Acids and bases

    Cards (73)

    • Topics covered
      • Bronsted-Lowry acid base equilibria
      • Definition and determination of pH
      • Kw (ion product of water)
      • Ka for weak acids and bases
      • pH curves, titrations and indicators
      • Buffers
    • Bronsted-Lowry acids
      Proton donors
    • Bronsted-Lowry bases
      Proton acceptors
    • Acids produce hydronium ions (H3O+) which make the solution acidic
    • Bases produce hydroxide ions (OH-) which make the solution basic
    • Examples of strong acids
      • Hydrochloric acid
      • Sulfuric acid
      • Nitric acid
    • Examples of weak acids
      • Carboxylic acids (e.g. ethanoic acid)
    • Examples of strong bases

      • Sodium hydroxide
      • Potassium hydroxide
    • Examples of weak bases

      • Ammonia (NH3)
    • Strong acids and bases dissociate/ionise almost completely
    • Weak acids and bases dissociate/ionise poorly
    • Acid-base reactions

      1. Protons are exchanged
      2. Forms a salt (BH+ and A-)
      3. Equilibrium can shift based on amounts of reactants/products added
    • Water behaves as a base when an acid is added to it
    • Ionic product of water (Kw)

      Equilibrium constant for the dissociation of water into H+ and OH- ions
    • Kw = [H+][OH-] = 1 x 10^-14 at 25°C
    • In pure water, [H+] = [OH-]
    • pH
      Logarithmic scale that measures the concentration of H+ ions in solution
    • pH = -log[H+]
    • pH 0 is very acidic, pH 7 is neutral, pH 14 is very basic
    • Calculating pH of hydrochloric acid: pH = -log[H+]
    • pH
      A logarithmic scale that measures the concentration of H+ ions in solution
    • pH scale ranges from 0 (very acidic) to 14 (very basic), with 7 being neutral
    • Calculating pH

      pH = -log[H+]
    • Calculating [H+] from pH
      [H+] = 10^(-pH)
    • Strong acids

      • Fully dissociate in water, [H+] = [acid]
    • Calculating pH of strong acid

      pH = -log[acid]
    • Diprotic acids

      • Produce 2 H+ per acid molecule, [H+] = 2[acid]
    • Strong bases

      • Fully dissociate in water, [OH-] = [base]
    • Calculating pH of strong base

      Use Kw = [H+][OH-] to find [H+], then pH = -log[H+]
    • Ka
      Acid dissociation constant, used for weak acids
    • For weak acids, [H+] ≠ [acid] so Ka must be used
    • Calculating pH of weak acid
      Use Ka expression: Ka = [H+]^2 / [acid]
    • Calculating [acid] from pH and Ka
      Rearrange Ka expression to solve for [acid]
    • Concentration of methanoic acid

      We need to bring up our k expression
    • Rearranging the equation

      Concentration of methanoic acid = h+ squared / ka
    • ka
      Value of 1.77
    • Concentration of weak acid is 2.96 x 10^-3
    • To calculate ka, we just put the figures in and work it out, no need to rearrange
    • pH
      Measure of acidity
    • pKa
      Another way of measuring the strength of an acid, using ka instead of concentration of H+