Acids & Bases
Three definitions
Arrhenius
Acids are substances that, when
dissolved in water, increase the
concentration of H+ ions. Likewise, bases
are substances that, when dissolved in
water, increase the concentration of OHions.
Bronsted-Lowry
An acid is a substance that can donate a
proton to another substance while a base
is a substance that can accept a proton.
Lewis
An acid is an electron pair acceptor
while a base is an electron pair donor.
Arrhenius vs Bronsted-Lowry
HCl( g ) + H 2O( l ) → H 3O + ( aq ) + Cl − ( aq )
HCl( g ) + NH 3 ( l ) → NH 4+ ( l ) + Cl − ( g )
An acid and a base that differ only in the
presence or absence of a proton are
called a conjugate acid-base pair.
HNO2 ( aq ) + H 2O( l ) U NO2− ( aq )
acid
base
conjugate base
NH 3 ( aq ) + H 2O( l ) U
base
acid
+
NH 4+ ( aq )
H 3O + ( aq )
conjugate acid
+
OH − ( aq )
conjugate acid conjugate base
Conjugate Acid & Base
Acid
Base
HClO4
?
H2S
?
PH 4+
?
HCO3−
?
Base
Acid
CN -
?
SO4−
?
H 2O
?
HCO3−
?
Autoionization of Water
H 2O( l ) + H 2O( l ) U H 3O + ( aq ) + OH − ( aq )
Keq = K w = [ H 3O + ][ OH − ] = [ H + ][ OH − ]
K w = 1.0 x10 −14
In pure water
[ H + ] = [ OH − ] = 1.0 x10−7
+
pH = − log[ H ]
In pure water
pH = 7
Strong Acids & Bases
Acid
Name
HCl
HBr
hydrochloric
hydrobromic
HI
HNO3
hydroiodic
nitric
HClO3
chloric
HClO4
perchloric
HNO3 ( aq ) + H 2O( l ) → H 3O + ( aq ) + NO3− ( aq )
HNO3 ( aq ) → H + ( aq ) + NO3− ( aq )
Base
NaOH
Name
sodium hydroxide
KOH
Ca(OH)2
potasium hydroxide
calcium hydroxide
Na2O
sodium oxide
CaO
calcium oxide
H-
hydride ion
N -3
nitride ion
Ca( OH )2 ( aq ) → Ca +2 ( aq ) + 2OH − ( aq )
CaO( s ) + H 2O( l ) → Ca +2 ( aq ) + 2OH − ( aq )
N −3 ( aq ) + 3H 2O( l ) → NH 3 ( aq ) + 3OH − ( aq )
Weak Acids
HA( aq ) + H 2O( l ) U A− ( aq ) + H 3O + ( aq )
[ A− ][ H + ]
K eq = K a =
[ HA]
K a = acid dissociation constant
pH determines Ka
or
Ka determines pH
Calculating Ka from pH
a. Given that a 0.1 M solution of formic
acid has a pH of 2.38, calculate Ka
b. What % of the acid is dissociated?
Calculating pH from Ka
a. Given that the Ka for acetic acid is
1.8x10-5, what is the pH of a 0.30M
solution?
b. What % of the acid is dissociated?
Effect of Dilution on % Dissociation
Polyprotic Acids
More than one ionizablbe H atom
H 2 SO3 ( aq ) U H + ( aq ) + HSO3−
HSO3− ( aq ) U H + ( aq ) + SO32− ( aq )
K a1=1.7x10 -2
K a2 =6.4x10 -8
The solubility of CO2 in pure water 250C
and 0.1 atm pressure is 0.0037 M.
CO2 ( aq ) + H 2O( l ) U H 2CO3 ( aq )
? pH
H 2CO3 ( aq ) U H + ( aq ) + HCO3− ( aq )
K a1=4.3x10 -7
HCO3− ( aq ) U H + ( aq ) + CO32− ( aq )
K a2 =5.6x10 -11
Predict whether the equilibrium is
predominantly to the left or to the right
HSO4− ( aq ) + CO32 − ( aq ) U SO42− ( aq ) + HCO3− ( aq )
HPO42 − ( aq ) + H 2O( l ) U H 2 PO4− ( aq ) + OH − ( aq )
NH 4+ ( aq ) + OH − ( aq ) U NH 3 ( aq ) + H 2O( l )
Weak Bases
B( aq ) + H 2O( l ) U BH + ( aq ) + OH − ( aq )
A solution is made by adding solid
sodium hypochlorite (NaClO) to water to
make a 2.0L solution. If the solution has
a pH of 10.5, how many moles of
(NaClO) were added to the water?
ClO − ( aq ) + H 2O( aq ) U ClOH ( aq ) + OH − ( aq )
[ HClO ][ OH − ]
−7
K eq =
.
x
=
3
33
10
[ ClO − ]
The solubility of CO2 in pure water at
250C and 0.1 atm pressure is 0.0037 M.
CO2 ( aq ) + H 2O( l ) U H 2CO3 ( aq )
? pH
H 2CO3 ( aq ) U H + ( aq ) + HCO3− ( aq )
HCO3− ( aq ) U H + ( aq ) + CO32 − ( aq )
K a1=4.3x10 -7
K a2 =5.6x10 -11
Three Broad Classes of Acids
1. Binary Acids
H atom + one other element; H-X
HF ,HCl,H 2O,H 2 S ,etc
2. Oxyacids
One or more OH groups attached to a
central atom. Often additional oxygen
atoms will be attached to the central
atom.
HOCl ,HOClO,HOClO2 ,HOClO3 ,H 2 SO4 ,H 2 SeO3 ,etc
3. Carboxylic acids
Contain the carboxyl group
Representative Questions
1. Make an aqueous solution of each of
the following
NaCl, NH4Cl, & NaClO
Which solution is acidic, basic or neutral?
2. Dissolve Na2HPO4 in water.
Is the solution acidic, basic or neutral?
3. An unknown salt is either NaF, NaCl,
or NaOCl. A solution of 0.05 mol of salt
in 0.50 L of H2O has a pH=8.08.
Which salt is it?