Percent dissociation of weak acids

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Percent dissociation of weak acids

• Percent dissociation
• HAdissociated x
  100
• 
  HAinitial
• Increases as Ka increases
• For given acid, increases with dilution

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Dissociation increases with dilution
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Polyprotic acids

• Dissociations are stepwise
• H2CO3 H2O H3O(aq) HCO3-(aq)
• HCO3-(aq) H2O H3O(aq) CO32-(aq)

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Ka decreases with each step

• It is harder to remove a proton from a negative
  ion than from a neutral molecule
• Solution contains mixture of all species
• Strongest acid is the unionized molecule HnX

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Use the same strategy described earlier

• What are concentrations of species present in
  0.020 M solution of carbonic acid?
• Step 1 initial species are H2CO3 and H2O
• Step 2 Ka1 Kw principal process is ionization
  of H2CO3

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ICE Age Two
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Step 5 Obtain x from Ka
X ltlt0.02
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Step 6 Big concentrations

• H3O HCO3- x 9.3 x 10-5 M
• H2CO3 0.020 x 0.020 M

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Step 7 small concentrations

• Obtained from subsidiary equilibria
• HCO3-(aq) H2O H3O(aq) CO32-(aq)
• CO32- Ka2 5.6 x 10-11 M
• In general, A2- Ka2

Ignore H3O generated in second ionization
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Step 7 contd.

• OH- from dissociation of water

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Step 8 Calculate pH

• pH -log10H3O
• -log(9.3x10-5)
• 4.03

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Weak base equilibria

• Treated in an analogous way to weak acids

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Worked example

• Calculate pH and concentrations of species in
  0.0012 M solution of codeine
• Cod H2O CodH(aq) OH-(aq)
• Kb 1.6x10-6
• H2O H2O H3O(aq) OH-(aq)
• Kw 1.0 x 10-14

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Principal reaction is protonation of codeine
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Step 5 dissociation
X ltlt0.0012
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Step 6 Big concentrations

• CodH OH- x 4.4x10-5 M
• Cod 0.0012 x 0.0012 M (x0.0012)

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Step 7 small concentrations

• H3O from dissociation of water

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Step 8 Calculate pH

• pH -log10H3O
• -log(2.3x10-10)
• 9.64

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Relationship between Ka and Kb for conjugate
acid-base pair

• Acid
• HA H2O H3O(aq) A-(aq)
• Base
• A- H2O HA(aq) OH-(aq)

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K for the overall reaction is product of Ks for
individual reactions

• In general
• Knet K1 x K2 x K3 x
• For conjugate acid-base pairs
• Ka x Kb Kw

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Salts

• Products of acid-base neutralization
• HCl NaOH NaCl H2O
• HCl KOH KCl H2O
• HNO3 KOH KNO3 H2O
• 2HCl Ca(OH)2 CaCl2 2H2O
• HCN NaOH NaCN H2O

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Unequally yoked

• Consequences for neutralized solutions of the
  following combinations
• Strong acid strong base
• NEUTRAL
• Strong acid weak base
• ACIDIC
• Weak acid strong base
• BASIC

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Salts that yield neutral solutions

• Group 1A cations
• Group 2A cations (except Be2)
• Anions from strong monoprotic acids
• (Cl-, Br-, I-, NO3-, ClO4-)

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Salts that yield acid solutions

• Salts of weak bases
• NH4(aq) H2O NH3 H3O(aq)
• Hydrated small polarizing metal cations

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• What is pH of a 0.10 M solution of AlCl3
• Ka 1.4 x 10-5
• Using prior strategy
• Al(H2O)63, Cl- and H2O are initial species
• Al(H2O)63 is stronger acid than H2O

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• pH -log(1.2 x 10-3) 2.92

X ltlt 1
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Salts that yield basic solutions

• Salts of weak acid and strong base
• CN-(aq) H2O HCN(aq) OH-(aq)
• Calculation of pH follows same strategy as for
  the salt of strong acid and weak base except use
  expression for Kb

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Weak acid and base what then?

• Competition between relative acid strength of
  cation and base strength of anion
• Consider (NH4)2CO3
• NH4(aq) H2O H3O(aq) NH3(aq) Ka
• CO32-(aq) H2O HCO3-(aq) OH-(aq) Kb
• If Ka gt Kb then pH lt 7
• If Ka lt Kb then pH gt 7
• If Ka Kb then pH 7

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Summing up
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Factors affecting acid strength

• How easily is the H X bond broken?
• the less easily dissociated, the weaker the acid.
• Bond strength
• Bond strength increases, acid strength decreases
• Polarity
• For a given bond strength, the more polar the
  more easily dissociated closer to H

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In a group, bond strength is the more important
factor

• Polarity is not a good predictor H-X polarity
  decreases down the group as acid strength
  increases
• Bond strength decreases more strongly down the
  group
• Bond length increases sharply

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Electronegativity more reliable indicator in a
period

• H-X polarity increases across period
• Acid strength increases across period
• Bond strength shows weaker trend
• Bond lengths vary little

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Summing up
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Oxoacids and strength where are the electrons
going

• How to weaken the OH bond and increase its
  polarity?
• Increase electronegativity of Y
• Increase oxidation number of Y (number of O atoms
  around Y
• Both serve to withdraw electrons from the H
  making it more positive closer to H

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Increasing electronegativity of Y

• In a homologous series, increasing
  electronegativity of Y increases strength
• Y draws charge away from the H atom, more easily
  ionized
• NOTE Opposite dependence to series HA, where A
  F, Cl, Br, I

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Increasing oxidation number (O atoms)

• More O atoms around Y, more acidic
• The O atoms draw charge away from the H atom,
  more easily ionized
• Also think in terms of relative stabilities of
  the oxoanions formed in the process

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Lewis acids no protons required

• Lewis acid an electron pair acceptor (BF3)
• Lewis base an electron pair donor (NH3)

BF3
NH3
DONOR
ACCEPTOR
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Lewis definition casts the net further

• All Arrhenius acids are Brønsted acids
• All Brønsted acids are Lewis acids
• The converses are not true
• Examples
• Al3(acid) H2O (base)
• Cu2(acid) NH3 (base)
• SO3 (acid) H2O (base)