SCH4U - Chemistry 12 (2024-25) - A

SCH4U-25A

4-3D: Summary

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4-3D: Summary

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  • Arrhenius acids & bases are classified based on their ability to ionize and form H+ or OH- ions
  • Brønsted-Lowry classified acids and bases based on their ability to donate or accept a proton (hydrogen ion).  According to Brønsted-Lowry an acid is classified as a proton donor while a base is classified as a proton acceptor
  • A strong acid or base is one that ionizes completely in water
  • A weak acid or base is one that ionize slightly in water, forming an equilibrium between the forward reaction in which ionization occurs and the reverse reaction in which the ions recombine
  • The equilibrium constant of a generic weak acid (HA) is called the acid dissociation constant (Ka) and is calculated by the formula: Ka = ([H3O+] * [A-]) / [HA]
  • The equilibrium constant of a generic weak base (B) is called the base dissociation constant (Kb) and is calculated by the formula: Kb = ([BH+] * [OH-]) / [B]
  • Water can act as an acid or a base according to Brønsted-Lowry theory as H2O ionizes to a small degree to form H+ and OH- ions
  • The equilibrium constant for the ionization of water (Kw) is calculated by the formula: Kw = [H+]*[OH-], which has a known value at 25 ºC of 1 x 10-14
  • pH and pOH are measures of the strength of an acid or base. 
  • Pure water has a pH of 7 is considered neutral
  • Acids have pH values less than 7 while bases have pH values greater than 7
  • The lower the pH value, the stronger the acid, the greater the pH value the stronger the base
  • pH = -log[H+]
  • [H+] = 10-pH
  • pOH = -log[OH-]
  • [OH-] = 10-pOH
  • pH + pOH = 14
  • Buffers are solutions that maintain a constant pH
  • Buffers are made up of a weak base and its conjugate acid or a weak acid and its conjugate base
  • Titration is a laboratory technique that can be used to determine the concentration of unknown acids & bases
  • In a titration, an unknown sample is placed in an erlenmeyer flask and an indicator that changes colour with pH is added.
  • The titrant is a solution with a known concentration that is added drop-by-drop into the unknown sample
  • The equivalence point is the point at which the titrant has completely neutralized the unknown sample