Acids & Bases
Part I: Some Definitions
Jespersen Chap. 16
Sec 1 & 2
Dr. C. Yau
Fall 2014
1
Different Definitions of Acid/Base
Arrhenius Definition:
An acid is a substance that increases
the H3O+ concentration when placed in
water.
HCl (g) + H2O (l)
H3O+(aq) + Cl-(aq)
A base is a substance that increases
the OH- concentration when placed in
water.
NaOH (s)
Na+(aq) + OH-(aq)
2
Different Definitions of Acid/Base
Brønsted-Lowry Definition:
An acid is a proton donor.
..
H O
.. H
+
water acting
as a base
..
H Cl :
..
H
+
H O
.. H
.. _
: Cl
.. :
+
proton donor
A base is a proton acceptor.
_
..
:O
.. H
proton
acceptor
+
..
H Cl :
..
H
:O
.. H
+
.. _
: Cl
.. :
Typically a Brønsted base has
a lone pair to attract the proton.
3
Brønsted-Lowry Definitions
Why do we need new definitions?
The Arrhenius definitions were based on
placing substances in water, but not all
reactions are in water.
e.g. NH3 (g) + HCl (g)
NH4Cl (s)
This is an acid-base reaction and yet no
H3O+ nor OH- ions are involved.
However, proton transfer is involved.
Write the Lewis structures for the
reaction above and draw in curved
arrows to show the flow of electrons.
4
Brønsted-Lowry Acids/Bases
Organic carboxylic acids have –COOH group
attached. Acetic acid has the structure…
H O
H C C O H
Which H will be
donated as a proton?
H
Recognize all the different ways of writing its
formula:
HC2H3O2 CH3COOH CH3CO2H
Remember: -COOH does NOT mean C-O-O-H
5
Write the equation using Lewis structures
and curved arrows to show the reaction of
acetic acid with ammonia.
Which is the proton donor, which is the
proton acceptor? Which is the proton
being transferred?
6
Conjugate Acids & Conjugate Bases
We will abbreviate "Brønsted-Lowry" to just
"Brønsted."
Write the equation for the reaction of acetic
acid and ammonia using molecular formulas:
HC2H3O2 + NH3
(This is ammonium acetate.)
HC2H3O2 + NH3
C2H3O2- + NH4+
conjugate conjugate
acid
base
base
acid
How is the acetate a base? and NH4+ an acid?
7
Identify the Conjugate Partner for Each
The clue is to remember what the
functions of Brønsted acids and bases
are.
conjugate base
conjugate acid
HCl
NH3
HC2H3O2
CNHF
8
8
Conjugate Acid-Base Pairs
Write the equation for the reaction of formic
acid with water using molecular formulas:
HCO2H + H2O
Identify which is the acid, base, conjugate
acid, conjugate base.
Give the conjugate acid-base pairs:
9
Conjugate Acid-Base Pairs
Example 16.1 p. 744
What is the conjugate base of nitric acid and
what is the conjugate acid of the hydrogen
sulfate ion?
Do Pract Exer 1, 2, 3 on p. 744
10
Conjugate Acid-Base Pairs
Example 16.2 p. 745
The anion of sodium hydrogen sulfate reacts as
follows with the phosphate ion. Identify the 2
conjugate acid-base pairs.
HSO4 (aq) + PO43 (aq)  SO42 (aq) + HPO42 (aq)
Do Pract Exer 4, 5, 6 p. 745.
11
Amphoteric Substances
An amphoteric substance is one that can
act either as an acid or a base.
Water is such a substance.
Although it is exists primarily as a
molecule, there are some ions present
due to the self-ionization of water:
HOH + HOH
H3O+ + OHWrite the equation for water reacting with
ammonia.
Write the equation for water reacting with
hydrochloric acid.
12
Amphoteric Substances
Write the equation for the reaction of the
bicarbonate ion with hydroxide.
Write the equation for the reaction of the
bicarbonate ion with hydronium ion.
Why is bicarbonate considered amphoteric?
Do Pract Exer 7, 8 p. 746
13
Acid and Base Strengths
First recall what an acid is supposed to
do…. proton donor… gives away a
proton.
A strong acid is one that can willingly
"gives away" its proton.
HCl + H2O 100% H3O+ + ClA weak acid is one that "hangs on" to its
proton.
HC2H3O + H2O
H3O+ + C2H3O214
Acid and Base Strengths
Recall what a base is supposed to do…
proton acceptor…. grabs up protons.
A strong base is one that "greedily grabs"
protons.
NH2- + H2O
NH3 + OHA weak base is one that does not
particularly want to grab a proton.
Cl- + H2O
HCl + OH15
H3O+ and OHH3O+ is the strongest acid that can exist in
water.
Note that mineral acids (nonorganic acids)
such as HCl, HNO3, H2SO4, are stronger
acids, but as soon as they are placed in
water, they dissociate 100% and no
longer exist in water as such.
They become H3O+ and the corresponding
anions.
16
H3O+ and OHOH- is the strongest base that can exist in
water.
O2- is a stronger base but as soon as it
goes into the water, it is converted to
OH- and does not exist as O2-.
17
Relationship between strengths
of acid and conjugate base
HC2H3O + H2O
H3O+ + C2H3O2-
Identify the acid, base, conjugate acid,
conjugate base.
Is the acid strong or weak?
Is its conjugate base strong or weak?
Is the base strong or weak?
Is the conjugate acid strong or weak?
18
Relationship between strengths
of acid and conjugate base
HC2H3O + H2O
H3O+ + C2H3O2-
The pattern we see is…
A weak acid produces a strong conjugate
base.
A strong acid produces a weak conjugate
base.
A weak base produces a strong conjugate
acid, & a strong base produces a weak
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conjugate acid.
Relationship between strengths
of acid and conjugate base
HC2H3O + H2O
H3O+ + C2H3O2-
Another pattern we see is that the position
of the equilibrium lies in favor of the weaker
acid and base.
REMEMBER: Equilibrium goes
from Strong acid to Weaker acid.
20
Example 16.3 p. 749
In this rxn, will the position of equilibrium lie
to the left or the right, given the fact that
acetic acid is known to be a stronger acid
than the hydrogen sulfite ion?
HSO3(aq) + C2H3O2 (aq)
HC2H3O2 (aq) + SO32 (aq)
Do Pract Exer 9, 10 p. 750
21
Which is the stronger acid?
Which “acids” are we talking about?
How can we tell which is the stronger acid?
22