Water as an Acid and Water as an Acid and Base Base Chapter 15.1C
Mar 26, 2015
Water as an Acid and Water as an Acid and BaseBaseChapter 15.1C
How can it be?How can it be?How can a substance be both an
acid and a base?Substances that can behave
either as an acid or as a base are called amphoteric substances.
Water is the most common amphoteric substance.
Ionization of WaterIonization of WaterLet’s look at the following equation
to understand why water is amphoteric:
H2O + H2O H3O+ + OH-
In this reaction, one water molecule acts as an acid by donating a proton, the other water molecule acts as a base by accepting the proton.
However, this reaction only happens to a small extent
Concentration of IonsConcentration of IonsIn any solution you will have a
certain concentration of H3O+ and OH- ions.
To demonstrate concentration, we place ions in brackets
Example: [H+] and [OH-]Note: to simplify we often write H3O+
as H+
The proportion of H3O+ and OH- ions determines whether your solution is acidic or basic
Ion-Product ConstantIon-Product ConstantIn a solution, [H+][OH-] = 1.0 x 10-14
This number is known as Kw, the ion-product constant for water
If your solution has more [H+] than [OH-], the solution is acidic
If your solution has more [OH-] than [H+], the solution is basic.
If [H+] and [OH-] are equal, your solution is neutral
You can use Kw and [H+] or [OH-] to solve for the other
Example 1Example 1Calculate the [H+] if [OH-]= 1.0 x 10-5
M, and identify whether this solution is acidic, basic, or neutral
[H+][OH-] = 1.0 x 10-14
[H+][1.0 x 10-5 M] = 1.0 x 10-14
Solve for [H+]:[H+] = (1.0 x 10-14)/ (1.0 x 10-5 )= 1.0
x 10-9 MSince we have more [OH-] than [H+],
the solution is basic.
Example 2Example 2Calculate [OH-] if [H+]= 10.0 M, and
identify whether this solution is acidic, basic, or neutral
[H+][OH-] = 1.0 x 10-14
[10.0 M][OH-] = 1.0 x 10-14
Solve for [OH-][OH-]= (1.0 x 10-14)/ (10.0 M) = 1.0 x
10-15 MSince we have more [H+] than [OH-]
the solution is acidic.
Book Work!Book Work!Complete the following
assignment:P 590 #22, 23