understanding acid-base and redox chemistry through “goldilocks” diagrams william h. myers...
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Understanding Acid-Base and Redox Chemistry Through
“Goldilocks” Diagrams
William H. MyersChemistry DepartmentUniversity of Richmond
• Acid = proton (H+) donor Has to have at
least HA one proton (H+) to donate
• Base = proton (H+) acceptorHas to have a place to put
:B a proton (H+)
• Acid-Base Reaction:
HA + :B HB+ + :A-
acid 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 H3O+ + :A-
acid reference conjugate conjugate base 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+ + :A-
more or mostly this
• If H2O < :A- as a base, then HA + H2O H3O+ + :A-
more 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 H3O+ + :A-
strong more or mostly this
acid 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 HB+ + OH-
base reference conjugate conjugate base acid acid of 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+ + OH-
more or mostly this
• If :B < OH- as a base, then :B + H2O HB+ + OH-
more or mostly this
• And using the same logic – Anchor point #2
Strong bases are bases that are stronger than OH-
Thus :B + H2O HB+ + OH-
strong more or mostly this
base 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 basepairs for which neither the conjugate acid nor the conjugate base is strong
-- “weak, but not too weak”
Base strength
very strong acids
very weak bases
weakbut not
too weak bases
conjugate acids
conjugate basesAcid strength
very weak acids
very strong bases
weakbut not
too weak acids
• And this can be displayed in a Goldilocks diagram
H3O+ H2O
H2O OH-
conjugate acids
conjugate basesAcid strength
Base strength
very strong acids
very weak bases
very weak acids
very strong bases
weakbut not
too weak acids
weakbut not
too weak bases
HCl
HF
HBrHI I-
Br-
Cl-
F-
NH2-NH3
CH4 CH3-
NH4+ NH3
• Oxyacids HnXOm
• Oxyacid notation (HO)nXOm-n
• Examples:
H2SO4 -- (HO)2SO2
H3PO4 -- (HO)3PO
H2SO3 -- (HO)2SO
HNO3 -- (HO)NO2
HNO2 -- (HO)NO
H3BO3 -- (HO)3B
“free oxygens”
2 “free oxygens”
1 “free oxygen”
1 “free oxygen”
2 “free oxygens”
1 “free oxygen”
0 “free oxygens”
H3O+ H2O
H2O OH-
conjugate acids
conjugate basesAcid strength
Base strength
HCl
HF
HBrHI I-
Br-
Cl-
F-
NH2-NH3
CH4 CH3-
NH4+ NH3
H2SO4
H3PO4
all 2 “free oxygens”oxyacids
all 1 “free oxygen”oxyacids
HSO4-
H2PO4-
H3O+ H2O
H2O OH-
conjugate acids
conjugate bases Acid
strength
Basestrength
HCl
HF
HBrHI I-
Br-
Cl-
F-
NH2-NH3
CH4 CH3-
NH4+ NH3
HClO4 ClO4-
H2SO4, HNO3,
HClO3, H2SeO4
HSO4-, NO3
-,
ClO3-, HSeO4
-
H3PO4, HNO2, HClO2,H2SO3, H5IO6, HSO4
-
H2PO4-, NO2
-, ClO2-,
HSO3-, H4IO6
-, SO4
2-
H3BO3, HClO,
H2PO4-, HSO3
-
H2BO3-, ClO-
,
HPO42-, SO3
2-
HPO42- PO4
3-
CH3CO2H CH3CO2-
CH3CH2OH CH3CH2O-
pKa = 0 >>>>
pKa = 14 >>>>
<<<< pKb = 14
<<<< pKb = 0
Q1 Stronger acid:NH3 or HF?
Q2 Stronger base:NO2
- or NO3- ?
Q3 Stronger acid:H2O or HF?
Q4 Stronger base:NO2
- or ClO- ?
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?)
H3O+ H2O
acid strength
basestrength
conjugate acids
conjugate bases
H2O OH–
HCl
HF F–
Cl–
NH3
CH4
NH2–
CH3–
HBrHI
Br–I–
NH3NH4+
H2SO4, HNO3
HClO3, H2SeO4
H3PO4, HNO2, HClO2
H2SO3, H5IO6, HSO4-
H2PO4-, NO2
-, ClO2-
HSO3-, H4IO6
-, SO42-
HSO4-, NO3
-
ClO3-, HSeO4
-
H2BO3-, ClO-
HPO42-, SO3
2-
H3BO3, HClO
H2PO4-, HSO3
-
HPO42- PO4
3-
HClO4 ClO4-
CH3CO2H CH3CO2–
CH3CH2OH CH3CH2O–
pKa = 0 >>>>
pKa = 14 >>>
<<<< pKb = 14
<<<< pKb = 0
• 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 than causes oxidation of something else
Thus, a species that is reduced during a redox reaction
• Reducing agent = a species than causes reduction of something else
Thus, a species that is oxidized during a redox reaction
oxidized form + e-’s reduced formreduced form – e-’s oxidized form
Strength of oxidizing agent
very strong
very poor
Reduced form
Oxidized form
Strength of reducing agent
in between in between
very poor
very strong
Reduced form
Oxidized form
Strength of reducing
agent
Strength of reducing
agent
Strength of oxidizing
agent
Strength of oxidizing
agent
H2 H2O
H2O O2 H2 H2O
H2 H3O+
Reduced form
Oxidized form
OR
H2 H3O+
H2 H2O
Li
Mg
NaK K+
Na+
Li+
Mg2+
CuAgAu, Pt
Pb, Fe, Sn
Cu2+
Ag+
Au3+, Pt2+
Pb2+, Fe2+, Sn2+
Ca Ca2+
Zn Zn2+
Strength of oxidizing agent
very strong
very poor
Reduced form
Oxidized form
Strength of reducing agent
in between in between
very poor
*
Fe3+ would be at another place
very strong
H2 H3O+
H2 H2O
Li
Mg
NaK K+
Na+
Li+-
Mg2+
CuAgAu, Pt
Pb, Fe, Sn
Cu2+
Ag+
Au3+, Pt2+
Pb2+, Fe2+, Sn2+
Ca Ca2+
Zn Zn2+
Strength of oxidizing agent
Reduced form
Oxidized form
Strength of reducing agent Q1 Stronger reducing agent:
Zn or Cu?
Q2 Stronger oxidizing agent:Cu2+ or Mg2+ ?
Q3 Which (if any) would react in acid?Fe, Ca, Ag?
Q4 Predict products of
a) Cu2+(aq) + Zn(s)
b) Fe2+(aq) + Mg(s)
c) Pb2+(aq) + Ag(s)
d) Ca2+(aq) + H3O+