4.6 Acids and Bases: General Principles 1

4.6 Acids and Bases: General Principles Brønsted-Lowry definition an acid is a proton donor a base is a proton acceptor 1

Proton Transfer from HBr to Water hydronium ion H Br O . – .. + base acid conjugate conjugate acid base 2

A strong acid is one that is completely ionized in water. 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 3

Equilibrium constant for proton transfer H . H . .. + Br – . .. . . . O + H Br O H + .. [H3O+][Br–] K = [HBr] [H+][Br–] K a = [HBr] pK a = – log Ka 4

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

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

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

Table 4.2 Strength of Some Brønsted Acids K a p C on j . B se NH 3 ~ 1 - 36 6 2 – ( CH ) N ammonia and amines are very weak acids; their conjugate bases are very strong bases 5

Proton Transfer to Alcohols alkyloxonium ion H Br O R . – .. + base acid conjugate conjugate acid base 2

4.7 Acid-Base Reactions: A Mechanism for Proton Transfer 1

Proton Transfer from HBr to Water . – .. + base acid conjugate conjugate acid base 2

Mechanism: single step transition state Potential energy activation energy reactants products Reaction coordinate 22

Mechanism: single step d+ d- H2O H Br bimolecular: both H2O and HBr involved at transition state Potential energy H2O + H—Br H2O—H + Br – + Reaction coordinate 22