1. Acid-Base Concepts Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology

2. 7/13/2015 Acid-Base Concepts The definitions of an acid and a base range from proton and hydroxide donors to electron pair donors and acceptors to specialized definitions for non-solvent systems.

3. 7/13/2015 Acid-Base Definitions Arrhenius - Acids are proton donors, bases are hydroxide donors. Bronsted-Lowry - Acids are proton donors, bases are proton acceptors. Lux-Flood - Acids are oxide acceptors, bases are oxide donors.

4. 7/13/2015 Lux-Flood Reactions Acids are oxide acceptors, bases are oxide donors CaO + SiO 2  CaSiO 3 CaO + H 2 O  Ca 2+ + 2 OH - base acid basic anhydride SiO 2 + H 2 O  H 2 SiO 3 acidic anhydride

5. Acid-Base Definitions Don’t Have To Be Limited to H & O (CH 3 ) 4 N + Cl - bases are chloride donors FeCl 3 acids are chloride acceptors OPCl 3 or OP(OEt) 3 (CH 3 ) 4 N + Cl - + FeCl 3 (CH 3 ) 4 N + + FeCl 4 -

6. 7/13/2015 Solvent Systems Autoionization 2 H 2 OH 3 O + + OH - 2 NH 3 NH 4 + + NH 2 - 2 OPCl 3 OPCl 2 + + OPCl 4 - strongest acid allowed in solvent strongest base allowed in solvent

7. 7/13/2015 Solvent Systems Acid Leveling 2 H 2 OH 3 O + + OH - acids stronger than H 3 O + are leveled to H 3 O + bases stronger than OH - are leveled to OH - HNO 3 + H 2 O  H 3 O + + NO 3 - CaO + H 2 O  Ca 2+ + 2 OH -

8. 7/13/2015 Solvent Systems Acid Leveling 3 HFH 2 F + + HF 2 - What is the strongest acid and the strongest base allowed in liquid hydrofluoric acid?

9. 7/13/2015 Other Acid-Base Definitions Lewis - Acids are electron pair acceptor, bases are electron pair donors. Usanovich - Acids are negative charge acceptors, bases are negative charge donors.

10. metalloids are amphoteric General Acid-Base Concepts  more basic metal oxides are basic nonmetal oxides are acidic

11. General Acid-Base Concepts Beryllium oxide is amphoteric high charge density causes polarization of any electron source BeO is more acidic and less basic than other alkaline earth oxides. BeO + 2 H + Be 2+ + H 2 O BeO + H 2 O + 2 OH - Be(OH) 4 2-

12. General Acid-Base Concepts Hydration and Hydrolysis coordination of the solvent (solvation) releases energy and stabilizes the product high charge density ions have large hydration (solvation) energies Na + + n H 2 O[Na(H 2 O) n ] +

13. General Acid-Base Concepts Hydration and Hydrolysis Al 3+ + 6 H 2 O[Al(H 2 O) 6 ] 3+ If the cation is acidic enough, it can polarize the oxygen of water away from the hydrogen. [Al(H 2 O) 5 (OH)] 2+ + H 3 O + H2OH2O the pH of 0.1 M Al(NO 3 ) 3 is 3.07 Why is 0.1 M iron(III) ion even more acidic?

14. General Acid-Base Concepts Oxyacids H m XO n The acidity of oxyacids depends on formal charge on the central atom inductive effects of substituents charge on the complex

15. General Acid-Base Concepts Oxyacids H m XO n if n-m  2, strong acid pK a << 0 = 1, weak acid pK a  2.1 ± 0.9 = 0, very weak acid pK a  8.5 ± 1 sequential proton loss leads to pK 2 values of +5 greater Examples: HNO 3 HNO 2 HSO 4 - HClO 2 H 3 PO 4 H 2 PO 4 - HClO 3 HClO

16. HNO 3 HNO 2 HSO 4 - HClO 2 H 3 PO 4 H 2 PO 4 - HClO 3 HClO << 0 3.3 1.9 HClO 2 2.1 7.2 HClO 3 7.5

17. General Acid-Base Concepts Oxyacids H m XO n The acidity of oxyacids depends on formal charge on the central atom inductive effects of substituents charge on the complex pK a = 10.5 - 5.0 n -  x for X(OH) m O n

18. General Acid-Base Concepts Oxyacids Special Cases H 2 SO 3 pK a = 1.81 H 3 PO 3 pK a = 2.00 H 2 CO 3 pK a = 6.37 H 3 BO 3 pK a = 9.1

19. General Acid-Base Concepts Substituted Amines NR 3 The basicity of amines depends on inductive effects of substituents steric effects NF 3 no basic character NH 3 NH 2 CH 3 NH(CH 3 ) 2 N(CH 3 ) 3 4.74 pK b 3.363.294.28

20. 7/13/2015