11

AcidAcid-Base -Base ChemistryAn 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-HOR:B + H2O HB+ + –OH

••••

••••

••

55ExamplesHHNO3 + 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 + – OHammonia ammonium

ion basebase conjugate acidconjugate acid

77Strengths of Acids & BasesA 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 gives 1 mole H1 mole H33OO+ +

+ 1 mole A+ 1 mole A––

88The common strong acids are:HClHCl hydrochloric acidhydrochloric acidHBr hydrobromic acidHIHI hydroiodic acidhydroiodic acidHNO3 nitric acidHH22SOSO44 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+ + –OH1 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 equationsfor the reaction of each

chemical as astrong acid strong acid

or strong base.strong base.

1313What 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 of1 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.

2121MonoproticMonoprotic 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

2424Dihydrogen 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/literin neutral water.

2727pH is defined aspH = -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)

3030Again: 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+]