Acids and Bases

• Acid (definition)

  substance defined as a proton donor according to Bronsted-Lowry theory
• Base (Definition)

  Substance defined as a proton acceptor according to Bronsted-Lowry theory
• Conjugate Acid (definition)

  species formed from a Bronsted-Lowry base by the addition of a proton
• Conjugate acid of NH3

  NH4+
• COnjugate base of CH3COOH

  CH3COO-
• Neutral Solution

  solution with an equal number of H+ and OH- ions
• Monoprotic (definition)

  1 mol of acid produces one mol of H+ ions
• Diprotic (definition)

  1 mol of acid produces 2 mol of H+
• Species that can act as both a BL acid and base

  • Hydrogenphosphate
  • Dihydrogenphosphate
  • Hydrogencarbonate
• Is [H20] included in the calculation for Kw? Why (not)?

  The value of [H20] is very big compared to [H+] and [OH-], so it is effectively constant and incorporated into Kw
• Does the value of Kw vary with temperature?

  Yes
• Does Kw vary with temperature? Why (not)?

  • Yes
  • As water is heated, the bonds holding it together dissociate more. More H+ is produced, so [H+] increases and pH decreases.
  • Although heated water has a lower pH, it is NOT more acidic as an equal amount of [OH-] is also produced.
• Value of Kw at 298K

  1 * 10^-14 mold^2 dm^-6
• Using Kw to calculate the pH of a strong base 


  1. Use equation Kw = [H+][OH-] to find [H+]
  2. Plug value of [H+] into pH = -log base 10 ([H+])
  3. Find [OH-]
  4. Plug in [OH-] into pOH = -log base 10 ([OH-])
• Bromothymol Blue

  Lower - yellow

  Upper - blue

  Colour change in pH range - 1.2 to 2.8
• Methyl Orange

  Lower - red

  Upper - yellow

  Colour change in pH range - 3.1 to 4.4
• Methyl Red

  Lower - red

  Upper - yellow

  Colour change in pH range - 4.4 to 6.2
• Phenolphthalein

  Lower - colourless

  Upper - pink

  Colour change in pH range - 6 to 7.6
• Thymol Blue

  Lower - red

  Upper - yellow

  Colour change in pH range - 8.3 to 10.0
• Buffers


  Buffers are solutions that can resist changes in pH when small quantities of acid or base are added.
• Acidic Buffer


  HA <=> H+ + A-
• Acidic Buffer - adding small amounts of acid 


  If H+ is added the equilibrium moves to the left, removing the H+ ions and restoring the pH
• Acidic buffers - adding small amounts of alkali



  • If OH- is added, the OH- ions react with H+ ions removing them.
  • This makes the equilibrium move to the right, replacing the H+ and restoring the pH
• In an EQ, you should use the equilibrium for the weak acid in the question when you give your answer.
• Basic buffer - adding small amounts of acid

  If H+ is added it reacts with ammonia and the equilibrium moves to the right, removing the H+ ions and restoring the pH.
• Basic buffers - small amount of alkali added


  • If OH- is added, the OH- ions react with H+ ions removing them and forming water (H+ + OH- à H2O)
  • This makes the equilibrium move to the left, replacing the H+ and restoring the pH
• Methods of preparing acidic buffers

  • Addition of a solid salt of the weak acid to a solution of the weak acid.
  • Addition of a solid strong bases such as sodium hydroxide to a solution of the weak acid.
  • Addition of a solution of the salt of a weak acid to a solution of the weak acid.
  • Addition of a strong base such as sodium hydroxide solution to a solution of an excess of the weak acid.
• As both HA and A- are in the same volume of solution, the amount in moles of HA and A- may be used in the calculations instead of concentration.
• Blood 


  In the human body, one important buffer system in blood involves the hydrogencarbonate ion, HCO3-, and carbonic acid, H2CO3, which is formed when carbon dioxide dissolves in water.

   

  Blood acts as a buffer to maintain a constant pH of 7.41 even if a small amount of acid enters the bloodstream.
• Blood

  • Human body - important buffer system in blood
  • involves the hydrogencarbonate ion, HCO3-,
  • carbonic acid, H2CO3 (formed when CO2 dissolves in water)
  • Blood acts as a buffer
  • maintains a constant pH of 7.41 even if a small amount of acid enters the bloodstream.
• Blood (p.2)


  • Addition of small amounts of acid send the equilibrium to the left.
  • Ratio of [carbonic acid]/[hydrogencarbonate ion] remains constant
  • Hence [H+] and pH remain constant.
• Blood p.3

  
• Strong Acid and Base Dissociation


  Here it is

  
• Acid Dissociation Equations


  for HCl

  
• For strong acids, [H+] = ...


  [HA]