Module 5.1.3- Acids, bases and buffers

• Bronsted-Lowry acid

  proton donor
• Bronsted-Lowry base

  proton acceptor
• Conjugate acid-base pair


  consists of two substances related to each other by the donating and accepting of a single hydrogen ion

  
• Monobasic acid


  Produce a H+ ion in solution
• Dibasic acid


  Produces two H+ ions in solution
• Tribasic acid


  Produces three H+ ions in solution
• Ionic equation for neutralisation


  H+ + OH- -> H2O
• Acid + Metal Oxide

  salt + water
• Acid + Alkali

  salt + water
• Acid + Carbonate

  salt + water + carbon dioxide
• Acid + Metal

  salt + hydrogen
• formula for pH


  -log[H+]

  
• formula for [H+]

  10^-pH

  
• Calculating pH of strong acids


  They ionise 100%, so if given the concentration, then they will be plugged straight into the pH equation
• dilution formula

  C1V1=C2V2

  
• Strong acid


  an acid that dissociates completely in aqueous solution
• Weak acid


  an acid that dissociates partially in aqueous solution
• Acid dissociation constant Ka


  the ratio of the concentration of the dissociated form of an acid to the undissociated form; stronger acids have larger Ka values than weaker acids

  
• formula for pKa

  -log(Ka)

  
• The pH of weak acids


  Find Ka and use an RICE table to determine H+

  Ka = [H+][A-]/ [HA]

  Ka= [H+]^2/ [HA]

  √Ka x [HA] = [H+]

  then use -log[H+] for pH
• Approximations in calculations involving weak acids


  • dissociation of water is negligible
  • concentration of acid is much greater than the H+ concentration at equilibrium
• What is Kw

  The ionic product of water

  Kw = [H+][OH-]
• Kw at 25 C


  1.0 x 10^-14

  
• Acidic solution


  [H+] > [OH-]
• Alkali solution


  [OH-] > [H+]
• Neutral solution


  [H+] = [OH-]
• pH of a strong base can be calculated from

  - the concentration of the base

  - the ionic product of water Kw

  convert [OH-] to [H+]

  then use pH= -log[H+]
• Convert [OH-] to [H+]


  Kw= [OH-][H+]

  Kw / [OH-] = [H+]
• For neutralisation reactions: calculating the pH of a solution created by the reaction of a strong acid and base


  1) calculate moles of H+

  2) calculate moles of OH-

  3) calculate excess moles of H+ or OH- (subtract smaller from larger)

  4) convert to a concentration in the final solution ( [H+] or [OH-] )

  5) calculate pH
• Calculating pH of weak base

  Similar method to calculating the pH of weak acid
• Buffer solution


  a solution made from a weak acid and its conjugate base that neutralizes small amounts of acids or bases added to it
• The weak acid in a buffer solution...

  ...removes added alkali
• The conjugate base in a buffer solution...


  ...removes added acid
• Preparation of weak acid buffer solutions (1)

  Weak acid and its salt

  e.g. CH3COOH + CH3COONa
• Preparation of weak acid buffer solutions by neutralisation

  Weak acid and aqueous solution of an alkali

  e.g. CH3COOH + NaOH
• Buffer solution equilibrium


  HA (aq) ⇌ H+(aq) + A- (aq)
• Adding acid to a buffer equilibrium


  [H+] increases

  H+ reacts with conjugate base A-

  Equilibrium shifts to the left removing most of the H+ ions
• Adding alkali to a buffer equilibrium


  [OH-] increases

  OH- reacts with small concentration of H+ ions to form water

  Equilibrium shifts to the right
• Calculating pH of a buffer solution

  Calculate H+ using equation

  Ka x [HA]/[A-]

  Simple conversion

  
• When concentration of [HA] and [A-] are the same:


  Ka = [H+] AND pKa= pH