HC2H3O2 + Na2CO3

2 NaC2H3O2 + H2O + CO2

Effects of Acid Rain on Marble(calcium carbonate)

George Washington:BEFORE

George Washington:AFTER

Properties of Bases Bases are proton (hydrogen ion, H+) acceptors Bases have a pH greater than 7 Bases taste bitter Bases effect indicators

Red litmus turns blue Phenolphthalein turns purple

Solutions of bases feel slippery Bases neutralize acids

Bases Effect Indicators

Red litmus paper turns blue in contact with a base.

Phenolphthalein turns bright pink in a base.

A. A

B. B

C. C

D. D

Section 18.1 Assessment

A conjugate acid is formed when:

A. a base accepts a hydrogen ion

B. an acid accepts a hydrogen ion

C. an acid donates a hydrogen ion

D. a base donates a hydrogen ion