Acids and Bases: Definitions, Strength, Dissociation, and Amphoteric Water

Chapter 10: Acids and Bases

10.1 Definitions

This section introduces the fundamental definitions of acids and bases, focusing on the hydronium ion and classical models.

• Hydronium Ion (H3O+): The ion formed when an acid reacts with water. It is central to acid-base chemistry in aqueous solutions.

• Arrhenius Definition:

  • Acid: A substance that produces hydronium ions (H3O+) when dissolved in water. Example Reaction: $\mathrm{HCl} + \mathrm{H_2O} \rightarrow \mathrm{H_3O^+} + \mathrm{Cl^-}$

  • Base: A substance that produces hydroxide ions (OH-) when dissolved in water. Examples:

    • Metal Hydroxides: Ionic compounds (e.g., NaOH) that release OH- directly.

    • Molecular Compounds: Substances like ammonia (NH3) that react with water to produce OH-.

• Brønsted-Lowry Definition:

  • Acid: Proton donor (gives a hydrogen ion, H+, to another molecule or ion). Solvent Independent: The reaction does not need to occur in water. H+ Transfer: The defining characteristic is the transfer of a proton to a base.

  • Base: Proton acceptor (accepts H+ from an acid). Lone Pair Requirement: Must have a lone pair of electrons to bond with the incoming proton (e.g., NH3).

• Polyprotic Acids: Acids that can donate more than one proton per molecule.

  • Monoprotic: HCl (1 proton)

  • Diprotic: H2SO4 (2 protons)

  • Triprotic: H3PO4 (3 protons)

• Conjugate Acid-Base Pairs:

  • Pairs of chemical species found on opposite sides of a chemical reaction, differing by one hydrogen ion (H+).

  • General Reaction: $\mathrm{HA} + \mathrm{B} \rightleftharpoons \mathrm{A^-} + \mathrm{BH^+}$

10.2 Acid and Base Strength

This section analyzes the strength of acids and bases and how it affects chemical equilibria.

• Strong Acids:

  • Give up a proton easily; nearly 100% dissociated in water.

  • Reaction goes essentially to completion.

• Weak Acids:

  • Give up a proton with difficulty; only partially dissociated.

  • Equilibrium favors the undissociated form.

• Strong Bases:

  • Hold a proton tightly; high affinity for H+.

• Weak Bases:

  • Have little affinity for a proton; do not hold H+ tightly.

• Stepwise Dissociation:

  • Polyprotic acids lose protons in steps. Step 1: Nearly 100% extent for strong first-stage acids. Step 2: Much lesser extent due to electrostatic difficulty in removing a proton from a negatively charged ion.

• Inverse Strength Relationship (Seesaw Rule):

  • Stronger Acid → Weaker Conjugate Base

  • Weaker Acid → Stronger Conjugate Base

• Predicting Equilibrium:

  • Equilibrium favors the reaction of the stronger acid with the stronger base.

  • The reaction proceeds to form the weaker acid and the weaker base.

10.3 Acid Dissociation Constants (Ka)

This section explores the acid dissociation constant as a predictor of acid strength.

• Expression for Ka: $K_a = \frac{[\mathrm{H_3O^+}][\mathrm{A^-}]}{[\mathrm{HA}]}$

• Interpreting Ka Values:

  Type	Ka Value	Interpretation
  Strong Acid	Ka >> 1	Dissociation is highly favored.
  Weak Acid	Ka << 1	Dissociation is not favored.
  Organic Acids	~10-5	Typically weak acids (contain –COOH).

10.4 Amphoteric Water

This section examines water's unique role acting as both an acid and a base.

• Amphoteric Substance: Can react as either an acid or a base.

• As a Base: Accepts a proton when in contact with an acid.

• As an Acid: Donates a proton when in contact with a base.

Ion-Product Constant (Kw)

Water undergoes autoionization, forming hydronium and hydroxide ions in equal amounts.

• Autoionization Reaction: $2\mathrm{H_2O} \rightleftharpoons \mathrm{H_3O^+} + \mathrm{OH^-}$

• Value of Kw at 25°C: $K_w = 1.0 \times 10^{-14}$

Solution Classification

Solutions are classified based on the relative concentrations of hydronium and hydroxide ions.

Type	[H3O+]	[OH-]
Acidic	> 10-7	< 10-7
Neutral	= 10-7	= 10-7
Basic	< 10-7	> 10-7

Example:

If [H3O+] = 1.0 × 10-4 M and [OH-] = 1.0 × 10-10 M, the solution is acidic.

Additional info: These notes expand on the provided slides with definitions, equations, and examples for clarity and completeness.