3.9 ACID-BASE EQUILIBRIA

• Lowry-Bronsted Theory

  acids are proton donors and bases are proton acceptors
• conjugate acid [1+Eg]

  -species formed when an H+ is donated to a base

  eg. HCl + NH3 ->NH4+ + Cl-

  Acid + base -> conjugate acid + conjugate base

  ACIDS AND BASES FLIP
• conjugate base [1 + Eg]

  -species formed by the loss of an H+ from an acid

  eg. HCl + NH3 ->NH4+ + Cl-

  Acid + base -> conjugate acid + conjugate base
• strength of acid

  extent of dissociation
• conc. of acid

  volume of acid dissolved in 1dm^3 of solution
• strong vs weak acids [1]

  -fully vs partially dissociate in water
• concentrated vs dilute acid

  large/small quantity in small/large quantity of water
• Ka, pKa, and acid strength

  Ka= [products]/[reactants]

  pKa= -log10Ka

  the lower the pKa value, the stronger the acid

  
• Ka value and acid strength

  the higher the Ka value, the stronger the acid
• pH formula strong acids

  pH=-log[H+]

  
• [H+] formula strong acids

  [H+]=10^-pH

  
• Kw, Ionic product constant for water

  Kw= [H+ ] [OH-]

  H2O ⇌ H+ + OH- (reverse reaction is ionic equation for neutralisation)

  
• Kw at 25 degrees C

  1.0 x 10^-14 mol2dm-6

  1.0 x 10^-7 moldm-3

  
• Ka weak acids

  [H+]^2/[HA]

  
• pH weak acids

  -log(sq root Ka x [acid])
• buffer solutions

  solutions that resist change in their pH, even when small amounts of acid or base are added
• acid buffer mechanism [3+3]

  ~Weak acid + salt of same acid~

  ADDING H+

  -eqm shifts to left

  -HA formed

  -eqm restored pH unchanged

  ADDING OH-

  -OH- reacts with H+

  -eqm shifts to right

  -produces more H+
• basic buffer mechanism [3+3]

  ~Weak base and salt of same base~

  ADDING H+

  -H+ reacts with OH-

  -eqm shifts to rights

  -produces more OH-

  ADDING OH-

  -eqm shifts to left

  -more weak base formed

  -pH unchanged
• pH for buffer formula

  
• buffer solution equation


  HA -> H+ + A-
• Example of a buffer system


  biological
• equivalence point (vertical region on pH curve)


  occurs when the moles of acid equal the moles of base in a solution

  
• end point on pH curve

  The point at which the indicator changes colour during a titration.

  
• Strong acid-strong base titration curve, and what indicator suitable? [3]

  -positive S slope and very sharp

  -most indicators suitable

  -start at pH 1

  
• Strong acid-weak base titration curve, and what indicator suitable? [2]

  -methyl orange suitable

  -start at pH 1

  
• Weak acid-strong base titration curve, and what indicator suitable?

  -phenolphthalein suitable

  -start at pH 3

  
• Weak acid-weak base titration curve [3]

  -no real vertical section

  -no real suitable indicators

  -must use pH probe
• pKa


  The negative logarithm of Ka, a measure of an acid's strength.