Chapter 20 – Acids and Bases

Describing Acids and BasesHydrogen Ions and AcidityAcid-Base TheoriesStrengths of Acids and Bases

20.1 Describing Acids & Bases Properties of Acids and Bases

The very word “acid” conjures all kinds of images: skin eating, drugs, caustic… But, what are some properties of acids and how do we begin to understand them?

Some properties: sour, conduct electricity (electrolytes), can react with different reactants and get different results, etc. Acid: a compound that produces hydrogen ions when

dissolved in water.

20.1 Describing Acids & Bases (cont.) Properties of Acids and Bases

Naming acids…”HX compounds”… Add hydro- and –ic for “ide” anion compounds. Add –ous for “ite” anion compounds. Add –ic for “ate” anion compounds.

Example: H2SO4 = hydrogen sulfate…sulfuric acid. Others:

1. 2. 3.

20.2 Hydrogen Ions and Acidity Hydrogen Ions from Water

Recall the polarity of a water molecule… Sometimes the collisions between molecules are

energetic enough to transfer a hydrogen ion from one molecule to another…

Figure 20.3, page 580

20.2 Hydrogen Ions and Acidity (cont.) Hydrogen Ions from Water

(cont.) Self-ionization, neutral solutions, ion-

product constants and acid/base solutions… Self-ionization: A reaction in

which two water molecules produce ions (H2O H+ + OH- ).

Neutral solution: a solution in which the [H+] and [OH-] are equal.

Obviously, not all ion concentrations are equal, the solution can be described either as acidic or basic.

This can be determined by the ion-product concentration: Kw = [H+][OH-]

[H+] < [OH-] = basic, [H+] > [OH-] = acidic

20.2 Hydrogen Ions and Acidity (cont.) The pH Concept

The measurement of this concentration can be calculated and described according to pH value, the negative log of the hydrogen-ion concentration: pH = -log[H+]

Examples… pH < 7 is acidic, pH > 7 is

basic, pH = 7 is neutral

Chapter 20 Assignments

CPQ’s # 1 pg. 609 #34,36,37,39,41,42

20.3 Acid-Base Theories

Arrhenius Acids and Bases In the first part of this chapter, we defined an acid as “a

compound that produces hydrogen ions when dissolved in water.” Defining an acid (or a base) is much more than that however…

Svante Arrhenius… Not all compounds containing hydrogen are acids. The number of hydrogen atoms in an acid determines how

many hydrogen ions may be released: Monoprotic, diprotic, tripotic

Bronsted-Lowry Acids and Bases Hydrogen-ion donor…hydrogen-ion acceptor Conjugate acids and bases…

20.3 Acid-Base Theories (cont.) Lewis Acids and Bases

Lewis acid: a substance that can accept a pair of electrons to form a covalent bond.

Lewis base: a substance that can donate a pair of electrons to form a covalent bond. Example: H+ + -O-H HOH

Table 20.6, short look at acid/base theory…

20.4 Strengths of Acids and Bases Strong and Weak Acids and Bases Strong acids –

completely ionized in aqueous solution… HCl, H2SO4

Weak acids – ionize slightly in aqueous solution… CH3COOH

20.4 Strengths of Acids and Bases (cont.) Strong and Weak Acids and Bases (cont.)

An acid dissociation constant (Ka) gives a more accurate representation of acid/base strength: Ka : the ratio of concentration of the dissociated form of

an acid to the concentration of the undissociated form. Example: CH3COOH

Chapter 20 Assignments

CPQ’s # 1 pg. 609 #34,36,37,39,41,42

CPQ’s # 2 pg. 609 #