Understanding Acid-Base and Redox
Chemistry Through “Goldilocks”
Diagrams
William H. Myers
Chemistry Department
University of Richmond
• Acid = proton (H+) donor
Has to have at
least
one proton (H+) to donate
HA
• Base = proton (H+) acceptor
Has to have a place to put
:B
a proton (H+)
• Acid-Base Reaction:
HA + :B  HB+ + :Aacid
base
conjugate
conjugate
acid of base base of acid
• Thus, 2 acids and 2 bases – one of each on each side
• Standard Acid Reaction:
HA + H2O
acid
reference
base
 H3O+ + :Aconjugate
conjugate
acid of
base of acid
reference base
• Can think of reaction as 2 bases (H2O and :A-)
competing for the proton
• If H2O > :A- as a base, then H2O wins
• If H2O < :A- as a base, then A- wins
So:
• If H2O > :A- as a base, then HA + H2O 
H3O+ + :Amore or mostly this
• If H2O < :A- as a base, then HA + H2O  H3O+ + :Amore or mostly this
• Note the logical conclusion:
 The stronger base has the weaker conjugate acid
 The weaker base has the stronger conjugate acid
and
• Always and every time – the side that dominates “at equilibrium”
is the side with the weaker base (it lost the competition) and the
weaker acid (the conjugate acid of the stronger base that won the
competition )
• So – Anchor point #1
 Strong acids are acids that are stronger than H3O+
Thus
HA + H2O
strong
acid

H3O+ + :Amore or mostly this
in solution
• By convention:
we assume that strong acids react with water to produce
~100% H3O+ + conjugate base
• And note that the conjugate base of a strong acid will not be able
to take a proton away from H3O+ , much less H2O
• Standard Base Reaction:
:B + H2O
base
reference
acid

HB+
OH-
+
conjugate
acid of base
conjugate base
of reference acid
• Again - 2 bases compete for a proton, :B and OH• If :B > OH- as a base, then :B wins
• If :B < OH- as a base, then OH- wins
So:
• If :B > OH- as a base, then :B + H2O  HB+ + OHmore or mostly this
• If :B < OH- as a base, then :B + H2O  HB+ + OHmore or mostly this
• And using the same logic – Anchor point #2
 Strong bases are bases that are stronger than OHThus
:B + H2O
strong
base

HB+ + OHmore or mostly this
in solution
• And by convention:
we again assume that strong bases react with water to
produce ~100% OH- + conjugate acid
• Note, though that the conjugate acid of a strong base will not be
able to protonate OH- , much less H2O
• Anchor point #3
 Strong acids have very weak conjugate bases
 Strong bases have very weak conjugate acids
• Corollary
There is, then, a group of conjugate acid/conjugate base
pairs for which neither the conjugate acid nor the conjugate
base is strong
-- “weak, but not too weak”
• And this can be displayed in a Goldilocks diagram
Acid strength
conjugate
acids
conjugate
bases
very
strong
acids
very
weak
bases
weak
but not
too weak
acids
weak
but not
too weak
bases
very
weak
acids
very
strong
bases
Base strength
Acid strength
very
strong
acids
conjugate
acids
HI
HBr
HCl
IBrCl-
H3O+
H2O
HF
weak
but not
too weak
acids
conjugate
bases
very
weak
bases
Fweak
but not
too weak
bases
NH4+
NH3
H2O
OH-
NH3
NH2-
very
weak
acids
very
strong
bases
Base strength
CH4
CH3-
• Oxyacids
HnXOm
• Oxyacid notation
(HO)nXOm-n
“free oxygens”
• Examples:
H3PO4 -- (HO)3PO
2 “free
oxygens”
1 “free oxygen”
H2SO3 -- (HO)2SO
1 “free oxygen”
HNO3 -- (HO)NO2
HNO2 -- (HO)NO
2 “free
oxygens”
1 “free oxygen”
H3BO3 -- (HO)3B
0 “free oxygens”
H2SO4 -- (HO)2SO2
Acid strength
all 2 “free oxygens”
oxyacids
all 1 “free oxygen”
oxyacids
conjugate
acids
conjugate
bases
HI
HBr
HCl
H2SO4
H3O+
IBrClHSO4H2O
H3PO4
H2PO4-
HF
F-
NH4+
NH3
H2O
OH-
NH3
NH2Base strength
CH4
CH3-
conjugate
acids
Acid
strength
conjugate
bases
HClO4
pKa = 0 >>>>
H2SO4, HNO3,
HClO3, H2SeO4
H3PO4, HNO2, HClO2,
H2SO3, H5IO6, HSO4H3BO3, HClO,
H2PO4-, HSO3-
ClO4HI
HBr
HCl
IBrCl-
H3O+
H2O
HF
CH3CO2H
NH4+
FCH3CO2NH3
HPO42pKa = 14 >>>>
HSO4-, NO3-,
ClO3-, HSeO4-
<<<< pKb = 14
H2PO4-, NO2-, ClO2-,
HSO3-, H4IO6-, SO42H2BO3-, ClO-,
HPO42-, SO32-
PO43H2O
CH3CH2OH
NH3
CH4
OHCH3CH2O-
NH2-
CH3-
<<<< pKb = 0
Base
strength
conjugate
bases
conjugate
acids
ClO 4 -
HClO 4
acid
strength
pK a = 0 >>>>
H 2SO 4, HNO 3
HClO 3 , H 2SeO 4
H 3 PO 4 , HNO 2 , HClO 2
H 2 SO 3 , H 5 IO 6 , HSO 4-
HI
HBr
HCl
I–
Br–
Cl–
H3O+
H 2O
HF
CH 3CO 2H
HPO 4 2-
Q2
-
-
HSO 4 , NO 3
ClO 3 -, HSeO 4-
<<<< pK b = 14
H 2 PO 4 -, NO 2 -, ClO 2 HSO 3-, H 4IO 6 -, SO 4 2-
F–
CH 3CO 2 –
H 2 BO 3 -, ClO HPO 42-, SO 3 2-
H 3 BO 3, HClO
H 2 PO 4 -, HSO 3-
pK a = 14 >>>
Q1 Stronger acid:
NH3 or HF?
NH 4 +
NH 3
H 2O
OH –
CH 3CH 2OH
NH 3
CH 4
CH 3 –
Q3 Stronger acid:
H2O or HF?
Q4
Stronger base:
NO2- or ClO- ?
PO 43<<<< pK b = 0
CH 3CH 2 O –
NH 2 –
Stronger base:
NO2- or NO3- ?
base
strength
Q5 (more challenging)
Will HSO3- act as an acid or as a
base in water solution?
( Variation for Q5:
Will a water solution of NaHSO3
be acidic or basic?)
• Now a quick move to redox species:
 oxidation = loss of electrons (oxidation number becomes
more positive in an atom in the species)
 reduction = gain of electrons (oxidation number becomes more
negative in an atom in the species)
• Oxidizing agent = a species that causes oxidation of
something else
Thus, a species that is reduced during a
redox reaction
• Reducing agent = a species that causes reduction of
something else
Thus, a species that is oxidized during a
redox reaction
oxidized form + e-’s  reduced form
reduced form – e-’s  oxidized form
Strength of
reducing agent
very
strong
in between
very
poor
Reduced
form
Oxidized
form
very
poor
in between
very
strong
Strength of oxidizing
agent
Strength of
reducing
agent
Reduced
form
H2
Oxidized
form
Strength of
reducing
agent
H2O
Reduced
form
Oxidized
form
H2
H3O+
H2
H2O
OR
H2O
O2
Strength of
oxidizing
agent
Strength of
oxidizing
agent
Strength of
reducing agent
very
strong
Reduced
form
Oxidized
form
K+
Na+
Li+
Ca2+
H3O+
K
Na
Li
Ca
H2
Mg
Mg2+ Zn2+
Zn
in between
in between
Pb2+, Fe2+, Sn2+
Pb, Fe, Sn
H2
very
poor
very
poor
Fe3+ would be at
another place
H2O
Cu
Ag
Au, Pt
Cu2+
Ag+
Au3+, Pt2+
*
very
strong
Strength of oxidizing
agent
Strength of
reducing agent
Reduced
form
Q1 Stronger reducing agent:
Zn or Cu?
K+
Na+
Li+Ca2+
H3O+
K
Na
Li
Ca
H2
Mg
Oxidized
form
Zn
Q2
Stronger oxidizing agent:
Cu2+ or Mg2+ ?
Q3 Which (if any) would react in acid?
Fe, Ca, Ag?
Mg2+ Zn2+
Q4
Predict products of
a) Cu2+(aq) + Zn(s) 
Pb, Fe, Sn
Pb2+, Fe2+, Sn2+
b) Fe2+(aq) + Mg(s) 
c) Pb2+(aq) + Ag(s) 
H2
H2O
d) Ca2+(aq) + H3O+ 
Cu
Ag
Au, Pt
Cu2+
Ag+
Au3+, Pt2+
Strength of oxidizing
agent