Acids and Bases: Properties, Reactions, and Classification

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.

	Acid	Base
Taste	sour	bitter
Litmus	blue litmus turns red	red litmus turns blue
Other	dissolves metals readily	slippery feel
Ions	H+ (hydronium ion)	OH- (hydroxide ion)
Common forms	HCl, H2SO4, HNO3, HCOOH, HF	NaOH, KOH, NaHCO3, NH3

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: $\mathrm{HCl} + \mathrm{H_2O} \rightarrow \mathrm{H_3O^+} + \mathrm{Cl^-}$

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: $\mathrm{HCl} + \mathrm{NaHCO_3} \rightarrow \mathrm{NaCl} + \mathrm{CO_2} + \mathrm{H_2O}$

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

• Example: $\mathrm{Mg} + 2\mathrm{HCl} \rightarrow \mathrm{MgCl_2} + \mathrm{H_2}$

• 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: $\mathrm{HCl} \rightarrow \mathrm{H^+} + \mathrm{Cl^-}$

• 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 $1.0 \times 10^{-7}$ M.

• Acidic solution: $[\mathrm{H^+}] > [\mathrm{OH^-}]$

• Basic solution: $[\mathrm{OH^-}] > [\mathrm{H^+}]$

• In all aqueous solutions: $[\mathrm{H^+}][\mathrm{OH^-}] = K_w = 1.0 \times 10^{-14}$