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1.12 Acids and Bases: The Arrhenius View

1.12 Acids and Bases: The Arrhenius View. Definitions. Arrhenius An acid ionizes in water to give protons. A base ionizes in water to give hydroxide ions. Brønsted-Lowry An acid is a proton donor. A base is a proton acceptor. Lewis

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1.12 Acids and Bases: The Arrhenius View

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  1. 1.12Acids and Bases:The Arrhenius View

  2. Definitions Arrhenius An acid ionizes in water to give protons. A base ionizes in water to give hydroxide ions. Brønsted-Lowry An acid is a proton donor. A base is a proton acceptor. Lewis An acid is an electron pair acceptor. A base is an electron pair donor.

  3. . . + .. .. – – . . A + H A + OH M + OH M H .. .. Arrhenius Acids and Bases An acid is a substance that ionizes to give protons when dissolved in water. A base is a substance that ionizes to give hydroxide ions when dissolved in water.

  4. . . + .. .. – – . . A H A + OH OH M H .. .. Arrhenius Acids and Bases Strong acids dissociate completely in water. Weak acids dissociate only partially. Strong bases dissociate completely in water. Weak bases dissociate only partially. + M +

  5. [H+][A–] . Ka= . [HA] + – A H A + H Acid Strength is Measured by pKa pKa= – log10Ka

  6. 1.13Acids and Bases:The Brønsted-Lowry View Brønsted-Lowry definitionan acid is a proton donora base is a proton acceptor

  7. . . A Brønsted Acid-Base Reaction A proton is transferred from the acid to the base. + – . . + B H A H A B + base acid

  8. . . A Brønsted Acid-Base Reaction A proton is transferred from the acid to the base. + – . . + B H A H A B + base acid conjugate acid conjugate base

  9. . . . . . . . . Proton Transfer from HBr to Water hydronium ion base acid conjugate conjugate acid base H H .. + – .. . . . . O H Br H Br O + + .. .. H H

  10. H H – .. . . . . . . . . . . Br .. H H Equilibrium Constant for Proton Transfer .. + . . + O + H Br H O .. • Takes the same form as for Arrhenius Ka, but H3O+ replaces H+. H3O+ and H+ are considered equivalent, and there is no difference in Ka values for Arrhenius and Brønsted acidity. [H3O+][Br–] Ka = [HBr]

  11. H H – .. . . . . . . . . . . Br .. H H Equilibrium Constant for Proton Transfer .. + . . + O + H Br H O .. [H3O+][Br–] Ka = [HBr] pKa= – log10Ka

  12. . . . . . . Water as a Brønsted Acid H base acid conjugate conjugate acid base H .. – .. – . . + N H OH H OH N + .. .. H H

  13. stronger acid A c i d p K C on j . B a se a – H I I -10.4 – HB r B r -5.8 – HS O H S O 4 -4.8 2 4 – Cl HCl -3.9 weaker acid + H O H O -1.7 3 2 Dissociation Constants (pKa) of Acids* strong acids are stronger than hydronium ion *For a more detailed list click here for Table 1.8

  14. stronger acid A c i d p K C on j . B a se a – H I I -10.4 – HB r B r -5.8 – HS O H S O 4 -4.8 2 4 – Cl HCl -3.9 weaker acid + H O H O -1.7 3 2 Important Generalization! The stronger the acid, the weaker the conjugate base. *For a more detailed list click here for Table 1.8

  15. A c i d p K C on j . B a se a + H O – 1 .7 H O 3 2 – H F 3. 5 F – CH C O H 4. 6 CH C O 3 2 3 2 + NH 9. 2 NH 4 3 – H O 1 5 .7 H O 2 Dissociation Constants (pKa) of Acids* weak acids are weaker than hydronium ion *For a more detailed list click here for Table 1.8

  16. A c i d p K C on j . B a se a – 1 5 .2 CH O H CH O 3 3 – H O 1 5 .7 H O 2 – CH CH O H ~ 1 6 CH CH O 3 2 3 2 – ( CH ) CH O H ~ 1 7 ( CH ) CH O 3 2 3 2 – ( CH ) C O H ~ 1 8 ( CH ) C O 3 3 3 3 Dissociation Constants (pKa) of Acids* alcohols resemble water in acidity; their conjugatebases are comparable to hydroxide ion in basicity *For a more detailed list click here for Table 1.8

  17. A c i d p K C on j . B a se a – NH ~ 3 6 NH 3 2 – ( CH ) NH ~ 3 6 ( CH ) N 3 2 3 2 Dissociation Constants (pKa) of Acids* ammonia and amines are very weak acids;their conjugate bases are very strong bases *For a more detailed list click here for Table 1.8

  18. – H H H H H H H – H H H2C H2C CH CH2 H H – HC HC C CH – CH3CH2 Dissociation Constants (pKa) of Acids* Acid p K C on j . B a se a 26 Most hydrocarbons are extremely weak acids. 43 45 62 CH3CH3 *For a more detailed list click here for Table 1.8

  19. 1.14What Happened to pKb?

  20. About pKa and pKb • A separate “basicity constant” Kb is not necessary. • Because of the conjugate relationships in the Brønsted-Lowry approach, we can examine acid-base reactions by relying exclusively on pKa values.

  21. N •• H H N •• H Example • Which is the stronger base, ammonia (left) or pyridine (right)? • Recall that the stronger the acid, the weaker the conjugate base. • Therefore, the stronger base is the conjugate of the weaker acid. • Look up the pKa values of the conjugate acids of ammonia and pyridine in Table 1.8.

  22. + N H Example H H + weaker acid N H pKa = 9.3 H pKa = 5.2 stronger acid Therefore, ammonia is a stronger base than pyridine

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