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