19.2 Hydrogen Ions and
Acidity
Self-Ionization of Water
Water is very polar
 When the water molecules collide too fast
a proton can break off and two ions form
H2O → H+ + OH Usually the H+ ion reacts with an intact H2O
molecule to form an hydronium ion H3O+
H+ + H2O → H3O+

Algebra Review
10000 = 104
___1___ = 10-4
10000
Which number is larger 104 or 106?
106
Which number is larger 10-4 or 10-6?
10-4
think about it what is the larger number
1/10,000 or 1/1,000,000
Acidic or basic
[H+] > 10-7 solution is acidic
 [H+] = 10-7 solution is neutral
 [H+] < 10-7 solution is basic
Or other way around
 [OH-] > 10-7 solution is basic
 [OH-] = 10-7 solution is neutral
 [OH-] < 10-7 solution is acidic

H+/OH- concentrations in acids and
bases
H+ OH-
acid
H+ OH-
neutral
H+ OH-
base
Algebra
What’s a logarithm?
 Writing exponents w/o superscript
Example:
log 103 = 3
log 10-3
= -3
-log 10-3
=3

How much alkalinity/acidity
Easier way of expressing acidity/alkalinity
 Focuses only on [H+] concentration, called
pH (little p, big H!!!)
 The pH of a solution is the negative
logarithm of the hydrogen ion
concentration
pH = -log [H+]
 Artificial scale ranging from 0 to14

Example H2O
A few water molecules do this
H2O → H+ + OHIs water an acid or a base?
 How many molecules break up?
 10-7
 Water has [H+] = 10-7
 pH = -log 10-7
 pH= 7
 Because water is considered neutral, pH 7 is the
neutral pH

pH of Household Liquids
Addtl. definition of an acid/base
Acid has a pH < 7 (0 - 6.9)
 Base has a pH > 7 (7.1 - 14)
