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 Lets look at the following equation to understand why water is amphoteric: H 2 O + H 2 O H 3 O + + 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 H 3 O + and OH - ions. To demonstrate concentration, we place ions in brackets Example: [H + ] and [OH - ] Note: to simplify we often write H 3 O + as H + The proportion of H 3 O + and OH - ions determines whether your solution is acidic or basic

Ion-Product Constant In a solution, [H + ][OH - ] = 1.0 x This number is known as K w, 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 K w and [H + ] or [OH - ] to solve for the other

Example 1 Calculate the [H + ] if [OH - ]= 1.0 x M, and identify whether this solution is acidic, basic, or neutral [H + ][OH - ] = 1.0 x [H + ][1.0 x M] = 1.0 x Solve for [H + ]: [H + ] = (1.0 x )/ (1.0 x )= 1.0 x 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 [10.0 M][OH - ] = 1.0 x Solve for [OH - ] [OH - ]= (1.0 x )/ (10.0 M) = 1.0 x M Since we have more [H + ] than [OH - ] the solution is acidic.

Book Work! Complete the following assignment: P 590 #22, 23