7.2 - Acids

Acidity and pH

  • Acidity: Quantifies the concentration of hydrogen ions (H+) in a solution.

  • pH Scale: Ranges from 0 (highly acidic) to 14 (highly basic); a pH of 7 is neutral (e.g., pure water).

  • The formula to calculate pH is: pH = −log ([H+]).

  • At 25°C: The concentration of H+ and OH- in pure water is 1 x 10^-7 mol/L, leading to a pH of 7.

Acid/Base meanings

  1. Arrhenius Theory (1887):

    • Defines acids as substances that increase the concentration of H+ ions in aqueous solutions.

    • Bases are defined as substances that increase the concentration of OH- ions in aqueous solutions.

  2. Bronsted-Lowry Theory (1923):

    • Expands upon Arrhenius by defining acids as proton donors and bases as proton acceptors.

      • Acids: donors

      • Bases: acceptors

    • Example: In the reaction HCl + NH3 → NH4+ + Cl–, HCl donates a proton (acid), and NH3 accepts it (base).

  3. Lewis Theory (1923):

    • Defines lewis acids as electron-pair acceptors and bases as electron-pair donors.

    • Example: The interaction NH3 + BF3 demonstrates coordinate bonding where NH3 donates an electron pair to BF3 (the Lewis acid).

Amphoteric Substances

  • Amphoteric Compounds: Substances that can act as either acids or bases, depending on the environment.

  • Water (H2O): Is a prime example, acting as a base when reacting with acids (e.g., HCl) and as an acid when reacting with bases (e.g., NaOH).

Carboxylic Acids

  • Definition: A family of organic acids characterized by the presence of a carboxyl group (R-COOH).

  • Properties:

    • Carboxylic acids are weak acids that are only partially ionised in aqueous solutions, making them weak electrolytes.

    • React with bases to form salts and water in neutralisation reactions.

  • Examples:

    • Methanoic Acid (HCOOH, R=H), Ethanoic Acid (CH3COOH, R=CH3), Propanoic Acid (C3H6O2, R=CH3CH2).

Structures of Polymers

  • Polymers: Large macromolecules formed by chemically bonding small units called monomers. They can be classified as either natural (e.g., proteins, cellulose) or synthetic (e.g., nylon, polyethylene).

  • Types of Polymer Structures:

    • Linear: Chains of monomers connected in a single line, resulting in specific properties (e.g., increased tensile strength).

    • Branched: Chains with side branches that can affect the melting point and viscosity.

    • Crosslinked: Monomers linked together in a complex network, enhancing rigidity and thermal stability, often used in thermosetting plastics.