Acid/Base Properties
• In the past, we have classified acids and
bases according to their observed properties
ACIDS
BASES
• Sour taste
bitter taste
• Watery feel
slippery feel
• Reactive with metals
can be corrosive
• Water soluble
may be soluble in
H2O
Extending Acid/Base Theories
Arrhenius Theory (Svante Arrhenius 1859-1927)
•Proposed that when salts dissolve in water, they
dissociate completely into ions
Acids- dissolve in water to produce H+(aq) ions
Eg. HCl, HNO3 , H3PO4 , H2SO4 , HF
Bases- dissolve in water to produce OH-(aq) ions
Eg. NaOH , Ca(OH)2 , Mg(OH)2
Neutralization- acid
+
base
 water
+
a salt
HCl(aq) + NaOH(aq)  HOH(ℓ) + NaCl(aq)
Extending Acid/Base Concepts
Although Arrhenius theory is very successful and useful for
explaining acid/base chemistry, it has limitations
WHY is Na2CO3(aq) BASIC?!?
Bronsted-Lowry Theory
(1923 Johannes Bronsted, Thomas Lowry)
• Independently noted that some acid-base
neutralization reactions can occur without
dissolving ions in water
Eg. HCl(g) + NH3(g)  NH4Cl(s)
• They concluded that acid/base reactions simply
involve the transfer of a hydrogen ion from one
molecule to another
Extending Acid/Base Concepts
According to Bronsted-Lowry definitions:
ACIDS- are proton (H+) donors
BASES- are proton (H+) acceptors
Comparing
Acid
theories
Base
Arrhenius Produces H+ in Produces OH– in
(grade 11)
solution
solution
HCl  H+ + Cl– NaOH  OH– + Na+
BronstedLowry
H+ donor
H+ acceptor
HCl + NaOH  H2O + NaCl
Extending Acid/Base Concepts
Bronsted-Lowry theory could be used to explain neutralization
reactions that could not be explained by Arrhenius theory.
H
H
+
H
N + H
Cl
H
H
N
H
+
Cl
H
HCl is an acid because it donates H+, NH3
accepts H+ and therefore is the base
(NH4+ and Cl– then form an ionic compound)
So, Na2CO3 is basic because…
CO32-(aq) + H2O  HCO31-(aq) + OH1-(aq)
Strong vs. Weak Electrolytes
• DEMO conductivity of acids and bases
1.0 mol/L HCl(aq) and 1.0 mol/L CH3COOH(aq)
1.0 mol/L NaOH(aq) and 1.0 mol/L NH3(aq)
• The conductivity of these solutions varies eventhough
they are all of the same concentration
• Substances that dissolve in water and completely
dissociate into ions are called strong electrolytes
• Acids/Bases that dissolve in water and completely
dissociate into ions are called strong acids/bases
Strong vs. Weak Electrolytes
• Some strong acids and bases:
Hydronium ion
HCl(aq) + H2O(ℓ)  H3O+(aq) + Cl1-(aq)
NaOH(s) + H2O(ℓ)  Na1+(aq) + OH1-(aq)
fully dissociated!
Strong vs. Weak Electrolytes
• Some substances are highly water soluble,
yet they do not fully dissociate into ions upon
dissolution. These substances are
weak
electrolytes
• Acids and bases that do not fully dissociate
into ions upon dissolving are weak
acids/bases
• When weak acid/bases are dissolved in
water, a chemical equilibrium is established
in the solution
Strong vs. Weak Electrolytes
• Weak acid/base equilibria(CLICK HERE!)
CH3COOH(aq) + H2O(ℓ)  H3O+(aq) + CH3COO1-(aq)
NH3(aq) + H2O(ℓ)  NH4+(aq) + OH1-(aq)
• The reactant sides of these equilibria are favoured, so complete
dissociation of the acid/base molecules does not occur.
Oxyacids
• Oxyacids are mineral acids which contain oxygen
combined with another non-metal
• Eg. HNO3(aq), H2SO4(aq), H3PO4(aq)
Monoprotic
diprotoic
triprotic
• The relative strength (ability to donate protons) of an
oxyacid increases with the number of oxygen atoms that
are not bonded to an hydrogen atom in the acid molecule.
• Eg. H2SO4(aq) sulfuric acid is stronger than H2SO3(aq)
Oxyacids
• The ionization of polyprotic acids can be
expressed using more than one chemical
equation. Eg. H3PO4(aq) ionization:
• H3PO4(aq) + H2O(ℓ)  H3O+(aq) + H2PO41-(aq)
• H2PO41-(aq) + H2O(ℓ)  H3O+(aq) + HPO42-(aq)
• HPO42-(aq) + H2O(ℓ)  H3O+(aq) + PO43-(aq)
Conjugate Acid/Base Pairs
• In any equilibrium expression involving
1-(aq) is the conjugate base of CH3COOH(aq)
CHa3COO
weak acid/base more than one
+(aq) is the conjugate acid of H2O(ℓ)
H3O
Bronsted/Lowry
acid and base can be
identified
CH3COOH(aq) + H2O(ℓ)  H3O+(aq) + CH3COO1-(aq)
Conjugate acid/base pair
Conjugate acid/base pair- two substances that differ from
each other by just one proton (H+).
Conjugate Acid/Base Pairs
HCN(l) + H2O  CN–(aq) + H3O+(aq)
Forward: HCN is acid, H2O is base
Reverse: H3O+ is acid, CN– is base
Relative strengths of conjugate acid/base pairs
•The conjugate base of a weak acid must be a strong base
•The conjugate base of a strong acid must be a weak base
•The conjugate acid of a weak base must be strong acid
•The conjugate acid of a strong base must be a weak acid
Q. Explain whether you would expect a solution of NH4Cl(aq) to be
acidic or basic.
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Amphoterism
• Any substance that can behave either as an acid
or a base is amphoteric (amphiprotic)
Eg. Bicarbonate buffer in blood, HCO31-(aq)
HCO31- + OH1-  H2O + CO32- (as an acid)
HCO31- + H3O+  H2CO3 + H2O (as a base)
Homework
p.532 #1, 2
p.549 #2, 3
Read p.532 to 549 for
next class