Equilibrium acid-base systems

• Kw


  Water ionization constant
• Water in a liquid state, is eliminated from the equilibrium expression, water is indicated by the resulting ions are in an aqueous state
• Pure water has slight conductivity
• Water also has the ability to self ionize, however ionization occurs only 0.000001% of the time, therefore reaction greatly favours the reactants under SATP
• Acid (according to the modified Arrhenius theory)

  A substance that reacts with water to form hydronium ions
• Base (according to the modified Arrhenius theory)

  A substance that reacts with water to form hydroxide ions
• pH


  The power of the hydronium ions and is a measure of the activity of hydronium ions in a solution
• pH meter


  1. Measures the concentration of hydronium ions using an electrochemical cell

  2. Measures the potential difference between a pH probe placed in a solution of hydronium ions and a reference half cell

  3. Reduction potential difference is then converted into pH
• pOH


  The power of the hydroxide ion and is a measure of the activity of the hydroxide ions in a solution and therefore its alkalinity
• Strong and weak acids are not defined by concentrations, they are defined by their ability to ionize
• Strong acid

  Ionizes quantitatively with water to form hydronium ions and its conjugate base
• Weak acid


  Only ionizes partially (less than 50%, <50%) with water to form hydronium ion and its conjugate base
• Percent ionization


  Relates the concentration of hydronium ions at equilibrium to the original concentration of the acid
• Strong acid

  • High conductivity
  • Fast reaction rates with active metals and carbonates
  • pH closer to 0
• Weak acid

  • Low conductivity
  • Slow reaction rates with active metals and carbonates
  • pH closer to 7
• Arrhenius theory

  Acids ionize in water to produce hydronium ions plus an anion, Bases dissociate to produce hydroxide ions plus a cation
• Limitations of the Arrhenius theory: failed to predict the acidic or basic properties of some compounds
• modified Arrhenius theory


  Acids are substances that react with water to produce hydronium ions and bases are substances that react with water to produce hydroxide ions
• Limitaions of the modified Arrhenius theory


  - no provisions for reactions that do not occur in an aqueous solution - some substances have both acidic and basic properties
• Amphoteric 


  substances that have both acidic and basic properties
• Brønsted–Lowry concept 


  An entity is referred to as acting as an acid or acting as a base in the context of a specific reaction.
• A Brønsted–Lowry acid


  a proton donor in a specific reaction.
• A Brønsted–Lowry base


  a proton acceptor in a specific reaction.
• Brønsted–Lowry reaction


  involves a single proton transfer from one entity (the acid) to another (the base). Resulting in an acid–base dynamic equilibrium, where the forward and reverse reactions occur at the same time and at the same rate.
• Acetic acid


  Weak acid with strong attraction for its own proton
• Acetate ion


  Conjugate base of acetic acid, stronger base than water
• The weaker an acid


  The stronger its conjugate base
• Hydrogen chloride molecule


  Much weaker attraction for its own proton than water, making HCl(aq) a strong acid
• The stronger an acid


  The less it attracts its own proton
• The stronger a base


  The more it attracts another proton
• The stronger an acid


  The weaker its conjugate base
• Strongest base


  Reacts with the substance that gives up its proton or protons most easily (i.e. the strongest acid)
• Strong Electrolytes


  Write in dissociated form
• Weak Electrolytes


  Write as is
• All ionic hydroxides completely dissociate upon dissolving, so they are considered to be strong bases
• Acid ionization constant, Ka

  Indicates the extent to which an acid will react with water, ratio of dissociated form to undissociated form
• The advantage of using Ka over % ionization to express strength is that once determined, it is valid over a wide range of concentrations, unlike % ionization which is concentration specific
• Base ionization constant, Kb


  Value of a weak base determined using an equilibrium law expression for a Bronsted-Lowry reaction between the weak base and water
• Kw


  Ka x Kb
• Rule Of 1000

  The value of x in the denominator can be omitted whenever the original concentration of the acid is at least 1000 times the numerical value of the Ka or Kb