Acids are substances that ionize in aqueous solutions to form hydrogen ions, thereby increasing the concentration of H+ ions.

 Because hydrogen atom consists of a proton and an electron, H+ is simply a proton.

 Thus, acids are often called proton donors.

 Molecules of different acids can ionize to

form different numbers of H+ ions.

 Both hydrochloric acid and nitric acid are

monoprotic acids, which yield one per

molecule of acid.

 Sulfuric acid is a diprotic acid, one that

yields two H+ per molecule of acid.

 Bases are substances that accept H+

ions.

 Bases produce hydroxide ions when

they dissolve in water.

 When dissolved in water, they

dissociate into their component ions,

introducing OH- ions into the solution.

ACID BASE

Sour taste Bitter taste

Neutralizes bases Neutralizes acids

Turns litmus paper blue to red Turns litmus paper red to blue

Soapy and slippery feeling

Indicator Color in strongly acidic

solution

pH at which color changes

Color in strongly alkaline

solution

Methyl orange Red 4 Yellow

Litmus Red 7 Blue

Phenolphthalein Colorless 9 Red

Screened Methyl Orange

Red 4 Green

 Acids and bases that are strong

electrolytes (completely ionized in

solution) are called strong acids and

strong bases.

  Those that are weak electrolytes (partly

ionized) are called weak acids and weak

bases.

STRONG ACIDS STRONG BASES

Hydrochloric acid Lithium hydroxide

Hydrobromic acid Sodium hydroxide

Hydroiodic acid Potassium hydroxide

Chloric acid Rubidium hydroxide

Perchloric acid Cesium hydroxide

Nitric acid Calcium hydroxide

Sulfuric acid Strontium hydroxide Barium hydroxide

  To classify a soluble substance as a strong

electrolyte, weak electrolyte, or nonelectrolyte,

we simply use the following table:

Strong Electrolyte

Weak Electrolyte

Nonelectrolyte

Ionic All None None

Molecular Strong acids Weak acids

Weak bases All other compounds

  If an acid is not listed, it is probably a weak electrolyte.   NH3 is only a weak base that we consider.

 When a solution of an acid and that of

a base are mixed, a neutralization

reaction occurs.

 The products of the reaction have

none of the characteristics properties

of either the acidic and the basic

solutions.

 By analogy to this reaction, the term salt

has come to mean any ionic compound

whose cation comes from a base and

whose anion comes from an acid.

 A neutralization reaction between an acid

and a metal hydroxide produces water

and salt.

1.  Arrhenius Acids and Bases

2.  Bronsted-Lowry Acids and Bases

3.  Lewis Acids and Bases

  Swedish chemist Svante Arrhenius (1859-1927)

proposed a revolutionary way of defining and

thinking about acids and bases.

  He said that acids are hydrogen-containing

compounds that ionize to yield hydrogen ions (H+)

in aqueous solutions.

  He also said that bases are compounds that ionize

to yield hydroxide ions (OH-) in aqueous solutions.

  Acids that contain one ionizable hydrogen, such as

nitric acid, are called monoprotic acids.

  Acids that contain two ionizable hydrogens, such

as sulfuric acid, are called diprotic acids.

  Acids that contain three ionizable hydrogens, such

as phosphoric acid, are called triprotic acids.

  In 1923, the Danish chemist Johannes Bronsted

and the English chemist Thomas Lowry

independently proposed a new definition.

  Defines an acid as a hydrogen-ion donor.

  Defines a base as a hydrogen-ion acceptor.

  A conjugate base is the particle that remains when

an acid has donated a hydrogen ion.

  A conjugate base is the particle that remains when

an acid has donated a hydrogen ion.

  Conjugate acids and bases are always paired with

a base or an acid, respectively.

  A conjugate acid-base pair consists of two

substances related by the loss or gain of a single

hydrogen ion.

  The third theory of acids and bases was proposed

by Gilbert Lewis.

  Lewis focused on the donation or acceptance of a

pair of electrons during a reaction.

  This concept is more general than either the

Arrhenius theory or the Bronsted-Lowry theory.

  A Lewis acid is a substance that can accept a pair

of electrons to form a covalent bond.

  A Lewis base is a substance that can donate a

pair of electrons to form a covalent bond.

  A hydrogen ion (Bronsted-Lowry acid) can accept

apair of electrons in forming a bond.

  A hydrogen ion, therefore, is also a Lewis acid.

  A Bronsted-Lowry base, or a substance that

accepts a hydrogen ion, must have a pair of

electrons available and is also a Lewis base.

  A widely used system for expressing [H+] is the pH

scale, proposed in 1909 by the Danish scientist

Soren Sorenson.

  It ranges from 0-14, neutral solutions have a pH of

7.

  A pH of 10 is strongly basic.

  The pH of a solution is the negative logarithm of

the hydrogen-ion concentration.

 The pH may be represented

mathematically using the following

equation:

pH = - log [H+]

 Similarly, the pOH of a solution equals

the negative logarithm of the

hydroxide-ion concentration.

pOH = - log [OH-]

  A neutral solution has a pOH of 7.

  A solution with a pOH less than 7 is basic.

  A solution with a pOH greater than 7 is acidic.

  A simple relationship between pH and pOH makes

it easy to find either one when the other is known.

pH + pOH = 14

pH = 14 – pOH

pOH = 14 - pH

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

10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-1110-12 10-1310-14

NE

UT

RA

L

Increasing Basicity Increasing Acidity

 What is the pH of a solution with a hydrogen-ion concentration of 1.0 x 10-10 M?

 Find the pH of each solution: ◦  [H+] = 1.0 x 10 -4 M ◦  [H+] = 0.0010 M

◦  [H+] = 1.0 x 10 -9 M ◦  [H+] = 1.0 x 10 -12 M

◦  [H+] = 0.010 M