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Understanding Acid-Base and Redox Chemistry Through “ Goldilocks ” Diagrams

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 HA one proton (H + ) to donate. Base = proton (H + ) acceptor

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Understanding Acid-Base and Redox Chemistry Through “ Goldilocks ” Diagrams

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  1. Understanding Acid-Base and Redox Chemistry Through “Goldilocks” Diagrams William H. Myers Chemistry Department University of Richmond

  2. Acid = proton (H+) donor • Has to have at least • HA one proton (H+) to donate • Base = proton (H+) acceptor • Has 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

  3. 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

  4. 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 )

  5. 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

  6. 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

  7. 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

  8. 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”

  9. And this can be displayed in a Goldilocks diagram conjugate acids conjugate bases Acid strength 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

  10. conjugate acids conjugate bases Acid strength very weak bases HI I- very strong acids HBr Br- HCl Cl- H3O+ H2O HF F- weak but not too weak bases weak but not too weak acids NH4+ NH3 H2O OH- very strong bases very weak acids NH3 NH2- Base strength CH4 CH3-

  11. Oxyacids HnXOm • Oxyacid notation (HO)nXOm-n “free oxygens” • Examples: • H2SO4 -- (HO)2SO2 • H3PO4 -- (HO)3PO • H2SO3 -- (HO)2SO • HNO3 -- (HO)NO2 • HNO2 -- (HO)NO • H3BO3 -- (HO)3B 2 “free oxygens” 1 “free oxygen” 1 “free oxygen” 2 “free oxygens” 1 “free oxygen” 0 “free oxygens”

  12. conjugate acids conjugate bases Acid strength HI I- HBr Br- all 2 “free oxygens” oxyacids HCl Cl- H2SO4 HSO4- H3O+ H2O all 1 “free oxygen” oxyacids H3PO4 H2PO4- HF F- NH4+ NH3 H2O OH- NH3 NH2- Base strength CH4 CH3-

  13. conjugate acids conjugate bases Acid strength HClO4 ClO4- HI I- HBr Br- HCl Cl- H2SO4,HNO3, HClO3, H2SeO4 HSO4-, NO3-, ClO3-, HSeO4- pKa = 0 >>>> H3O+ H2O <<<< pKb = 14 H3PO4, HNO2, HClO2, H2SO3, H5IO6, HSO4- H2PO4-, NO2-, ClO2-, HSO3-, H4IO6-, SO42- HF F- CH3CO2H CH3CO2- H3BO3, HClO, H2PO4-, HSO3- H2BO3-, ClO-, HPO42-, SO32- NH4+ NH3 HPO42- PO43- pKa = 14 >>>> H2O OH- <<<< pKb = 0 CH3CH2OH CH3CH2O- Base strength NH3 NH2- CH4 CH3-

  14. 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?)

  15. 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

  16. Strength of reducing agent Reduced form Oxidized form very poor very strong in between in between very strong very poor Strength of oxidizing agent

  17. Strength of reducing agent Strength of reducing agent Reduced form Reduced form Oxidized form Oxidized form H2 H2O H2 H3O+ OR H2O O2 H2 H2O Strength of oxidizing agent Strength of oxidizing agent

  18. Reduced form Oxidized form Strength of reducing agent K K+ very poor very strong Na Na+ Li Li+ Ca Ca2+ H2 H3O+ Mg Zn Mg2+ Zn2+ in between in between Pb, Fe, Sn Pb2+, Fe2+, Sn2+ Fe3+ would be at another place H2 H2O very poor very strong Cu Ag Au, Pt Cu2+ Ag+ Au3+, Pt2+ Strength of oxidizing agent *

  19. 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+  K K+ Na Na+ Li Li+- Ca Ca2+ H2 H3O+ Mg Zn Mg2+ Zn2+ Pb, Fe, Sn Pb2+, Fe2+, Sn2+ H2 H2O Cu Ag Au, Pt Cu2+ Ag+ Au3+, Pt2+ Strength of oxidizing agent

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