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Branches of Chemistry

• Chemistry is the science that studies matter and its properties.
• There are five main branches of Chemistry:
  • Organic Chemistry: studies carbon compounds containing carbon-hydrogen bonds (e.g., CH4).
  • Inorganic Chemistry: studies the structure, properties, and reactions of non-carbon compounds.
  • Analytical Chemistry: deals with the identification, separation, and quantification of chemical substances.
    • Qualitative Analysis: identifies chemical substances in a sample.
    • Quantitative Analysis: determines the concentration or amount of a substance.
  • Physical Chemistry: covers the physical properties of chemical compounds using physics concepts.
  • Biochemistry: studies biological structure, composition, and chemical reactions at the cellular and molecular level.

Acids and Bases - Historical Perspective

• Early chemists categorized substances based on physical and chemical properties.
• Substance A:
  • Sour taste
  • Reacts with carbonates to produce CO2
  • Reacts with metals to produce hydrogen (H2)
  • Turns blue litmus pink
  • Reacts with substance B to produce salt and water.
• Substance B:
  • Bitter taste
  • Reacts with fats to produce soaps
  • Does not react with most metals
  • Turns red litmus blue
  • Reacts with substance A to produce salt and water.

Arrhenius Theory

• Arrhenius proposed the first definition of acids and bases.
• Acids: substances that dissociate in water to produce H+ ions.
• Bases: substances that dissociate in water to produce OH- ions.
  • Example: NaOH (aq) → Na+(aq) + OH-(aq)
  • Example: HCl (aq) → H+(aq) + Cl-(aq)

Brønsted-Lowry Theory

• Brønsted and Lowry revised Arrhenius's theory to include other solvents besides water.
• Acid: a hydrogen-containing species that donates a proton.
• Base: a substance that accepts a proton.
• Example: HCl (aq) + H₂O (l) → Cl⁻(aq) + H₃O⁺(aq) ( In this case, HCl is the Brønsted Acid and H₂O is the Brønsted Base.).

Conjugate Acid-Base Pairs

• Brønsted acids and bases always exist as conjugate acid-base pairs.
• Their formulas differ by only one proton.
• In the reverse reaction, Cl⁻ acts as a base and H₃O⁺ acts as an acid.

Strong vs. Weak Acids

• Strong acids ionize completely.
• Weak acids do not ionize completely
• Single arrows indicate complete ionization (strong acids)
• Double arrows indicate equilibrium (weak acids).

Common Strong Acids and Bases

• A list of common strong acids and bases is provided.

Autoionization of Water

• Water molecules can act as both acids and bases (amphoteric).
• In pure water: H₂O (l) + H₂O (l) ⇌ H₃O⁺ (aq) + OH⁻ (aq). -For pure water, [OH⁻] = [H⁺].

Ionization Constant of Water (Kw)

• Kw is the ionization constant of water.
• Kw is temperature dependent.
• Kw = [H+][OH-].

pH Scale

• Aqueous solutions are classified as neutral, acidic, or basic based on the relative concentrations of H+ and OH- ions.
  • Neutral: [H⁺] = [OH⁻]
  • Acidic: [H⁺] > [OH⁻]
  • Basic: [H⁺] < [OH⁻]
• pH is the power of hydrogen, a measure of the concentration of H+.

Lewis Theory

• Lewis theory focuses on electron transfer instead of proton transfer.
• Acids accept electron pairs, while bases donate electron pairs.
• The reaction of a Lewis acid and a Lewis base produces a coordinate covalent bond.

Classification of Lewis Acids

• Various classifications of Lewis acids are provided.