Chapter 18 Notes
Acids and Bases: An Introduction
Acids are found in food, your stomach, and the
environment. Bases are found in soap, household
cleaners, and antacid tablets. Acids often taste
sour, are corrosive, and will turn blue litmus paper
(an indicator) red. Acids react with most metals to
give off H2 gas (remember all those labs…). Bases
often taste bitter, feel slippery, are corrosive, and will
turn red litmus paper blue. Acids typically come
from gases that are dissolved in water. Bases
typically come from crystalline solids and are
dissolved in water.
Common acids:
Citrus fruits have citric acid.
Yogurt has lactic acid.
Vinegar is acetic acid.
Your stomach uses hydrochloric acid.
Car batteries use sulfuric acid.
Some soft drinks use phosphoric acid.
Nitric acid is used to make fertilizer.
Common bases:
Glass cleaner may contain ammonia.
Draino uses sodium hydroxide.
Tums uses calcium hydroxide.
Milk of magnesia uses magnesium hydroxide.
Baking soda is sodium bicarbonate.
Most common fertilizers use ammonium nitrate.
Ammonia is used to make fertilizer.
What makes an acid an acid or a base a base can
vary depending on definition being used. The first
definition was created by Svante Arrhenius in 1883.
Arrhenius said compounds that will ionize to give off
H+ ions (makes H3O+ - hydronium ions) are acids and
compounds that will ionize to give off OH- ions are
bases. (Of course to ionize they must be dissolved
in water - aquated!)
HCl → H+ + ClNaOH → Na+ + OH-
HCl → H+ + ClACID
NaOH → Na+ + OHBASE
With the Arrhenius definitions only compounds with
hydrogen can be acids and only compounds with
hydroxide can be bases. This definition sometimes
leaves out compounds that also seem to act like
bases, so two other chemists came up with a new
definition for a base. The Bronsted-Lowry definition
for a base is that it is an H+ acceptor. The definition
of an acid is still the same as for Arrhenius - an acid
gives off H+
HCl + NH3 → NH4+ + Cl-
ACID
BASE
HCl + NH3 → NH4+ + Cl-
In the above reaction the HCl gives off H+ and the
NH3 takes the H+. Therefore, HCl is an acid and NH3
is a base. Now watch what happens when the
reaction above is reversed:
NH4+ + Cl- → HCl + NH3
ACID
BASE
NH4+ + Cl- → HCl + NH3
Now the NH4+ gives off H+ (is an acid) and Claccepts the H+ (is a base). Notice how the HCl starts
as an acid and makes Cl- which is a base, and NH3
starts as a base and makes NH4+ which is an acid.
This makes HCl and Cl- and NH3 and NH4+ conjugate
acid-base pairs.
Another interesting conjugate acid-base effect
happens with water.
HCl + H2O → H3O+ + ClNH3 + H2O → NH4+ + OH-
In the first reaction water acts as a base to accept
the H+ from HCl, and in the second reaction water
acts as an acid to give H+ to NH3. Substances that
can act as both an acid and base are called
amphoteric.
A third definition for acids and bases is often useful
– a Lewis acid is an electron pair acceptor and a
Lewis base is an electron pair donator.
H
H–N:
H
H
H
H
B–H → H–N–B-H
H
H
H
A third definition for acids and bases is often useful
– a Lewis acid is an electron pair acceptor and a
Lewis base is an electron pair donator.
H
H–N:
H
Base
H
H
H
B–H → H–N–B-H
H
Acid
H
H
There are also acids and bases that can act as an
acid or base more than one time. For example:
H3PO4 + H2O → H2PO4- + H3O+
H2PO4- + H2O → HPO4-2 + H3O+
HPO4-2 + H2O → PO4-3 + H3O+
Acids that can give off more than one H+ (protons –
make more than one hydronium ion) are called
polyprotic. H3PO4 is a triprotic acid, and H2SO4
would be a diprotic acid.
Strengths of Acid and Bases
The strength of an acid or base has nothing to do
with the concentration. A strong acid is a compound
where 100% of all the little molecules will ionize. A
weak acid is a compound where much less than
100% of all the little molecules will ionize. Thus a
strong acid falls apart completely and a weak acid
still has some of the molecules stuck together.
A concentrated acid has a lot of acid dissolved in
water, and a dilute acid has only a little. The
concentration is usually measured in molarity
(moles divided by liters). It is possible to have a
concentrated strong acid (12M HCl) and a
concentrated weak acid (12M HCN). It is also
possible to have a dilute strong acid (0.1M HCl) and
a dilute weak acid (0.1M HCN).
There is NO connection between
concentrated/dilute and
strong/weak!
What is pH?
Because you cannot use the concentration to know
the strength of the acid, and because you cannot
use the strength to know the concentration of the
acid, chemists have determined a way to get one allinclusive number to use. This number is called pH.
(pH stands for the “potential of Hydrogen”)
The pH scale goes from 0 to 14. 7 is considered
neutral. Anything from 0-7 on the pH scale is an
acid and anything from 7-14 is a base.
In addition, anything from 0-4 is a strong acid and
anything from 4-7 is a weak acid. Anything from 710 is a weak base and anything from 10-14 is a
strong base.
0
1
2
3
Strong Acid
4
5
6
Weak
Acid
7
8
9
Weak
Base
10
11
12
13
Strong Base
14
To determine the pH of a substance requires an
indicator. Indicators are substances that turn colors
at certain pH’s. Most indicators can only turn one
color. For example, phenolphthalein is colorless in
acids and pink-purple with bases.
To determine the pH of a substance requires an
indicator. Indicators are substances that turn colors
at certain pH’s. Most indicators can only turn one
color. For example, phenolphthalein is colorless in
acids and pink-purple with bases. In order to
quickly determine a pH, pH paper has many different
indicators soaked into it. This allows pH paper to
turn 5 or more colors so 5 or more pHs can be
determined.
The best way to determine the pH is with a pH meter
or calculation. pH = -log [H+], which means the
negative logarithm of the concentration of the H+
ions. If the [H+] is known (in molarity), then a
calculator can provide the pH number, but if the [H+]
is not known, determining it without the use of
technology is a long and grueling process.
pH meters have been created to do these
calculations quickly and show the exact pH results
almost immediately. Extreme care must be taken
with pH meters as they are fragile and will not give
correct results if not properly cared for.
Often it is useful with bases to calculate a pOH
instead of a pH. For this pOH = - log [OH-]. Also a
pOH scale could be created, and is literally the
reverse of the pH scale.
On the pOH scale, low numbers are strong bases
and high numbers are strong acids. The pH and
pOH of a particular solution are inversely related, so
for any solution, the pH + pOH = 14.
When water ionizes, it forms equal amounts of H+
and OH-. It has been measured that [H+][OH-] = 1 ×
10-14 M2.
In acidic solutions [H+] > [OH-] and in basic solutions
[H+] < [OH-], but for all solutions it still works that:
[H+][OH-] = 1 × 10-14 M2
pH = -log [H+ ]
[H+ ]
pH
pH + pOH = 14
[H+][OH-] = 1 × 10-14 M2
[H+ ] = 10-pH
pOH = -log [OH- ]
[OH- ]
pOH
[OH- ] = 10-pOH
Neutralization
When an acid and base are added together in
appropriate amounts, the resulting pH is near 7 neutral. This is why acids and bases are said to
“neutralize” each other. In a neutralization reaction
the acid and base (according to the Arrhenius
definition) react to form water and a salt.
HCl + NaOH → HOH + NaCl
Water
Salt
The salt does not have to be table salt (NaCl). A salt
is almost any metal with an anion. What’s the salt in
the below reaction?
H2SO4 + Ba(OH)2 → 2HOH + BaSO4
If the base contains carbonate ion instead of
hydroxide, a salt and water are still formed, but so is
carbon dioxide gas. Consider this volcano recipe:
HC2H3O2 + NaHCO3 → NaC2H3O2 + H2O + CO2
If the base contains carbonate ion instead of
hydroxide, a salt and water are still formed, but so is
carbon dioxide gas. Consider this volcano recipe:
HC2H3O2 + NaHCO3 → NaC2H3O2 + H2O + CO2
Vinegar
Baking Soda
Neutralization has many purposes. If a lake
becomes too acidic because of acid rain, then a
base can be added to return the pH to the level fish
need to live. If a base spills in lab an acid can be
added to make the spill less dangerous to clean up.
Certain compounds in your blood neutralize the
carbonic acid in your body to maintain homeostasis.
Most importantly for chemists, if an unknown base
is neutralized with a known acid, the amount of OHin the base can be calculated. Likewise if an
unknown acid is neutralized with a known base, the
amount of H+ in the acid can be calculated.
Neutralizing an acid and base to determine an
unknown amount of H+ or OH- is called titration. In a
titration a small amount of indicator is added that
will change color (end point) when the neutralization
is complete (equivalence point).