Section 16.2
Determining the Acidity of a Solution
Georgia Standards & Essential Questions:
•
SC7. Students will characterize
the properties that describe
solutions and the nature of
acids and bases.
b. Compare, contrast, and evaluate
the nature of acids and bases:
– Strong vs. weak acids/bases
in terms of percent
dissociation
– Hydronium ion & Hydroxide
ion concentration
– pH
– Acid-Base neutralization
• How do we measure
the strengths of acids
& bases?
Section 16.2
Determining the Acidity of a Solution
Objectives
1. To understand pH and pOH
2. To learn to find pH and pOH for various solutions
3. To use a calculator to find pH
4. To learn methods for measuring pH of a solution
5. To learn to calculate the pH of strong acids
Section 16.2
Determining the Acidity of a Solution
Georgia Performance Standards & Essential Questions
• SC7. Students will
characterize the properties
that describe solutions and
the nature of acids and
bases.
b. Compare, contrast, and
evaluate the nature of acids
and bases:
– Strong vs. weak
acids/bases in terms of
percent dissociation
– Hydronium ion
concentration
– pH
– Acid-Base neutralization
• How do we measure
the strengths of
acids & bases?
Section 16.2
Determining the Acidity of a Solution
B. Acid Strength
• Strong acid – completely ionized or completely
dissociated
Section 16.2
Determining the Acidity of a Solution
B. Acid Strength
•
•
Weak acid – most of the acid molecules remain intact
Does not fully dissociate
Section 16.2
Determining the Acidity of a Solution
B. Acid Strength
• A strong acid contains a
relatively weak conjugate
base.
Section 16.2
Determining the Acidity of a Solution
B. Acid Strength
• Common strong acids are
– Sulfuric Acid, H2SO4
– Hydrochloric acid, HCl
– Nitric acid, HNO3
– Perchloric acid, HClO4
– Hydrobromic acid, HBr
– Hydiodic acid, HI
• Common Weak Acids
– Formic acid HCOOH
– Acetic acid CH3COOH
– Trichloroacetic acid
CCl3COOH
– Hydrofluoric acid HF
– Hydrocyanic acid HCN
Section 16.2
Determining the Acidity of a Solution
B. Acid Strength
Section 16.2
Determining the Acidity of a Solution
Strong Acids
• You will recall that the seven strong acids are HCl, HBr, HI,
HNO3, H2SO4, HClO3, and HClO4.
• These are, by definition, strong electrolytes and exist totally as
ions in aqueous solution.
• For the monoprotic strong acids,
[H3O+] = [acid].
Section 16.2
Determining the Acidity of a Solution
Strong Bases
• Strong bases are the soluble hydroxides, which are the alkali metal
and heavier alkaline earth metal hydroxides (Ca2+, Sr2+, and Ba2+).
• Again, these substances dissociate completely in aqueous solution.
Section 16.2
Determining the Acidity of a Solution
Strong Base: fully dissociate in water
• Strong Bases:
– NaOH
– LiOH
– KOH
– Ca(OH)2
– Ba(OH)2
– CsOH
• Common Weak Bases
– ammonia NH3
– trimethylammonia N(CH3)3
– pyridine C5H5N
– Ammonium hydroxide
NH4OH
Section 16.2
Determining the Acidity of a Solution
How Do We Measure pH?
For more accurate
measurements, one uses a pH
meter, which measures the
voltage in the solution.
Section 16.2
Determining the Acidity of a Solution
B. Measuring pH
• Indicators – substances that exhibit different colors in
acidic and basic solutions
• EX: Phenolthalein
– In an acid solution the indicator will be clear
– In a basic solution the indicator will be blue
Section 16.2
Determining the Acidity of a Solution
B. Measuring pH
• Other methods
– Indicator paper
– Blue Litmus paper turn red with acids
– Red Litmus paper turn blue with bases
– pH meter
Section 16.2
Determining the Acidity of a Solution
A. The pH Scale
Section 16.2
Determining the Acidity of a Solution
A. The pH Scale
• Because the pH scale is a log scale based on 10, the pH
changes by 1 for every power of 10 change in the [H+].
Section 16.2
Determining the Acidity of a Solution
A. The pH Scale
• The “pH scale” is used to express small numbers.
• pH = log [H+]
Section 16.2
Determining the Acidity of a Solution
A. The pH Scale
• pOH scale
pOH = log [OH]
• pH + pOH = 14.00
Section 16.2
Determining the Acidity of a Solution
Calculating Percent Ionization
[H3O+]eq
[HA]initial
• Percent Ionization =
 100
• In this example
[H3O+]eq = 4.2  10-3 M
[HCOOH]initial = 0.10 M
Percent Ionization =
4.2  10-3
0.10
= 4.2%
 100
Section 16.2
Determining the Acidity of a Solution
Reactions of Anions with Water
• Anions are bases.
• Anions react with water in a hydrolysis reaction to
form OH- and the conjugate acid:
X- (aq) + H2O (l)
HX (aq) + OH- (aq)
Section 16.2
Determining the Acidity of a Solution
Reactions of Cations with Water
• Cations with acidic protons (like NH4+)
will lower the pH of a solution.
• Most metal cations that are hydrated in
solution also lower the pH of the solution.
Section 16.2
Determining the Acidity of a Solution
Reactions of Cations with Water
• Attraction between nonbonding electrons on
oxygen and the metal causes a shift of the
electron density in water.
• This makes the O-H bond more polar and the
water more acidic.
• Greater charge and smaller size make a
cation more acidic.
Section 16.2
Determining the Acidity of a Solution
Effect of Cations and Anions
1.
An anion that is the conjugate base
of a strong acid will not affect the
pH.
2.
An anion that is the conjugate base
of a weak acid will increase the pH.
3.
A cation that is the conjugate acid
of a weak base will decrease the
pH.
Section 16.2
Determining the Acidity of a Solution
Effect of Cations and Anions
4.
Cations of the strong Arrhenius bases will not affect the pH.
5.
Other metal ions will cause a decrease in pH.
Section 16.2
Determining the Acidity of a Solution
Factors Affecting Acid Strength
• The more polar the H-X bond and/or the weaker the H-X bond, the
more acidic the compound.
• So acidity increases from left to right across a row and from top to
bottom down a group.
Section 16.2
Determining the Acidity of a Solution
Factors Affecting Acid Strength
In oxyacids, in which an -OH
is bonded to another atom,
Y, the more electronegative
Y is, the more acidic the
acid.
Section 16.2
Determining the Acidity of a Solution
Factors Affecting Acid Strength
For a series of oxyacids, acidity increases with the number of oxygens.