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4.6 Acids and Bases: General Principles

This article explains the Brønsted-Lowry definition of acids and bases, acid strength, and acid-base reactions. It also provides a table of the strengths of some Brønsted acids.

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4.6 Acids and Bases: General Principles

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  1. 4.6Acids and Bases: General Principles

  2. 4.6Acids and Bases: General Principles Brønsted-Lowry definitionan acid is a proton donora base is a proton acceptor

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

  4. Acid Strength A strong acid is one that is completely ionized in water. A weak acid is one that ionizes in water to the extent of less than 100%. Acid strength is measured by Ka or pKa

  5. H H – .. . . . . . . . . . . Br .. H H Equilibrium constant for proton transfer .. + . . + O + H Br H O .. [H3O+][Br–] K = [HBr] [H+][Br–] Ka = [HBr] pKa = – log Ka

  6. A c i d K p K C on j . B a se a a 10 – H I ~ 1 0 – 1 0 I 9 – HB r ~ 1 0 – 9 B r 7 – HCl ~ 1 0 – 7 Cl 5 – H S O 1. 6 x 1 0 – 4 .8 HS O 2 4 4 + H O 5 5 .5 – 1 .7 H O 3 2 Table 4.2 Strength of Some Brønsted Acids strong acids are stronger than hydronium ion

  7. A c i d K p K C on j . B a se a a + H O 5 5 .5 – 1 .7 H O 3 2 –4 – H F 3. 5 x 1 0 3. 5 F –5 – CH C O H 1. 8 x 1 0 4. 6 CH C O 3 2 3 2 + –10 NH 5. 6 x 1 0 9. 2 NH 4 3 –16 – H O 1. 8 x 1 0 1 5 .7 H O 2 Table 4.2 Strength of Some Brønsted Acids weak acids are weaker than hydronium ion

  8. A c i d K p K C on j . B a se a a - 16 – H O 1. 8 x 1 0 1 5 .7 H O 2 - 16 – CH O H ~ 1 0 ~ 1 6 CH O 3 3 - 16 – CH CH O H ~ 1 0 ~ 1 6 CH CH O 3 2 3 2 - 17 – ( CH ) CH O H ~ 1 0 ~ 1 7 ( CH ) CH O 3 2 3 2 - 18 – ( CH ) C O H ~ 1 0 ~ 1 8 ( CH ) C O 3 3 3 3 Table 4.2 Strength of Some Brønsted Acids alcohols resemble water in acidity; their conjugatebases are comparable to hydroxide ion in basicity

  9. A c i d K p K C on j . B a se a a - 36 – NH ~ 1 0 ~ 3 6 NH 3 2 - 36 – ( CH ) NH ~ 1 0 ~ 3 6 ( CH ) N 3 2 3 2 Table 4.2 Strength of Some Brønsted Acids ammonia and amines are very weak acids;their conjugate bases are very strong bases

  10. R R .. + – .. . . . . . . . . . . . . O H Br H Br O .. .. H H Proton Transfer to Alcohols alkyloxonium ion base acid conjugate conjugate acid base

  11. 4.7Acid-Base Reactions:A Mechanism for Proton Transfer

  12. H H .. + – .. . . . . . . . . . . . . O H Br H Br O .. .. H H Proton Transfer from HBr to Water base acid conjugate conjugate acid base

  13. Mechanism: single step transition state Potentialenergy activation energy reactants products Reaction coordinate

  14. H2O H Br + H2O—H + Br – Mechanism: single step d+ d- bimolecular:both H2O and HBr involved at transition state Potentialenergy H2O + H—Br Reaction coordinate

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