CHAPTER 15 ACIDBASE EQUILIBRIA
Introduction to Acids, Bases and the Equilibrium Concept
Water (H O)2– the most important molecule on earth. Even in pure water, there are small
amounts of ions from the equilibrium below (“selfionization of water” or “autoionization of
H O2(l) ⇌ H (aq) + OH (aq)
H O (l) + H O (l) ⇌ H O (aq) + OH (aq)
2 2 3
H 3 (aq) = hydronium ion; often abbreviated as H (aq)
OH (aq) = hydroxide ion
[H 3 ] = [OH] in pure water
Definitions of Acids and Bases
1) ACIDS: give [H O ] >3[OH] in solution (vinegar, lemon juice)
2) BASES: give [H O ] <3[OH] in solution (bicarb)
Understanding “acidity” and “basicity” involves knowing the [H ] and [OH] concentrations in
solution. Historically, the first definitions of acids and bases were the “Arrhenius
1) ACID = a substance with H in its formula, and which dissociates in water to give
H 3 (aq) (= H (aq))
Generic acid = HA (e.g., HCl, HNO , H SO3, et2.) 4
2) BASE = a substance with OH in its formula, and dissociates to yield OH
Generic base = MOH (e.g., NaOH, Ca(OH) , etc.) 2
Neutralization: the reaction between an acid and a base
H (aq) + OH (aq) → H O (l)2 ΔH rxn = 55.9 kJ
Strengths of Acids and Bases: (i.e., the amount of H or OH produced per mole of substance
STRONG ACIDS AND BASES DISSOCIATE COMPLETELY
(~100%) IN SOLUTION – MUST KNOW THEM
Strong Acids: HCl, HBr, HI, HClO , HNO ,4H SO 3plu2 fe4 rare ones
Strong Bases: MOH and M(OH) , where M2= Li , Na , K , Rb , Cs , Ca , Sr , Ba 2+ 2+ 2+
** A LL OTHER ACIDS AND BASES ARE WEAK **
Because strong acids and bases dissociate “completely” (i.e. ~100%), we do not consider them
equilibria, (i.e.,cK >>> 1) and we write them as a onedirectional reaction.
Strong Acid: HA (g or l) + H 2 (l) → H O 3(aq) + A (aq)
Strong Base: MOH (s) + H O (2) → M (aq) + OH (aq)
Weak Acids: dissociate only partially in solution; it is an equilibrium.
HA (aq) + H O2(l) ⇌ H O 3(aq) + A (aq)
**Amount of dissociation varies, depending on K of the acid **
Section 15.1 The BrønstedLowry Definition
The Arrhenius definition of acids and bases doesn’t cover all possibilities, e.g., some bases do
not contain OH in their formula. The BrønstedLowry definition is much better.
Acid: an H don . Must contain H in its formula (all BrønstedLowry acids are also
15-2 Base: an H accept . Must contain a lonepair capable of binding an H (e.g., NH , F, 3
OH, etc.). BrønstedLowry bases are not necessarily Arrhenius bases, but
Arrhenius bases contain the BrønstedLowry base, OH.
This gives acid/base reactions: something can act as a BronstedLowry acid if there is a
BronstedLowry base to pick up the H .
**The acid is the H donor; the base is the H acceptor**
e.g. HCl (g) + H O (l) → Cl (aq) + H O (aq)
NH (g) + H O (l) ⇌ NH (aq) + OH (aq)
3 2 4
HCl (g) + NH (g)3 → NH Cl4(s)
NH 4aq) + H O (l2 ⇌ NH (g3 + H O (3q)
3+ 2+ +
[Fe(H O2 ] 6(aq) + H O (l)2⇌ [Fe(H O) (OH)]2(aq5 + H O (aq) 3
NOTE: H O is 2mphiprotic – it can be a Bronsted acid or base.
POLYPROTIC ACIDS Very common.
Acids with two or more ionizable H: diprotic (H A), tripro2ic (H A), etc 3
Ionization occurs stepwise: e.g. triprotic phosphoric acid, H PO
H P3 (a4 ⇌ H )aq) + H PO (aq) 2 4
H 2O (a4) ⇌ H (aq) + PO (aq) 4
2 + 3
HPO (a4 ⇌ H (aq) + P (aq) 4
Note: Each ionization step is more difficult because it is more difficult to remove H from a +
molecule as its negative charge increases.
Similarly, PO i4 a triprotic base – it can pick up three H . Also note that H PO and HPO 2 4 42
are further examples of amphiprotic species.
15-3 Conjugate Acid/Base Pairs
The BL definition introduces the idea of conjugate acids and bases. Consider NH /NH . +
H 2 + NH 3 ⇌ HS + NH 4+
(acid) (base) (base) (acid)
Forward rxn: NH is 3he base/H S is2the acid.
Reverse rxn: NH i4 the acid/HS is the base.
We say that NH /NH4 are a3conjugate acid/base pair.
H 2 + NH ⇌3 HS + NH 4
**The acid has one extra H and thus one unit greater positive charge (or one less negative
Similarly, H 2/HS are a conjugate acid/base pair.
“ NH 4s the conjugate acid of NH ” 3
“ NH 3s the conjugate base of NH ” 4+
“ HCl is the conjugate acid of Cl “
“ Cl is the conjugate base of HCl ”
“ MeCOOH is the conjugate acid of MeCOO “
(acetic acid) (acetate)
“MeCOO is the conjugate base of MeCOOH”
15-4 Sections 15.2 and 15.3. The pH Scale
Remember that even pure water contains H and OH ions from selfionization. 2H O (l) ⇌ 2
H O (aq) + OH (aq)
[H O + ][OH − ]
K = 3 Note that [H O] is (essentially) constant,
c [H O] 2 2
∴ K cH O]2 = [H O ] 3OH]
Putting K =wK [H c] g2ves K w= [H O 3 [OH]
K w= ionization constant for water
K w= [H O 3[OH] = 1.0 x 10 (at 25 ºC)
∴ in pure water, [H O ] = [OH] = 1.0 x 10 M 7
** The K equwtion tells us that the product [H O ][OH ] is a constant3(= 1.0
x 10 at 25 ºC). If one goes up, the other must come down **
∴ if [H O 3 increases, [OH] decreases
if [H O ] decreases, [OH] increases
This gives us the way to define “acidic” and “basic” in terms of [H O ] and [OH3 that was
[H O ] > [OH ] acidic solution
[H O3] > [HA], so it changes negligibly when HA dissociates
∴ we put it with K cnd we have
[H O + ] [A ]
K cH O2 = K a 3 (omitting (aq) for convenience)
The bigger is K , the stronger is
[H O + ] [A ] a
∴ K = 3 the acid HA; see Table 15.3. The
a [HA] biggerais K , the greater the dissociation.
Part of Table 15.3
HA Name K a
HCl hydrochloric acid very large
HF hydrofluoric acid 7.2 x 10
PhCOOH benzoic acid 6.3 x 10
MeCOOH acetic acid 1.8 x 105
HCN hydrocyanic acid 4.0 x 10
PhOH phenol 1.0 x 1010
15-8 Weak Bases: none are Arrhenius bases, because they do not have OH in formula. Most have
a N atom with a lone pair (e.g. H N:,3R HN:,2etc)
They react with H O to give OH – that’s why they are bases!
B (aq) + H O2(l) ⇌ BH (aq) + OH (aq)
Their equilibrium constant is called the Base Ionization Constant (K ) b
Part of Table 15.3
B Name K b
Cl chloride ion very small
F fluoride ion 1.4 x 10 11
PhCOO benzoate ion 1.6 x 10 10
MeCOO acetate ion 5.6 x 10
CN cyanide ion 2.5 x 10
PhO phenoxide ion 1.0 x 10 4
Note: These are conjugate bases of the acids on the previous page.
The stronger is a weak acid, the weaker is its conjugate base as a base.
Section 15.7 Relationship Between K and K a b
K = acid dissociation constant (e.g. for NH ) +
K b= base dissociation constant (e.g. for NH ) 3
Since NH a3d NH are 4 conjugate acid/base pair, it is not s