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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