Theories of Acids and BasesTopic 8.1

• concepts acids and bases were loosely defined as substances that change some properties of water

• criteria that was often used was taste– substances were classified

• salty-tasting • sour-tasting• sweet-tasting• bitter-tasting

• sour-tasting substances would give rise to the word 'acid', which is derived from the Greek word oxein, which mutated into the Latin verb acere, which means 'to make sour'

Interesting Background (not required)

Three Acid-Base Definitions

Arrhenius definition– form hydrogen ions (H+) in aqueous

solution

–with Arrhenius bases forming hydroxide ions (OH-) in solution

Brønsted – Lowry definition– involves the transfer of a proton (H+)• a “proton” is really just a hydrogen atom that

has lost its electron–acids: molecule or ion that acts as proton

(H+) donor–bases: molecule or ion that acts as proton

(H+) acceptor

• HCl (g) + H2O (l) ⇌ H3O+(aq) + Cl-(aq)–in the forward reaction, • HCl is the acid (will donate H+)• H2O is the base (will accept H+)

–in the reverse reaction, • H3O+ is the acid (will donate H+)• Cl- is the base (will accept H+)

acid base acid base

Conjugate Pairs• an acid-base reaction always involves (at

least) two conjugate pairs that differ by an H+

• conjugate acids and conjugate bases are compounds formed when a H+ ion is gained and a H+ is lost

• a conjugate pair is: – an acid and its conjugate base– a base an its conjugate acid

Brønsted-Lowry conjugate pairs

acid basebase acid

• Strong acid Weak conjugate base• Weak acid Strong conjugate base• Strong base Weak conjugate acid• Weak base Strong conjugate acid

Acid Strength Base Strength

H2SO4 Very Strong

Fairly Strong

Weak

Very Weak

HSO4- Very Weak

Weak

Less Weak

Fairly Strong

HCl Cl-

HNO3 NO3-

H3O+ H2O

HSO4- SO4

2-

CH3COOH CH3COO-

H2CO3 HCO3-

NH4+ NH3

HCO3- CO3

2-

H2O OH-

16

Water is amphiprotic/amphoteric (can act as acid or base)

Acid Base Conjugate Acid Conjugate Base

HCl + H2O H3O+ + Cl-

H2PO4- + H2O H3O+ + HPO4

2-

NH4+ + H2O H3O+ + :NH3

Base Acid Conjugate Acid Conjugate Base

:NH3 + H2O NH4+ + OH-

PO43- + H2O HPO4

2- + OH-

Donating protons…

• monoprotic acids contain a single proton that can be donated (HCl, HNO3, HNO2, CH3COOH)

• diprotic acids contain two protons that can be donated (H2CO3, H2SO4, H2SO3)

• triprotic acids contain three protons that can be donated (H3PO4)

• for a substance to be an acid, the hydrogen usually has to be attached to oxygen or a halogen– for example, in CH3COOH, only the H on “OH” is able to be

donated, the three hydrogens on carbon are non-acidic (do not write this C2H4O2)

Lewis Acid and Base Definitions– most general/encompassing definition• must have lone pairs (ligands) available to donate

– Lewis acids accept a pair of electrons to form a dative covalent bond

– Lewis bases donate a pair of electrons to form a dative covalent bond baseacid

LONE PAIR DONOR = Base

LONE PAIR ACCEPTOR = Acid

LONE PAIR DONOR = Base

LONE PAIR ACCEPTOR = Acid

• when ammonia donates a pair of electrons to hydrogen, its a Lewis base

• when boron trifluoride accepts a pair of electrons from nitrogen (in NH3) its a Lewis acid

N

H

H

H

N+

H

H

H

HH+

N

H

H

H

N+

H

B-

H

H

F

F

F

B

F

F

F

+

+