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 HAone proton (H + ) to donate Base = proton (H + ) acceptor Has to have a place to put :Ba 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 + H 2 O  H 3 O + + :A - acid reference conjugate conjugate base acid of base of acid reference base Can think of reaction as 2 bases (H 2 O and :A - ) competing for the proton If H 2 O > :A - as a base, then H 2 O wins If H 2 O < :A - as a base, then A - wins So: If H 2 O > :A - as a base, then HA + H 2 O  H 3 O + + :A - more or mostly this If H 2 O < :A - as a base, then HA + H 2 O  H 3 O + + :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 H 3 O + Thus HA + H 2 O  H 3 O + + :A - strong more or mostly this acidin solution By convention: we assume that strong acids react with water to produce ~100% H 3 O + + conjugate base And note that the conjugate base of a strong acid will not be able to take a proton away from H 3 O +, much less H 2 O

Standard Base Reaction: :B + H 2 O  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 + H 2 O  HB + + OH - more or mostly this If :B < OH - as a base, then :B + H 2 O  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 + H 2 O  HB + + OH - strong more or mostly this basein 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 H 2 O

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”

Base strength very strong acids very weak bases weak but not too weak bases conjugate acids conjugate bases Acid strength very weak acids very strong bases weak but not too weak acids And this can be displayed in a Goldilocks diagram

H3O+H3O+ H2OH2O H2OH2OOH - conjugate acids conjugate bases Acid strength Base strength very strong acids very weak bases very weak acids very strong bases weak but not too weak acids weak but not too weak bases HCl HF HBr HII-I- Br - Cl - F-F- NH 2 - NH 3 CH 4 CH 3 - NH 4 + NH 3

Oxyacids H n XO m Oxyacid notation (HO) n XO m-n Examples: H 2 SO 4 -- (HO) 2 SO 2 H 3 PO 4 -- (HO) 3 PO H 2 SO 3 -- (HO) 2 SO HNO 3 -- (HO)NO 2 HNO 2 -- (HO)NO H 3 BO 3 -- (HO) 3 B “free oxygens” 2 “free oxygens” 1 “free oxygen” 2 “free oxygens” 1 “free oxygen” 0 “free oxygens”

H3O+H3O+ H2OH2O H2OH2OOH - conjugate acids conjugate bases Acid strength Base strength HCl HF HBr HII-I- Br - Cl - F-F- NH 2 - NH 3 CH 4 CH 3 - NH 4 + NH 3 H 2 SO 4 H 3 PO 4 all 2 “free oxygens” oxyacids all 1 “free oxygen” oxyacids HSO 4 - H 2 PO 4 -

H3O+H3O+ H2OH2O H2OH2OOH - conjugate acids conjugate bases Acid strength Base strength HCl HF HBr HII-I- Br - Cl - F-F- NH 2 - NH 3 CH 4 CH 3 - NH 4 + NH 3 HClO 4 ClO 4 - H 2 SO 4, HNO 3, HClO 3, H 2 SeO 4 HSO 4 -, NO 3 -, ClO 3 -, HSeO 4 - H 3 PO 4, HNO 2, HClO 2, H 2 SO 3, H 5 IO 6, HSO 4 - H 2 PO 4 -, NO 2 -, ClO 2 -, HSO 3 -, H 4 IO 6 -, SO 4 2- H 3 BO 3, HClO, H 2 PO 4 -, HSO 3 - H 2 BO 3 -, ClO -, HPO 4 2-, SO 3 2- HPO 4 2- PO 4 3- CH 3 CO 2 HCH 3 CO 2 - CH 3 CH 2 OHCH 3 CH 2 O - pK a = 0 >>>> pK a = 14 >>>> <<<< pK b = 14 <<<< pK b = 0

Q1 Stronger acid: NH 3 or HF? Q2 Stronger base: NO 2 - or NO 3 - ? Q3 Stronger acid: H 2 O or HF? Q4 Stronger base: NO 2 - or ClO - ? Q5 (more challenging) Will HSO 3 - act as an acid or as a base in water solution? ( Variation for Q5: Will a water solution of NaHSO 3 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 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 form reduced form – e - ’s  oxidized form

Strength of oxidizing agent very strong very poor Reduced form Oxidized form Strength of reducing agent 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 H2H2 H2OH2O H2OH2OO2O2 H2H2 H2OH2O H2H2 H3O+H3O+ Reduced form Oxidized form OR

H2H2 H3O+H3O+ H2H2 H2OH2O Li Mg Na KK+K+ Na + Li + Mg 2+ Cu Ag Au, Pt Pb, Fe, Sn Cu 2+ Ag + Au 3+, Pt 2+ Pb 2+, Fe 2+, Sn 2+ CaCa 2+ ZnZn 2+ Strength of oxidizing agent very strong very poor Reduced form Oxidized form Strength of reducing agent in between very poor * Fe 3+ would be at another place very strong

H2H2 H3O+H3O+ H2H2 H2OH2O Li Mg Na KK+K+ Na + Li +- Mg 2+ Cu Ag Au, Pt Pb, Fe, Sn Cu 2+ Ag + Au 3+, Pt 2+ Pb 2+, Fe 2+, Sn 2+ CaCa 2+ Zn Zn 2+ Strength of oxidizing agent Reduced form Oxidized form Strength of reducing agent Q1 Stronger reducing agent: Zn or Cu? Q2 Stronger oxidizing agent: Cu 2+ or Mg 2+ ? Q3 Which (if any) would react in acid? Fe, Ca, Ag? Q4 Predict products of a) Cu 2+ (aq) + Zn (s)  b) Fe 2+ (aq) + Mg (s)  c) Pb 2+ (aq) + Ag (s)  d) Ca 2+ (aq) + H 3 O + 