Unit 25
Strengths of Acids and Bases (Chapter 7)
•Acid and Base Strength (7.4)
•pH Scale (7.6)
•Conjugate Acid-Base Pairs (7.7)
Acid Strengths (7.4)
•Recall that an acid reaction with water may be written generally as:
HA (aq) +
H2O (ℓ) →
A- (aq)
+
H3O+ (aq)
•Any acid reacting with water will undergo the same sort of reaction but possibly to
a different degree. Acids may be classified as strong or weak depending on the
extent to which they break apart.
•Strong acids: A strong acid will undergo the reaction above to about 100% to form the A- and the
H3O+. The only seven strong acids are HCl, HBr, HI, HNO3, H2SO4, HClO3, and HClO4.
•Weak acids: A weak acid will only undergo the reaction above to small extent – maybe 5%. Acids
that are not one of the seven above will be classified as weak acids. Examples include HF, HNO2,
HC2H3O2, and others
Base Strengths (7.4)
•The distinction between strong and weak bases is similar to that between strong and weak
acids:
•A strong base is completely ionized in water
•A weak base is only slightly ionized in water
•Examples:
•Examples of strong bases include NaOH as well as any of the other alkali metal hydroxides. Group
2 hydroxides are also strong bases except for Be(OH)2 and Ca(OH)2.
•One of the primary examples of a weak base is NH3. It reacts with water according to the following
equation:
NH 3 (aq)  H 2O( )
NH 4  (aq)  OH  (aq)
•The lopsided arrows indicate that most of the NH3 stays as that with only a small portion being
converted to NH4+ - thus, the ionization does not occur to a large extent making NH3 a weak base.
pH Scale (7.6)
•The concentration of H3O+ is usually fairly small, even in an acidic solution. The H3O+ are best expressed
in scientific notation, but that gets a little cumbersome sometimes.
•Acidity of a solution is usually expressed in terms of a scale called the pH scale. The scale ranges from 0
to 14, with lower numbers being acidic, 7 being neutral, and numbers greater than 7 representing basic
solutions. Pictorially:
Molarity
pH
1
0
10-1
10-2
10-3
10-4
10-5
10-6
10-7
10-8
10-9
10-10
10-11
10-12
10-13
1
2
3
4
5
6
7
8
9
10
11
12
13
10-14
14
•Mathematically the scale is logarithmic with the relationship governing it as:
pH   log[ H 3O  ]
where [ H 3O  ] is the molarity of the hydronium ion
•We won’t do any calculations with this, but it is interesting to note it is the same sort of scale used for the
Ricther scale for determining the magnitude of earthquakes and the decibel scale used to determine sound
levels.
•A difference in pH of one unit means the concentration of H3O+ in the solution with the lower pH is ten
times greater than that in the one with the higher pH. A difference in pH of three units (e.g., 8 compared to
5) means that the concentration of H3O+ in the pH 5 solution is 103 (=1000) times greater than that in the
pH 8 solution.
pH Scale Cont. (7.6)
•It is relatively easy to get an idea of [H3O+] from the pH. If the pH is 6.5, for example, the [H3O+] is
between 10-6 and 10-7 M. Just consider the exponents.
•Going the other way, from [H3O+] to pH, again involves looking at the exponent. If the [H3O+]=2.4 × 10-8 M
the pH will be between 7 and 8. It will be below the numeric value of the exponent.
Molarity
pH
1
0
10-1
10-2
10-3
10-4
10-5
10-6
10-7
10-8
10-9
10-10
10-11
10-12
10-13
1
2
3
4
5
6
7
8
9
10
11
12
13
10-14
14
•To get an idea of the acidity of different materials, go to the simulation at:
https://phet.colorado.edu/en/simulation/ph-scale
and either download it or hit the Run Now button.
•You can select different materials by selecting the drop-down box above the faucet on the left of the
simulation. As you select different substances, notice what happens to the red bar to the right of the
simulation (the [H3O+]) and notice that the blue bar [OH-] always goes in the opposite direction.
•Also observe the behavior of the red and blue bars as the pH is raised and lowered using the slider toward
the middle of the simulation.
Conjugate Acid-Base Pairs (7.7)
•A pair of compounds that differ only in the presence of a H+ are called Brønsted-Lowry are called a
conjugate acid-base pair. The member of the pair that has the H+ is called the acid of the pair and the other
is the base.
•Examples:
Acid (has the extra H+)
Base
HCl
H+
H2SO4
HSO4-
H2O
OH-
NH4+
NH3
HNO3
NO3-
•Solutions made by combining a weak acid with its conjugate base or a weak base with its conjugate acid
are called buffer solutions. A buffer solution is much more resistant to changes in pH with additions of
small amounts of strong acid or base than when the strong acids or bases are added to a solution of just
one of the components.