Chemisty - Lecture 9 Acid-base Reactions - Power Point

ACID-BASE REACTIONS

Page 114- 127, 614-619

ACIDS 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

BASES 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

COMMON PROPERTIES OF ACIDS AND BASES: 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 Litmus

Red

4

Yellow

Red

7

Blue

Phenolphthal ein Screened Methyl Orange

Colorless

9

Red

Red

4

Green

STRONG AND WEAK ACIDS AND BASES 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.

COMMON STRONG ACIDS AND 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

IDENTIFYING STRONG AND WEAK ELECTROLYTES  To

classify a soluble substance as a

strong electrolyte, weak electrolyte, or nonelectrolyte, we simply use the following table:

SUMMARY OF THE ELECTROLYTIC BEHAVIOR OF A COMMON SOLUBLE IONIC AND MOLECULAR COMPOUNDS Strong Electrolyte

Weak Electrolyte

Nonelectroly te

Ionic

All

None

None

Molecular

Strong acids

Weak acids Weak bases

 If

All other compounds

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

NEUTRALIZATION REACTIONS AND SALTS 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

HCl + NaOH (acid) (base) NaCl (salt) By

H2 O + (water)

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

ACID-BASE THEORIES 1.

Arrhenius Acids and Bases

2.

Bronsted-Lowry Acids and Bases

3.

Lewis Acids and Bases

ARRHENIUS 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

 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.

BRONSTED-LOWRY ACIDS AND BASES  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

LEWIS ACIDS AND BASES  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

pH Concept 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

pH 2 13

3 14

4

5

Increasing Acidity

6 NEUTRAL

1

7

8

9

10

11

12

Increasing Basicity

10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-1110-12 1013 10-14

[H ] +

PRACTICE EXERCISE: 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+] [H+] [H+] [H+] [H+]

= = = = =

1.0 x 10 -4 M 0.0010 M 1.0 x 10 -9 M 1.0 x 10 -12 M 0.010 M