Theories of Acids and Bases
General Characteristics
• Acids
– normally C, O, F, Cl and especially H covalently
bonded
• H2SO4, HCl, H2NO3
– they dissociate/ionize (break apart) to form at
least one cation (H+) and an anion
• Bases
– many contain a metal and a hydroxide ion (OH-)
• NaOH, Mg(OH)2, NH3
– many produce OH- when dissolved in water
Background
• 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
• it was once thought that oxygen was responsible
for a compounds acid properties
– it is now know that it is not
https://www.youtube.com/watch?v=_U1kPbvj8Q
Three (only need to know first
two for SL) Acid-Base Definitions
Nobel Prize in Chemistry 1903
Arrhenius definition
– too narrow (but works for most acids and
bases) because it had to include water
• left out some acids and bases
– acids ionize in water to form (H+) (or H30+
called hydronium ions)
– bases form hydroxide ions (OH-) in water
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
acid
base
acid
base
• 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+)
Conjugate (joined together) 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 and its conjugate acid
Brønsted-Lowry conjugate pairs
base
acid
acid
base
• Strong acid  Weak conjugate base
– ALL of the acid donates H+
– almost NO H+ is accepted back
• Weak acid  Strong conjugate base
• Strong base  Weak conjugate acid
• Weak base  Strong conjugate acid
Acid
Strength
Base
Strength
H2SO4
Very Strong
HSO4-
Very Weak
HCl
Cl-
HNO3
NO3-
H3O+
Fairly Strong
H2 O
HSO4-
SO42-
CH3COOH
CH3COO-
H2CO3
Weak
NH4+
HCO3H2 O
HCO3-
Weak
Less Weak
NH3
Very Weak
CO32OH-
Fairly Strong
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)
Amphiprotic substances can act as an acid or a
base (amphoteric substances are slightly different when
considering the Lewis definition in topic 17)
Acid
Conjugate Acid
Base
+
H2O

H2PO4- +
H2O

NH4+
H2O

HCl
+
Base
Conjugate Base

H3O+
+
Cl-

H3O+
+
HPO42-

H3O+
+
:NH3
Conjugate Acid
Acid
:NH3
+
H2O

PO43-
+
H2O

Conjugate Base

NH4+
+
OH-

HPO42- +
OH18