Acid-Base Chemistry
Today’s Class
•Definitions
•Arrhenius
Theory
•Bronsted-Lowry Theory
•Amphoteric/Amphiprotic Substances
•Polyprotic Acids
•Worksheet for practice
1661 - Robert Boyle
Characterized acids
as: corrosive, sour
tasting, changes
litmus from blue to
red, and becomes
less acidic when
mixed with base.
Characterized bases
as: slippery feeling,
turns litmus from red
to blue, and becomes
less basic when
mixed with acid.
Antoine Lavoisier
Believed that all
acids contained
oxygen after
studying several
acids.
Examples: Sulfuric
acid (H2SO4) and
Nitric acid (HNO3)
1811 - Humphry Davy
Challenged
Lavoisier’s belief
that all acids
contained oxygen
by noting that
Hydrochloric acid
(HCl) did not
contain oxygen but
still was an acid.
Other non-oxygen
acids were also
found
HBr, HF, HI.
•Arrhenius proposed that water could
dissolve many substances into their
individual ions.
•He suggested that acids are compounds
that contain hydrogen and can dissolve
into water to release hydrogen ions into
solution.
•He defined bases as substances that
dissolve in water to release hydroxide ions
into solutions.
•The Arrhenius definition also explains
Boyle’s observation that acids and bases
counteract each other. This idea that a base
can make an acid weaker, and vice versa
was later found to be called neutralization.
Arrhenius Definition

Acid = proton donor
HA = H+ + A-

Base = hydroxide donor
BOH = B+ + OH-

Dilemma: NH3
Dilemma
•
Though Arrhenius helped explain the
fundamentals of acid/base chemistry,
unfortunately his theories have limits. For
example, the Arrhenius definition does not
explain why some substances, such as
common baking soda (NaHCO3) or
ammonia(NH3), can act like a base even
though they do not contain hydroxide ions.
•Brønsted and Lowry separately proposed a new set of
definitions for acids and bases which are known as either
Brønsted acids and bases or Brønsted-Lowry acids and bases.
•For Acids: Any substance that can donate a proton, H+ ion to
a base. (Proton donor, or hydrogen-ion donor)
•For Bases: Any substance that can accept a proton, H+ ion
from an acid. (Proton acceptor or hydrogen-ion acceptor)

The Brønsted-Lowry model of acid and bases
helps develop the concept of Conjugate acid-base
pairs. The part of the acid remaining when an acid
donates a proton is called the conjugate base. The
Acid formed when a base accepts the proton is
called the conjugate acid.
Strong acids have weak conjugate bases.
Strong bases have weak conjugate acids.
Bronsted-Lowry Definition

Acid = proton donor

Base = proton acceptor
NH3 + H+ = NH4+
Amphoteric Substances
•
•
An Amphoteric substance can act as either an acid or a
base depending on their surroundings
Examples include: Water, Al(OH)3 and Be(OH)2
Amphiprotic Substances
•An amphiprotic substance can accept and donate
protons
•Examples include: Water, hydrogen
carbonate(HCO3-) and hydrogen sulfate(HSO4-)
Polyprotic Acids
Are acids that are able to donate more than one
proton per acid molecule
• Example H2SO4- Sulfuric acid
H2SO4 + H2O
HSO4- + H3O+ Ka1= very large
HSO4- + H2O
SO42- + H3O+ Ka2=1.3X10-2
• Ka1 is greater than Ka2
• Polyprotic acids can be diprotic(able to donate 2
protons), triprotic(able to donate 3 protons) or can
be more.
•
Conjugate acid-base example
Consider the reaction of acid hydrogen fluoride with
water.
HF+H2O
HF is donating its proton to the water molecule. In the
reaction a proton is transferred from HF to H2O :
HF+H2O
H3O+ + FH2O is acting as a base in the reaction because it is
accepting a proton. After it accepts the proton the
water becomes a hydronium ion. It now has a proton,
which it can donate, therefore it is an acid.
Example continued
•
•
•
•
When a base accepts a proton, it becomes an acid
because it now has a proton that it can donate.
When an acid donates a proton it becomes a base,
because it now has room to accept a proton.
These are what we call conjugate pairs of acids
and bases
So in this example the HF and F- are a conjugate
acid-base pair and H2O and H3O+ are a conjugate
acid-base pair.
Another Example
•
H2SO4 + H2O
•
So in this example H2SO4 is the acid so it will get
rid of a proton and form a conjugate base which is
willing to accept a proton: HSO4H2O is acting as the base so when it accepts a
proton it becomes a conjugate acid: H3O+
So the full reaction can be represented by:
H2SO4 + H2O
HSO4- + H3O+
•
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