Acids & Bases have distinctive properties…
Properties of Acids and BasesACIDS● Have a sour taste
● Change the color of many indicators
● Are corrosive (react with metals)
● Neutralize bases
● Conduct an electric current
BASES● Have a bitter taste
● Change the color of many indicators
● Have a slippery feeling
● Neutralize acids
● Conduct an electric current
● An acid is a compound that contains an H+, and produces hydrogen ion (H+) when dissolved in water.
Example: HCl H+ + Cl-
HCl + H2O H3O+(aq) + Cl-
(aq)
Hydronium Ion
(can be used interchangeably with H+)
● A base is a compound that contains an OH- group and produces hydroxide ions (OH-) when dissolved in water.
Example: NaOH Na+ + OH-
Neutralization● Neutralization: the combination of H+ with
OH- to form water.
● H+(aq) + OH-
(aq) H2O(l)
One problem with the Arrhenius theory is that it’s not comprehensive enough. Some compounds act like acids and bases that don’t fit the standard definition.
Bronsted-Lowry Theory of Acids & Bases:● An ACID is a proton (H+) donor
● A BASE is a proton (H+) acceptor
HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq)
Proton transfer
Acid
Base
NH3(aq) + H2O(l) NH4+
(aq) + OH-(aq)
BASE
ACID
CONJUGATE ACID
CONJUGATE BASE
Ammonia is a proton acceptor, and thus a
base
Water is a proton donor, and thus an
acid.
Conjugate acid-base pairs
● Conjugate acid-base pairs differ by one proton (H+)
**A conjugate acid is the particle formed when a base gains a proton.
**A conjugate base is the particle that remains when an acid gives off a proton.
Examples: In the following reactions, label the conjugate acid-base pairs:
H3PO4 + NO2- HNO2 + H2PO4
-
CN- + HCO3- HCN + CO3
2-
acid base c. acid c. base
acidbase c. acid c. base
Examples: In the following reactions, label the conjugate acid-base pairs:
HCN + SO32- HSO3
- + CN-
H2O + HF F- + H3O+
acid base c. acid c. base
acidbase c. acidc. base
● A substance that can act as both an acid and a base (depending on what it is reacting with) is termed AMPHOTERIC.
**Water is a prime example.
The Lewis Theory of Acids & Bases
● Lewis acid: a substance that can accept an electron pair to form a covalent bond (electron pair acceptor).
● Lewis base: a substance that can donate an electron pair to form a covalent bond (electron pair donor).
Example 1:Ionization of NH3:
NH3 + H2O NH4+ + OH-
N NH
H
H O HH+ H
H
H
H H+ O.. .....
.
.
.
+-
acidbase
Example 2:Auto-ionization of water:
H2O + H2O H3O+ + OH-
O OH H O HH+ H
HH H+ O.. .
.
.
...
.
.
+-
acidbase
.
.
.
.
Example 3:Reaction of NH3 with HBr (a Lewis AND
a Bronsted-Lowry acid-base reaction):
NH3 + HBr NH4+ + Br-
N NH
H
H BrH+ H
H
H
H + Br..
.
.
.
...
.
.
+-
acidbase
Lewis
Bronsted-Lowry
Arrhenius
SUMMARY OF ACID-BASE THEORIES:
Theory Acid Definition Base Definition
Arrhenius Theory
Any substance which releases H+ ions in water solution.
Any substance which releases OH- ions in water solution
Brǿnsted-Lowry Theory
Any substance which donates a proton (H+).
Any substance which accepts a proton (H+).
Lewis Theory
Any substance which can accept an electron pair.
Any substance which can donate an electron pair.