The pK a Scale. Acid Dissociation Constant, K a K a, is a measure of how readily H + ions are released HA (aq) + H 2 O (aq) H 3 O + (aq) + A - (aq) K.

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Presentation transcript:

The pK a Scale

Acid Dissociation Constant, K a K a, is a measure of how readily H + ions are released HA (aq) + H 2 O (aq) H 3 O + (aq) + A - (aq) K a = [H + ][A - ] [HA]

K a Values Stronger acids – higher K a values Weak acids – lower K a values AcidKaKa HCl1.0 x 10 7 HNO x 10 1 HF5.6 x HCOOH1.6 x CH 3 COOH1.7 x 10 -5

pK a Since K a values for weak acids are very small it is convenient to use the pK a scale where: pK a = -log 10 K a If K a = 1.70 x mol dm -3 Then pK a = -log 10 (1.70 x ) pK a = 4.77

K a Values & pK a Values Explain the trend in strength of these acids Calculate the pK a values of these acids AcidKaKa CCl 3 COOH0.2 HCOOH1.5 x CH 3 COOH1.6 x CH 3 CH 2 COOH1.3 x 10 -5

pK a Values Stronger acids – higher K a values Stronger acids – lower pK a values AcidKaKa pK a CCl 3 COOH HCOOH1.5 x CH 3 COOH1.6 x CH 3 CH 2 COOH1.3 x

Calculate the pH of a 0.1M solution of a weak acid given that it has a pK a value of 4.76 at 298K. pK a = -log 10 K a 4.76 = -log 10 Ka K a = x [H + ] = √ x x 0.1 K a ≈ [H + ] 2 [HA] total

Calculate the pH of a 0.1M solution of a weak acid given that it has a pK a value of 4.76 at 298K. pH = -log 10 [H + ] pH = 2.88 [H + ] = x mol dm -3

A 0.01M solution of a weak acid, X, has a pH of 3.37 at 200K. Calculate the pK a value for X at this temperature. pH = -log 10 [H + ] [H + ] = x K a ≈ [H + ] 2 [HA] total K a ≈ (4.266 x )

A 0.01M solution of a weak acid, X, has a pH of 3.37 at 200K. Calculate the pK a value for X at this temperature. pK a = -log 10 K a pK a = 4.74 K a ≈ x 10 -5