Acids and Bases

Basic Characteristics

Acids and bases are two fundamental classes of compounds in chemistry, distinguished by their properties and behaviors in aqueous solutions.

• Acids have a sour taste, turn blue litmus paper red, and often react with metals to produce hydrogen gas.

• Bases have a bitter taste, turn red litmus paper blue, and feel slippery to the touch.

Arrhenius Definition

The Arrhenius definition classifies acids and bases based on their behavior in water:

• Acid: Produces H+ ions in aqueous solution.

• Base: Produces OH- ions in aqueous solution.

• Acids and bases can neutralize each other to form water and a salt.

The Brønsted-Lowry Definition

This definition focuses on proton (H+) donors and acceptors:

• Acid: Proton donor.

• Base: Proton acceptor.

• Hydrogen ion (H+) is a proton; it has no neutrons or electrons.

• Acid will associate with H2O to form H3O+ (hydronium ion).

Additional info: According to Brønsted-Lowry, the proton donor and proton acceptor always act in pairs.

Amphoteric Substances

An amphoteric substance can act as either an acid (proton donor) or a base (proton acceptor), such as water.

Conjugate Acid-Base Pairs

In reversible acid-base reactions, acids and bases form conjugate pairs:

• When an acid donates a proton, it becomes a conjugate base.

• When a base accepts a proton, it becomes a conjugate acid.

• Example:

Acid-Base Reactions

General Reactions

Acids and bases react to form water and a salt. The salt results from the cation of the base and the anion from the acid.

• Gas evolution reactions: Acids react with carbonates or bicarbonates to produce water, salt, and carbon dioxide gas.

• Example:

• Acid-metal reactions: Acids react with metals to produce hydrogen gas and a salt.

• Example:

• Acid-metal oxide reactions: Acids react with metal oxides to produce water and a salt.

• Base reactions: Bases neutralize acids.

Acid-Base Titration

Definition and Process

Titration is the reaction of a substance in a solution of known concentration with another substance of unknown concentration.

• Slow process; small amounts are measured.

• Equivalence point: The point at which the number of moles of OH- added equals the number of moles of H+ originally in the solution.

• Indicators are used to determine the equivalence point.

Strong and Weak Acids

Strong Acids

Strong acids completely dissociate (ionize) in solution.

• Example:

• Strong acids are strong electrolytes.

• Common strong acids: HCl, HNO3, H2SO4, HBr, HI, HClO4

Weak Acids

Weak acids do not completely dissociate in solution.

• Example: Acetic acid (CH3COOH)

• Weak acids are weak electrolytes.

Additional info: The strength of an acid depends on its dissociation in water.

Strong and Weak Bases

Strong Bases

• Strong bases dissociate completely in water.

• Common strong bases: NaOH, LiOH, KOH, Sr(OH)2, Ca(OH)2, Ba(OH)2

Weak Bases

• Weak bases do not dissociate completely in water.

• Examples: NH3 (ammonia), pyridine, methylamine, bicarbonate ion

Water: Acid and Base in One

Autoionization of Water

Water can act as both an acid and a base. When water donates a proton, it acts as an acid; when it accepts, it acts as a base.

• Water is amphoteric.

• At 25°C, the concentration of H+ and OH- in pure water is M.

• Acidic solution:

• Basic solution:

• In all aqueous solutions: