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+