15.2-15.4
Acids and Bases – Acid Strength and Ka
The Nature of Acids
• Acids
–
–
–
–
–
Have a sour taste
Have the ability to dissolve many metals
Turns red litmus paper blue
Neutralize bases
Found in food, used to manufacture fertilizers, glues and dyes.
• Common acids:
–
–
–
–
Hydrochloric acid: HCl
Sulfuric acid: H2SO4
Nitric acid: HNO3
Acetic acid: HC2H3O2  this is a carboxylic acid (an organic acid that contains the
formula COOH. These are substances derived from living organisms.)
–
–
–
–
Citric Acid: H3C6H5O7
Carbonic acid: H2CO3
Hydrofluoric acid: HF
Phosphoric acid: H3PO4
The Nature of Bases
• Bases
–
–
–
–
–
Have a bitter taste
Feel slippery
Turns red litmus paper blue
Have the ability to neutralize acids
Are used in petroleum and cotton processing, and in soap
and plastic manufacturing. They are also found in
medication, in products such as Drano, and in the kitchen (an
example, baking soda).
• Common base:
–
–
–
–
–
Sodium hydroxide: NaOH
Potassium hydroxide: KOH
Sodium bicarbonate: NaHCO3
Sodium carbonate: NaCO3
Ammonia: NH3
Definitions of Acids and Bases
• There are three definitions of acids and bases, that together
correctly defines them.
– 1.) Arrhenius’s Definition – An acid is a substance that
produces hydrogen (H+) ions in solution, while bases are
substances that produce hydroxide (OH-) ions in
solution.
– 2.) Brønsted-Lowery’s Definintion (this theory is the focus in this course!
On the regents exam, this is known as the alternate acid-base
theory) – Acids are proton (H+) donors and they become conjugate
bases, and bases are proton (H+) acceptors, and they become
conjugate acids.
– 3.) Lewis’s Definition (This will not be covered on the AP
exam) – Acids are electron pair acceptors and bases are electron
pair donors.
Examples or Arrhenius Acids and Bases
• HCl is an Arrhenius acid because it produces H+ ions in solution:
HCl(aq)  H+(aq) + Cl-(aq)
• The hydrogen ions are very reactive, and they react with the water
molecules that form the aqueous solutions. Hydrogen ions always
associate with water molecules to form hydronium ions (H3O+).
• NaOH is a base because it produces hydroxide (OH-) ions in
solution: NaOH(aq)  Na+(aq) + OH-(aq).
• Arrhenius acids and bases combine to form water, neutralizing
each other in the process:
H+(aq) + OH-(aq)  H2O(l)
Examples of Brønsted-Lowery Acids and Bases
• According to Brønsted-Lowery, HCl is an acid because in solution it donates
protons:
HCl(aq) + H2O(aq)  H3O+(aq) + Cl-(aq)
• According to Brønsted-Lowery, NH3 is a base because in solution, it accepts protons
from water.
NH3(aq) + H2O(l)
NH4+(aq) + OH-(aq)
In the previous equation, if you look at it in reverse, we can see that NH4+
donates an proton (it is an acid) and OH- accepts a proton (it is a base). The
substance that was a base (NH3) becomes an acid (NH4+). The same this goes for the
substance that was an acid. These are called conjugate acid-base pairs. NH4+ an NH3
are a conjugate acid-base pair. (YES, YOU NEED TO KNOW THIS FOR THE AP EXAM!)
As you can see, in the two equations above, water acted both as an acid in one and a
base in the other. For this reason, water is considered amphoteric (or amphiprotic) –
meaning that it can act as either an acid or a base
Let’s Try a Couple of Practice Problems!
In the reaction below, identify the Brønsted-Lowery acid, and
base, and the conjugate acid, and the conjugate base.
C5H5N(aq) + H2O(l)
C5H5NH+(aq) + OH-(aq)
H2O is the acid, OH- is the conjugate base, C5H5N is the base, and
C5H5NH+ is the conjugate acid.
Which pair is not a conjugate acid-base pair?
a.) (CH3)3N; (CH3)3NH+ b.) H2SO4; H2SO3 c.) HNO2; NO2b.) H2SO4; H2SO3 is not a conjugate acid-base pair because they
are both acids.
Acid Strength and the Acid Ionization Constant (Ka)
• Just as strong and weak electrolytes were defined earlier in
the year, a similar definition applies to strong and weak acids.
Strong acids are those whose H+ ions dissociate fully (they
ionize fully), and weak acids consist of H+ ions that only
partially dissociate (they partially ionize).
Let’s look at the following equation:
HA(aq) + H2O(l)
H3O+(aq) + A-(aq)
HA is a general formula for an acid!
If the equilibrium lies far to the right, the acid is strong (it
completely ionizes). If it lies to the left, it is weak (only a small
percentage ionizes).
Strong vs. Weak Acids
•
A single arrow  indicates a strong acid (full ionization), while a double sided arrow
indicates a weak acid.
Examples:
HCl(aq) + H2O(l)  H3O+(aq) + Cl-(aq)
HCl is a strong acid, so 1.0 M HCl would yield 1.0 M H3O+
HF(aq) + H2O(l)
H3O+(aq) + F-(aq)
HF is a weak acid, so 1.0 M HF would yield a considerable smaller amount
of H3O+. Here [H3O+] is much less that 1.O M
•
If the attraction between HA is strong, you have a weak acid. If the attraction between HA is
weak you have a strong acid.
•
Generally, the stronger the acid the weaker the conjugate base, and vice versa.
•
Both HCl and HF are considered monoprotic acid, because they consist of one ionizable
proton. Polyprotic acids, those with two ionizable protons are considered diprotic, and
those with three are triprotic.
Acid Ionization Constant (Ka)
Let’s looks at the following two equations:
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
HA(aq) H+(aq) + A-(aq)
[H3O+] = [H+]
[H3O+][A-]
[H+][A-]
So, Ka = ----------------- = ---------------[HA]
[HA]
The smaller the ionization constant, the weaker the acid!
Let’s Try a Practice Problem!
Consider these two acids and their Ka values:
HF
Ka = 3.5X10-4
HClO Ka = 2.9X10-8
Which acid is stronger?
HF is stronger since its Ka value is larger (it has a larger
acid ionization constant).
15.2-15.4 pgs. 745-746 #’s 34, 36, 42, 43 and 44 (For the
questions that have multiple parts, only do a and b)
Read 15.5-15.6 pgs. 706-719