11

AcidAcid-Base -Base Chemistry

An acidacid is a HH++ (proton) donor (proton) donor.

General formula of acid = H-Aacid = H-A.

H-A HH-A H+ + + A + A--

acid proton conjugate base

22When H-A dissociates to H+ and A– in aqueous (water) solution, the “hydronium ion” or H3O

+ forms:

H-AH-A + H+ H22O HO H33OO++ + A + A––

H-AH-A + H-O-H H-O-H + AA––

HH

••

••

••

+

meansmeans

33A BaseBase is a HH++ (proton) acceptoracceptor.

A basebase has an unshared pair of unshared pair of electrons electrons to share with H+.

we can represent a basebase by the general formula :B :B or :Base.:Base.

:Base + :Base + HH++ H H-Base-Base++

from an conjugate acid acid

44When a Base accepts protons from water, it increases the concentration of HO– (hydroxide ions) in solution:

:B :B + H-O-H H-BB++ + – O-H

OR

:B + H2O HB+ + –OH

••

••

••

••••

55Examples

HHNO3 + H2O H3O+ + NO3

–

nitric acid nitrate ion acidacid conjugate baseconjugate base

HHCl + H2O H3O+ + Cl–

hydrochloric acid chloride ion acidacid conjugate conjugate basebase

66

:NH:NH33 + H2O +NH4 + – OH

ammonia ammonium ion

basebase conjugate acidconjugate acid

77Strengths of Acids & Bases

A strong acid HA completely strong acid HA completely dissociates dissociates to H+ (= H3O+) and A– in H2O:

H-AH-A + H2O H3O+ + A– .1 mole 1 mole to start to start

gives1 mole H1 mole H33OO+ +

+ 1 mole A+ 1 mole A––

88The common strong acids are:

HClHCl hydrochloric acidhydrochloric acid

HBr hydrobromic acid

HIHI hydroiodic acidhydroiodic acid

HNO3 nitric acid

HH22SOSO44 sulfuric acidsulfuric acid

[HClO4 perchloric acid] don’t

memorize [ ]

99A strong base strong base is one which completely dissociates to provide HO– in water.

These bases are ionic metal hydroxides:

Metal cation + hydroxide anionMetal cation + hydroxide anion

MOH M+ + –OH

1 mole 1 mole + 1 mole

1010There are 4 common strong There are 4 common strong bases:bases:LiOH lithium hydroxide

NaOHNaOH sodium hydroxidesodium hydroxide

KOHKOH potassium hydroxidepotassium hydroxide

Ba(OH)Ba(OH)2 2 barium hydroxidebarium hydroxide

1111

Some Advice...Some Advice...

It is best to memorizememorize the listslists of

strong acids & strong acids & strong strong bases --bases --

assume all others are weak.

1212But Also,

LEARN HOWLEARN HOW

to write the equations

for the reaction of each chemical as a

strong acid strong acid

or strong base.strong base.

1313

What about WEAK acids & bases?

A weakweak acid or weak base is incompletely ionized incompletely ionized or

dissociated in water solution.

The ionization reaction of a The ionization reaction of a weak acid or weak base is a weak acid or weak base is a

reversible equilibrium!!reversible equilibrium!!

1414Example

CH3CO-H + H2O CH3CO– + H3O+

O O

acetic acid acetateacetic acid acetatebefore equilibrium:before equilibrium:1 mole 0 0

after equilibrium:after equilibrium:0.98 mole 0.02 0.02

1515

The position of equilibrium is different for each weak acid

and weak base.

The equilibrium constant describes how much

ionization has occurred.

1616

CH3CO-H + H2O CH3CO– + H3O+

O OFor acetic acid:For acetic acid:

Keq = [CH3CO2–]•[H3O+]

[CH3CO2H]•[H2O]

OROR

Keq•[H2O] = K Kaa =[CH[CH33COCO22

––]•[H]•[H33OO++]]

[CH[CH33COCO22H]H]

1717We use Ka as a measure of the

strength of an acid because [H2O] is very large and does not effectively change as a result of

the ionization reaction.

Typical Ka values for weak acids are in the range of

1 x 10-1 to 1 x 10-10

1818Sample KSample Kaa values: values: do NOT memorize!!!!do NOT memorize!!!!WhatWhat KKaa==

CH3CO2H acetic acid 1.8x10-5

H2O water 1 x 10-16

CH3CH2OH alcohol 1 x 10-17

1919What do these small Ka values mean?

* Solutions of weak acids and bases at equilibrium contain both the unionized (H-A) and ionized forms (H+ and A–).

* The largest species present is H-A (unionized acid.)

2020And...* Any compound having Ka smaller than H2O is considered “neutral” =

“not acidic in water”.

Thus, alcohols are not acidic compounds. They are neutral substances.

2121

MonoproticMonoprotic acids are those which can release one Hrelease one H++ per per

molecule molecule of acid.

PolyproticPolyprotic acids can release release more than one Hmore than one H++ per molecule

of acid.

2222 Examples

Monoprotic HMonoprotic HCl Hydrochloric Acid

DiproticDiprotic HH22SO4 Sulfuric acid

TriproticTriprotic HH33PO4 Phosphoric Acid

2323Amphiprotic (Amphoteric) Amphiprotic (Amphoteric) Compounds Compounds can act as acids or bases.

ExamplesExamplesHCO3

– bicarbonate ion

as acidas acid HCO3– CO3

-2 + HH++

as base as base HCO3– + H+ HH2CO3

2424

Dihydrogen phosphate HDihydrogen phosphate H22POPO44–– is

another important amphotericamphoteric ion.

as acidas acid

HH22PO4–– HPO4

-2 + HH++

as baseas base

H2PO4–– + H+ HH3PO4

2525Water is the most important amphoteric compound of all.

H-O-HH + H-O-H HO– + H3O+

“ “acid” acid” “base”“base”

Keq = [HO–]•[H3O+] OROR

[H2O]•[H2O]

Keq•[H2O]2 = K Kww = [HO = [HO––]•[H]•[H33OO++] ]

2626

In pure H2O at room temperature, Kw has a value

of 1 x 10-14.

Since Kw = [HO–]•[H3O+], this means

[HO–]=[H3O+] = 1x10-7

moles/liter

in neutral water.

2727pH is defined as

pH = -log ([HpH = -log ([H33OO++]) ]) of a solution.

A logarithm tells what power of 10 = a certain number.That is, if y = 10y = 10xx, then log(y) = log(y) = xx.

Thus, 100 = 102 , so log(100) = 2.

2828We saw that for pure H2O,

[H3O+] = 1 x 10-7.

Since -log (1 x 10-7) = 7,

we say neutral water has pH = neutral water has pH = 77.

2929Recall: [HO–] and [H3O+] are related through the equilibrium

2 H2O HO– + H3O+ .

AcidsAcids are HH++ donors donors and cause an increaseincrease in [H[H33OO++]].

So: pH decreases. Also: [HO–] decreases.

Acidic solutions have pH <7. Acidic solutions have pH <7. (in the range 0 to 6.99)

3030

Again: 2 H2O HO– + H3O+ .A BaseBase increases [HO-] or

decreases [Hdecreases [H33OO++]].

So pH increases:a basicbasic or “alkaline” solution solution

has has a pH >7 a pH >7 (in the range 7+ to 14).

313114

0

7 Neutral [HO–] = [H3O+]pH

Acidic [HO–] < [H3O+]

Basic [HO–] > [H3O+]