ACID BASE UNIT
CHAPTER 19
The characteristic properties of acids
result from the presence of the H+ ion
generated when an acid dissolves in
water. It also explains why acids
neutralize bases and vice versa. Acids
provide the H+ ion; bases provide the
OH- ion; and these ions combine to
form water.
H+(aq) + OH-(aq) -> H2O(l)
II. Acid/Base Strengths
1. Strong Acids: Ionize completely, are good
conductors of electricity because they
produce the maximum numbers of ions.
Examples: Hydrochloric acid (HCl), Perchloric
acid (HCLO4), Nitric acid (HNO3), Hyrdoiodic
acid (HI), and sulfuric acid (H2SO4)
The general reaction that occurs when an acid
is dissolved in water can best be represented
as:
HX (aq) + H2O (l) -> H3O+ (aq) + X- (aq)
Acid + Base -> conjugate acid + conjugate base
Every acid has associated with it a conjugate
base, formed from the acid by the loss of a
proton.
The conjugate base is everything that remains of
the acid molecule after a proton is lost.
Every base has associated with it a conjugate
acid, formed from the base by the addition of a
proton.
HX (aq) + H2O (l) -> H3O+ (aq) + X- (aq)
The conjugate acid is formed when the proton is
transferred to the base. A conjugate acidbase pair consists of two substances related
to each other by the donating and accepting
of a single proton. In the example above,
there are two conjugate acid-base pairs:
HX and X-, and H2O and H3O+.
HX (aq) + H2O (l) -> H3O+ (aq) + X- (aq)
A conjugate acid-base pair consists of two
substances related to each other by the
donating and accepting of a single
proton. In the example above, there are
two conjugate acid-base pairs:
HX and X-, and H2O and H3O+.
Practice Problems
 Identify the conjugate acid-base pairs in the
following reactions.
1. NH4+(aq) + OH-(aq)  NH3(aq) + H20(l)
1. HBr(aq) + H20(l)H3O+(aq) + Br-(aq)
II. Strengths of Acids & Bases
1. Strength of acids
a.
b.
Strong Acids: Ionize completely
and are good conductors of
electricity
Examples: Hydrochoric (HCl),
Hydrobromic (HBr), Hydroiodic (HI),
Perchloic (HClO4), Nitric (HNO3),
Sulfuric (H2SO4)
c. Weak Acids: Partially ionize and are
weak conductors
d. Examples: Hydrofluoric (HF),
Hydrocyanic (HCN), Acetic
(HC2H3O2), Hydrosulfuric (H2S),
Carbonic (H2CO3), Hypochlorous
(HClO)
1. Strength of Bases
a.
b.
Strong Bases: dissociate entirely
into metal ions and hydroxide ions.
Examples: Calcium Hydroxide
(Ca(OH)2), Sodium Hydroxide
(NaOH), Potassium Hydroxide
(KOH), Barium Hydroxide
(Ba(OH)2)
c. Weak Bases: only partially ionize
into metal ions and hydroxide ions.
d. Examples: Ethylamine (C2H5NH2),
Methlyamine (CH3NH2), Ammoina
(NH3), Aniline (C6H5NH2)
pH Scale
Acidic and basic are two extremes that
describe a chemical property of
substances.
The pH scale measures how acidic or
basic a substance is. The pH scale
ranges from 0 to 14. A pH of 7 is
neutral. A pH less than 7 is acidic. A pH
greater than 7 is basic.
The pH scale is logarithmic and as a
result, each whole pH value below 7
is ten times more acidic than the next
higher value.
For example, pH 4 is ten times more acidic
than pH 5 and 100 times (10 times 10) more
acidic than pH 6.
The same holds true for pH values above 7,
each of which is ten times more alkaline
(another way to say basic) than the next
lower whole value.
For example, pH 10 is ten times more alkaline
than pH 9 and 100 times (10 times 10) more
alkaline than pH 8.
Definition of pH and pOH
The concentrations of hydrogen ions and
indirectly hydroxide ions are given by a
pH number. pH is defined as the
negative logarithm of the hydrogen
ion concentration. The equation is:
pH = - log [H+]
similarly, pOH = - log [OH-]
The following diagram illustrates the
relationship between pH, H+, OH-
Example: If an acid has an H+
concentration of 0.0001 M, find the pH.
Solution:
First convert the number to exponential
notation, find the log, then solve the pH
equation.
H+ = 0.0001M = 10-4; log of 10-4 = -4;
pH = - log [ H+] = - log (10-4)
= - (-4) = +4 = pH
 The purpose of the negative sign in the log
definition is to give a positive pH value.
Example: If the base has an OHconcentration of 0.001M, find the pH.
Solution:
First find the pOH, (similar to finding the
pH,) then subtract the pOH from 14.
OH- = 0.001M = 10-3;
pOH = -log [OH-] = -log (10-3) = +3 =
pOH
pH = 14 - pOH; pH = 14 - 3 = 11 = pH
pH Principle: pH and pOH must always
equal pKw (14).
Acid and Base Indicators
The most common indicator is found on
"litmus" paper. It is red below pH 4.5
and blue above pH 8.2.
Blue Litmus
Red Litmus
Acid
turns red
stays same
Base
stays same
turns blue
Other commercial pH
papers/solutions are able to give
different colors for every main pH
value.
Universal Indicator is a mixture of
indicators is able to also provide a
full range of colors for the pH scale.