Lesson 14.1 Acid-Base Concepts
Suggested Reading
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Zumdahl Chapter 14 Section 14.1 & 14.11
Essential Question

What are the basic properties of acids and bases?
Learning Objectives:




Define acids and bases according to the Bronsted & Lewis theories.
Deduce whether or not a species could act as a Bronsted &/or Lewis
acid or base.
Deduce the formula of the conjugate acid (or base) of any Bronsted
acid (or base).
Outline the characteristic properties of acids and bases in aqueous
solutions.
Introduction
Read the following introduction to acid-base chemistry.
Review of the Arrhenius Concept of Acids and Bases
In 1884, the Swedish chemist Svante Arrhenius proposed two specific
classifications of compounds, termed acids and bases. When dissolved in
an aqueous solution, certain ions were released into the solution. Recall
from lesson 4.4 on 9/24/12:
Arrhenius Acid: a substance that produces hydrogen ions, H+, when it
dissolves in water.
An example of an Arrhenius acid is nitric acid, HNO3, which dissolves in
water to give H+ and NO3-.
HNO3(aq) → H+(aq) + NO3-(aq)
Arrhenius Base: a substance that produces hydroxide ions, OH-, when it
dissolves in water. Sodium hydroxide is an example.
NaOH(s) → Na+ + OHThe molecular substance ammonia, NH3, is a base in the Arrhenius view
because it yields hydroxide ions when it reacts with water.
NH3(aq) + H2O(l) ↔ NH4+(aq) + OH-(aq)
Limitations of Arrhenius definitions:
1. Limits bases to compounds containing OH- ions.
2. Limits acids and bases to compounds.
3. Applies only to aqueous solutions.
Review of the Bronsted-Lowry Concept of Acids and Bases
Recall from lesson 4.4 (chapter 4):
In 1923, Johannes Bronsted and Thomas Lowry independently noted that
many reactions involve nothing more than a proton (H+) transfer. They
realized that they could use this idea to expand the definitions of acids and
bases to describe a large class of reactions. In this view, acid-base
reactions are proton-transfer reactions. This leads us to the BronstedLowry definitions of acids and bases.
Bronsted-Lowry acid: the species (molecule or ion) that donates a proton
to another species in a proton-transfer reaction.
Bronsted-Lowry base: the species (molecule or ion) that accepts a proton
in a proton-transfer reaction.
The dissolution of a strong acid, such as HCl or HNO3, in water is actually a
proton-transfer reaction.
New Content:
In any reversible acid-base reaction, both the forward and the reverse
reactions involve proton transfers. Consider the reaction of NH3 with H2O.
In the forward reaction, NH3 accepts the proton so it is the base and water
is the acid. However, in the reverse reaction NH3 donates the proton and
water accepts the protons. Thus, in the reverse reaction NH4+ is the acid
and OH- is the base. The species NH4+ and NH3 are a conjugate acid-base
pair. A conjugate acid-base pair are two species related to each other by
the donating an accepting of a proton. The acid in such a pair is called the
conjugate acid of the base, whereas the base is the conjugate base of
the acid. In the example above, NH4+ is the conjugate acid NH3, and NH3 is
the conjugate base of NH4+. Take another look at the diagram above.
Example: Identifying Acid and Base Species
In the reaction between acetic acid and water, label each species as an
acid or a base. Show the conjugate acid-base pairs.
The Bronsted-Lowry concept of acids and bases has a greater scope than
the Arrhenius concept. In the Bronsted concept:
1. A base is a species that accepts protons (Thus, OH- is only one
example of a base).
2. Acids and bases can be ions as well as molecules.
3. Acid-base reactions are not restricted to aqueous solutions.
4. Some species can act as either acids or bases depending on what
the other reactant is.
Lewis Concept of Acids and Bases
Certain reactions have characteristics of acid-base reactions but do not fit
the Bronsted concept. G. N. Lewis who proposed the electron-pair theory of
covalent bonding therefore realized that the concept of acids and bases
could be further generalized beyond the Arrehnius concept to include
reactions where there is no proton transfer. According his concept, a Lewis
acid is a species that can form a covalent bond by accepting an electron
pair from another species. A Lewis base is a species that can form a
covalent bond by donating an electron pair to another species.
The Lewis concept embraces many reactions that we might not otherwise
think of as acid-base reaction. The reaction of boron trifluoride with
ammonia is an example.
The formation of complex ions can also be looked at as a Lewis acid-base
reaction. Complex ions are formed when a metal bonds to electron paris
from molecules such as H2O or NH3, or from anions such as CN-. An
example of a complex ion is Al(H2O)63+. Hydrated ions like this are present
in compounds (hydrates) and in aqueous solutions. The formation of a
hydrated metal ions involves a Lewis acid-base reaction.
Characteristic Properties
ACIDS:
BASES:
Sting a wound.
Slippery feel.
Taste sour.
Taste bitter.
Colorless in
phenolphthalein.
Pink in
phenolphthalein.
Red on blue litmus
paper (a pH
indicator).
Blue on red litmus
paper (a pH
indicator).
pH<7.
pH>7.
Produce hydrogen
gas when reacted
with metals.
Produce carbon
dioxide when
reacted with
carbonates.
Examples: Lemons,
oranges, vinegar,
urine, sulfuric acid,
hyrdrochoric acid
Examples: Soap,
toothpaste, bleach,
cleaning agents,
limewater, ammonia
water, sodium
hydroxide.
HOMEWORK: Practice exercises 16.1-16.3. ***For question 16.3 use the
acid/base strength chart on page 4 in your notes.