10.5-10.6Ionization of Water and the pH Scale

Ionization of Water

10.5

3

In water,• H+ is transferred from one H2O molecule to another. • one water molecule acts as an acid, while another

acts as a base.

H2O + H2O H3O+ + OH− .. .. .. .. :O: H + H:O: H:O:H + + :O:H−

.. .. .. .. H H H water water hydronium hydroxide

ion (+) ion (-)

Ionization of Water

4

Pure Water is Neutral

In pure water, • the ionization of water

molecules produces small, but equal quantities of H3O+ and OH−

ions.• molar concentrations are

indicated in brackets as [H3O+] and [OH−].

[H3O+] = 1.0 x 10−7 M

[OH−] = 1.0 x 10−7 MCopyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Acidic Solutions

Adding an acid to pure water • increases the [H3O+].

• cause the [H3O+] to exceed 1.0 x 10-7 M.

• decreases the [OH−].

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Basic Solutions

Adding a base to pure water

• increases the [OH−].

• causes the [OH−] to exceed 1.0 x 10− 7M.

• decreases the [H3O+].

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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The ion product constant, Kw, for water

• is the product of the concentrations of the hydronium and hydroxide ions.Kw = [ H3O+] [ OH− ]

• can be obtained from the concentrations in pure water.Kw = [ H3O+] [ OH− ]

Kw = [1.0 x 10− 7 M] x [ 1.0 x 10− 7 M]

= 1.0 x 10− 14

Ion Product of Water, Kw

8

[H3O+] and [OH−] in Solutions

In neutral, acidic, or basic solutions, the Kw is always 1.0 x 10−14.

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Guide to Calculating [H3O+]

10

Calculating [H3O+]

What is the [H3O+] of a solution if [OH−] is 5.0 x 10-8 M?

STEP 1: Write the Kw for water.

Kw = [H3O+ ][OH− ] = 1.0 x 10−14

STEP 2: Rearrange the Kw expression.

[H3O+] = 1.0 x 10-14

[OH−]

STEP 3: Substitute [OH−]. [H3O+] = 1.0 x 10-14 = 2.0 x 10-7 M

5.0 x 10- 8

The pH Scale

10.6

LecturePLUS Timberlake 12

pH

Indicates the acidity [H3O+] of the solution

pH = - log [H3O+]

From the French pouvoir hydrogene (“hydrogen power” or power of hydrogen)

LecturePLUS Timberlake 13

In the expression for [H3O+]

1 x 10-exponent

the exponent = pH

[H3O+] = 1 x 10-pH M

pH

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pH Range

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Neutral

[H+]>[OH-] [H+] = [OH-] [OH-]>[H+]

Acidic Basic

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Some [H3O+] and pH

[H3O+] pH

1 x 10-5 M 5

1 x 10-9 M 9

1 x 10-11 M 11

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pH of Some Common Acids

gastric juice 1.0

lemon juice 2.3

vinegar 2.8

orange juice 3.5

coffee 5.0

milk 6.6

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pH of Some Common Bases

blood 7.4

tears 7.4

seawater 8.4

milk of magnesia 10.6

household ammonia 11.0

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pH on the Calculator

[H3O+] is 4.5 x 10-6 M

pH = 4.5 x EXP(or EE) 6+/- LOG +/-

= 5.35

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Acid Rain Unpolluted rain has a pH of 5.6

Rain with a pH below 5.6 is “acid rain“

CO2 in the air forms carbonic acid

CO2 + H2O H2CO3

Adds to H+ of rain

H2CO3 H+ (aq) + HCO3-(aq)

Formation of acid rain:1. Emission of sulfur and nitrogen oxides from the burning of fuels expecially coal with high S content, power stations, oil refineries, vehicles as well as bacterial decomposition, and

lighting hitting N2

SO2 26 million tons in 1980

NO and NO2 22 million tons in 1980

Mt. St Helens (1980) 400,000 tons SO2

2. Reactions in the atmosphere form SO3

2SO2 + O2 2 SO3

3. Reactions with atmosphere water form acidsSO3 + H2O H2SO4 sulfuric acid

NO + H2O HNO2 nitrous acid

HNO2 + H2O HNO3 nitric acid

4. Effects of Acid RainDecline in fish populations in rivers and lasts due to toxic effect of Al leached from soil by acid rainExtensive fish kills in spring from runoff due to accumulation of large amounts of acid on the snow Dissolves minerals Mg, Ca, and K from the soil and waxy coatings that protect leaves from bacteriaCorrodes metals, textiles, paper and leather

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Sources of Acid Rain

Power stationsOil refineries

Coal with high S content

Car and truck emissions

Bacterial decomposition, and lighting hitting N2

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SO2 26 million tons in 1980

NO and NO2 22 million tons in 1980

Mt. St Helens (1980) 400,000 tons SO2

Reactions with oxygen in air form SO3

2SO2 + O2 2 SO3

Reactions with water in air form acids

SO3 + H2O H2SO4 sulfuric acid

NO + H2O HNO2 nitrous acid

HNO2 + H2O HNO3 nitric acid

LecturePLUS Timberlake 22

Effects of Acid Rain

Leaches Al from soil, which kills fish

Fish kills in spring from runoff due to accumulation of

large amounts of acid in snow

Dissolves waxy coatings that protect leaves from

bacteria

Corrodes metals, textiles, paper and leather