water-as-an-acid-and-base by xiaopangnv

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Water as an Acid and Base
Chapter 15.1C
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 Water
 Let’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 Ions
 In 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
acidic or basic
Ion-Product Constant
   In 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 1
   Calculate 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 M
   Since we have more [OH-] than [H+], the
solution is basic.
Example 2
 Calculate [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 M
 Since we have more [H+] than [OH-] the
solution is acidic.
Book Work!
 Complete the following assignment:
 P 590 #22, 23

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