Definitions of Acids and Bases Green Damjii Chapter 8 Section 1 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction.

Definitions ofAcids and Bases

Green & Damjii – Chapter 8 – Section 1

Copyright © The McGraw-Hill Companies, Inc.  Permission required for reproduction or display.

4.3

There are multiple definitions for the terms ‘acid’ and ‘base’.

You will need to be familiar with three (3) of them:• Arrhenius acids and bases• Bronsted-Lowry acids and bases• Lewis acids and bases

4.3

An Arrhenius acid is a substance that produces H+ (H3O+) in water

Complete these reactions:

• Nitric acid plus water yields

• Acetic acid plus water yields

An Arrhenius base is a substance that produces OH- in water

4.3

Complete these reactions:• sodium hydroxide plus water yields

• ammonia plus water yields

• baking soda plus water yields

Why didn’t chemists just stick with the Arrhenius definitions of acids and bases?

• not all acid base reactions occur in an aqueous solvent

A Brønsted-Lowry acid is a proton donor• H atom attached by a bond that is easily broken

A Brønsted-Lowry base is a proton acceptor• contains a non-bonding electron pair – usually the

lone pair on an O or N atom

acidbase acid base

15.1

acid conjugate basebase conjugate

acid

15.4

Conjugate acid-base pairs:• The conjugate base of a strong acid is weak (has no measurable strength).

ex: HCl (strong acid): Cl− (weak conjugate base)• The conjugate acid of a strong base is weak (has no measurable strength)..

ex: NaOH (strong base): H2O (weak conjugate acid)

• The conjugate base of a weak acid is strong.

ex: CH3COOH (weak acid): CH3COO − (strong conjugate base)

• The conjugate acid of a weak base is strong.

ex: NH3 (weak base): NH4 + (strong conjugate acid)

15.4

O

H

H + O

H

H O

H

H H OH-+[ ] +

Acid-Base Properties of Water

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

H2O + H2O H3O+ + OH-

acid conjugate base

base conjugate acid

15.2

referred to as the autoionization of water

Conjugate acid-base pairs:

• H3O+ is the strongest acid that can exist in aqueous solution.

ex: H2O (weak base): H3O+ (strong conjugate acid)

• The OH- ion is the strongest base that can exist in aqueous solution.

ex: H2O (weak acid): OH- (strong conjugate base)

NOTE: Substances – like water – that can both accept and donate a proton are called amphiprotic.

15.4

Identify the Bronsted-Lowry conjugate acid base pairs in each reaction:

CH3COOH(aq) + H2O (l) ⇌ H3O+(aq) + CH3COO−

(aq)

NH4

+ + NH2− 2 NH3 (l)

H2SO4 (aq) H+(aq) + HSO4

− (aq) ⇌ 2 H+(aq) + SO4

2− (aq)

Why didn’t chemists just stick with the Bronsted-Lowry definitions of acids and bases?

Lewis recognized the special nature of the bond between a base and the proton it accepts…

• dative bond (aka coordinate covalent bond)• both of the electrons come from the base • proton is not contributing any electrons to the bond

(it doesn’t have any!)• these bonds are indicated by an arrow rather than a line

arrow points in the direction the electrons are donated• these covalent bonds are identical to other covalent bonds

So – acid base reactions now include reactions that involve the formation of a dative covalent bond.

A Lewis acid is a substance that can accept a pair of electrons

A Lewis base is a substance that can donate a pair of electrons

H+ H O H••••

+ OH-••••

••acid base

N H••

H

H

H+ +

acid base 15.12

N H

H

H

H+

Lewis acids include all Bronsted-Lowry acids (which include all Arrhenius acids)… but the term is typically used just for acids that are not already included in the other definitions.

EXs:

• boron trifluoride (see next slide)

• aluminum chloride

• any species that can accept an electron pair into its incomplete valence shell …

Lewis Acids and Bases

N H••

H

H

acid base

F B

F

F

+ F B

F

F

N H

H

H

No protons donated or accepted – just electron pairs !

Identify the dative bond ! Mark it with an arrow!

15.12

Lewis bases are any species with a non-bonding electron pair

EXs:

• molecules that are NOT hydrides of group 3 &/or 4 elements – generally containing an element with lone pair electrons

• anions

• includes ligands that combine with transition metal ions to form complex ions (we learned about this in Unit 2)

There are several metal hydroxides that are amphoteric – • they behave as both acids and bases.

Show how zinc hydroxide and/or aluminum hydroxide

• can act as a base (what type? Lewis)

• can act as an acid (what type? Lewis )