Acid-Base(a)

8/11/2019 Acid-Base(a)

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Acid-Base Equilibrium

Chapter 18


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Acids and Bases

Arrhenius

acids: generate H+in water

bases: generate OH-in water

Brnsted-Lowry

acids: H+donors

bases: H+acceptors

HCl + H2O Cl + H3O

+

acid base


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Conjugate acid-base pairs

Conjugate base:

remains after H+is lost

acid: HCl conj. base: Cl-

Conjugate acid:

remains after H+is gained

base: NH3 conj. acid: NH4+

Base Acid+Acid Base+

Conjugate Pair

Conjugate Pair


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Strong and Weak Acids

Strong: 100% dissociation

good H+donor

equilibrium lies far to right (HNO3)

generates weak base (NO3

-)

Weak:


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Acid Dissociation Constant

OHAOHHA 3-

2

[HA]

]][A[H

[HA]

]][AO[HK

--3

a


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Ka

Increases

Strength

vs. Ka


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Cat o25101]][OHO[HK 14-

3w

Acid Base Conj. Acid Conj. Base

Water is amphoteric


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[H3O+] and [OH-]

[H3O+] 1x100 to 1x10-14 in water[OH-] 1x10-14to 1x100 in water

]O[HK][OH3

w-

14-3w 101]][OHO[HK

][OH

K]O[H -w3


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Finding [H3O+] and [OH-]


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pH Scale

]log[H-]Olog[H-pH 3

14-3w 101]][OHO[HK

pH 0 to 14 in water

]log[OH-]log[OH-pOH pOH 14 to 0 in water

pKw = pH + pOH = 14


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H+ H+H+

OH-OH-

OH-

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Acidic Neutral Basic

pH and pOH

As [H3

O+] rises, [OH-] falls

As pH falls, pOH rises


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Indicators


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Strength of Bases

Strong:

100% dissociation

OH-supplied to solution

NaOH(s)

Na

+

(aq) + OH

-

(aq)

Weak:


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Base Dissociation Constant

OHBHOHB 2-

][B

][BH][OHK

-

-

b


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Kw, Kaand Kb

14-3w 101]][OHO[HK

14baw 101KKK

K

KK

b

w

a

KKK

a

wb


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Lewis Acids and Bases

Lewis Acid: electron pair acceptorLewis Base: electron pair donor

Al3+ + 6

H

O

H

Al O

H

H6

3+

Acid Base


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Buffers

Buffer solution: a mixture of conjugate acid and base

that resists pH changes.

A buffer contains a weak acid and a salt of the sameacid.

A buffer contains a weak base and a salt of the samebase.


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pH of buffers

For a weak acid, HA, we have

HA = H+ + A

[H+] [A]

Ka =

[HA]

[A]

log Ka=log [H+] log

[HA][A] [A]

pKa=pH log pH = pKa+ log

[HA] [HA]

[salt]

[acid


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pH = pKa + log salt / acid

pH = pKb + log salt / base


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The Common Ion Effect

Common

Ion Effect: the change in the

equilibrium that results from the addition of

an ion that is involved in the equilibrium.

Acid-Base(a)

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