Strong and Weak Acids

• The strength of an acid is determined by the extent to which it ionizes, its percent ionization, not the concentration of the acid, the concentration of its hydronium ions, or its ability to react with a metal

Strong Acid

• An acid that nearly completely dissociates

• All molecules of the acid break up to form the ions soluble in water

• If more than one proton is being removed, not all steps need to be complete dissociation.

Weak Acid

• An acid that only slightly dissociates in a water solution

• Only a small percent of acid molecules donate their hydrogen, and most remain the same.

• Example: CH3COOH

• A strong acid essentially ionizes 100%.

An example of a strong acid is hydrochloric acid, HCl (aq)

HCl(g) + H2O(l) H3O+(aq) + Cl-

(aq)

0.10 mol 0.10 mol 0.10 mol

100% ionization

few molecules many ions

An example of a weak acid is acetic acid, CH3COOH.

CH3COOH(l) + H2O(l) H3O+(aq) + CH3COO-

(aq)

0.10 mol << 0.10 mol << 0.10 mol

5% ionization at 25Cmany molecules few ions

Strong Base

• A base that dissociates almost completely into its ions.

• All oxides and hydroxides of group 1 and 2 are strong bases.

• Ex: NaOH

Weak Base

• Most bases are weak• They dissociate only

slightly in a water solution

• Example: NH3

• Strong acids are strong electrolytes and weak acids are weak electrolytes

• A strong base dissociates 100%.

An example of a strong base is sodium hydroxide, NaOH.

NaOH(s) + H2O(l) Na +(aq) + OH-

(aq) 0.10 mol 0.10 mol 0.10 mol

100% dissociationfew formula units (NaOH) many ions

• A weak base ionizes to a small extent.

An example of a weak base is NH3(g).

NH3(g) + H2O(l) NH4+

(aq) + OH-(aq)

0.10 mol << 0.10 mol << 0.10 mol

5% ionization at 25C

many molecules few ions

• Strong bases are strong electrolytes and weak bases are weak electrolytes.

Examples of Strong Acids and Bases

Strong Acids Strong BasesHClO4 perchloric acid LiOH lithium hydroxideHCl hydrochloric acid NaOH sodium hydroxideHNO3 nitric acid KOH potassium hydroxideH2SO4 sulfuric acid RbOH rubidium hydroxideHBr hydrobromic acid CsOH cesium hydroxideHI hydriodic acid Ca(OH)2 calcium hydroxide

Sr(OH)2 strontium hydroxideBa(OH)2 barium hydroxide

Ionization of Water

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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

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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

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[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+]

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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