Determining Strengths of Acids and Bases

Determining Strengths of Acids and Bases

(This section is in Chp 18 pages 605 – 612)(This section is in Chp 18 pages 605 – 612)

StrengthStrength

Strong Acids – Dissociate almost completely in water to produce HStrong Acids – Dissociate almost completely in water to produce H33OO++ ions ions Strong Bases – Strongly attract hydrogen ions in water, thus leaving OHStrong Bases – Strongly attract hydrogen ions in water, thus leaving OH--

- +

HCl

HNO3

H2SO4

NaOH

KOH

HBr

HI

HClO4

STRONG ACIDS "The Big Six” STRONG

BASES

Ca(OH)2

CaO

StrengthStrength

Weak Acids – Dissociate slightly in water to produce HWeak Acids – Dissociate slightly in water to produce H33OO++ ions. Do not readily dissociate. ions. Do not readily dissociate. Weak Bases – react only partially with water to form OHWeak Bases – react only partially with water to form OH-- ions. ions.

NH3

H2NNH2

WEAK

ACIDS

WEAK BASES

CO32-2-

PO43-3-

- +

HF

HC2H3O2

H3PO4

H2CO3

HCN

HCl + H2O H3O+ + Cl–

Strong Acid: SINGLE ARROW

DOUBLE ARROWWeak Acid:

HC2H3O2 + H2O H3O+ + C2H3O2–HC2H3O2 + H2O H3O+ + C2H3O2–

CaO Ca2+ + O2–

Strong Base: SINGLE ARROW

DOUBLE ARROWWeak Base:

CO32-

+ H2O

O2– + H2O 2OH–

HCO3- + OH–

Strong & WeakStrong & Weak

Acid Dissociation ConstantAcid Dissociation Constant

Ka = [H3O+][A-]

[HA]

Kais a measure of the strength of an acid.

HA(aq) + H2O (l)

The greater the Ka, the stronger the acid!

H3O+ (aq) + A–

(aq)

Each ionization resultsin a different Ka value.

If the acid is diprotic or triprotic, the loss of each hydrogen

is a separate ionization.

Base Dissociation ConstantBase Dissociation Constant

Kb = [HB][OH-]

[B]

Kbis a measure of the strength of a base.

B(aq) + H2O(l) HB(aq) + OH–

(aq)

The greater the Kb, the stronger the base!

Practice Problem #1Practice Problem #1

Acetic acid is a weak monoprotic acid. If the initial concentration of acetic acid is 0.200 M and the equilibrium concentration of H3O+ is 0.0019 M, calculate Ka for acetic acid.

Ka =[H3O+][A-]

[HA]

HA(aq) + H2O(l) H3O+ (aq) + A–

(aq)

Practice Problem #1Practice Problem #1

Ka =[H3O+][A-]

[HA]

HA(aq) + H2O(l) H3O+

(aq) + A– (aq)

Use Stoichiometry!The reaction produces an A- ion

for every H3O+ ion.

[H3O+] = [A-]

Practice Problem #1Practice Problem #1

Ka =[H3O+][A-]

[HA]

HA(aq) + H2O(l) H3O+

(aq) + A– (aq)

The equilibrium concentration of

HA equals its initial concentrationminus the amount

that ionizes.

[HA] = 0.200M – 0.0019M

[HA] = 0.1981M

Practice Problem #1Practice Problem #1

Ka =[H3O+][A-]

[HA]

HA(aq) + H2O(l) H3O+

(aq) + A– (aq)

[H3O+] = [A-] = 0.0019M

[HA] = 0.1981M

=(0.0019M)(0.0019M)

(0.1981M)

= 1.8 x 10-5

Acid-Base Properties of SaltsAcid-Base Properties of Salts

Salts dissociate in water to form H3O+ or OH-

Knowing what type of acid and base formed the salt helps predict salt solution acid-base properties.

Acid-Base Properties of SaltsAcid-Base Properties of Salts

RULES:

Strong Acid + Strong Base Neutral Salt

Strong Acid + Weak Base

Weak Acid + Strong Base

Weak Acid + Weak Base

Acidic Salt

Basic Salt

Cannot predictvery easily

HCl + NaOH Neutral Salt

NaClSTRONGACID & BASE

NH3 + HClAcidic

Salt

NH4Cl

STRONGACID

WEAKBASE

NaOH + H2CO3

BasicSalt

Na2CO3

WEAKACID

STRONGBASE