Download - Chapter 16
Chapter 16
Conjugate Acids and Bases
Differ by one and only one proton.
Acid form has the extra proton, base has one lessproton.
Examples :
HSO4- is conjugate base of H2SO4
HSO4- is conjugate acid of SO4
2-
SO42- is not conjugate base of H2SO4
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Natural pH Indicators
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Formulas of Acids and BasesACIDS EXAMPLE BASES EXAMPLE
HX HC2H3O2 M(OH)n Mg(OH)2 CationSpectator NH4Cl SpectatorAnion NaC2H3O2
Cationn+ Al3+ Anionn- F- Ammonium NH4
+ Ammonia NH3
•Ammonium (NH4+) and acetate (C2H3O2
-) are water-soluble ions (no exceptions), but are NOT spectators because they react with water.
•Ammonium is a weak acid.
•Acetate is a weak base.
•Cations are weak acids except H+ (strong acid) and spectators (not acidic).
•Anions are weak bases except OH- (strong base) and spectators (not basic).
•Always break formulas into left and right halves and analyze acid/base character of each half.
•Example: HC2H3O2 is combination of strong acid (H+) and weak base (C2H3O2-); combination
results in overall weak acid.
Strong and Weak Acids
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Introduction to Aqueous Acids
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Chemical Structure & Acid Strength
• Binary acids♦ More electroneg. partners stronger (left/right
Periodic Table)♦ Larger partners stronger (up/down Periodic
Table)• Oxyacids
♦ More electroneg. central atom stronger♦ More oxygens stronger (sam e # protons)♦ More H’s stronger (sam e # O’s)
• R em em ber base strength goes in oppositedirections than acid strength
Introduction to Aqueous Bases
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Ka and Kb
Equilibrium constants for acid-base reaction betweenacid or base (respectively) and water.
Neglect liquid water concentration when calculatingthese equilibrium constants.
Acid + H2O Conj. base + H3O+
[Acid]
[Conj. base][H3O+]Ka =
Base + H2O Conj. acid + OH -
Kb = [OH - ] [Conj. acid]
[Base]
Kw
Equilibrium constant for water undergoing acid-basereaction with itself.
Again liquid water concentration is ignored incalculating the equilibrium constant.
H2O + H2O → H3O+ + OH-
Kw = [H3O+] [OH-] = 1 x 10-14
Consider:Acid + H2O → Conj. base + H3O+ Ke q = Ka
Conj. base + H2O → Acid + OH- Ke q = Kb
Net: 2 H2O → H3O+ + OH- Ke q = Kw
The Meaning of “p”
Consider:
ΔG ° = -R T ln Ke q
= -(0.008314)(298)(2.303)log Ke q at T = 298 K= -5.71 log Ke q
= 5.71(-log Ke q)
ΔG ° 298 = 5.71 pKe q
At room tem perature “p” is an energy operation whichwhen m ultiplied by 5.71 (scaling factor) converts a Ke q
value into ΔG ° in kJ/m ol for a reaction.
pH is how unstable hydronium ion ispKa is how difficult it is for an acid to m ake hydronium .
Acid-base Calculations
• Acid to base or base to acid conversions♦ Concentration units♦ Energy units
• Concentration/energy conversions♦ Indian story♦ Concentration to energy♦ Energy to concentration
• Weak acid/base approxim ations♦ Weak acid approxim ation♦ Weak base approxim ation♦ Percent dissociation
• Strong acid/base calculations
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