📦 haribo's chem notes,

acid base

2 min read

further chem
when determining strength, consider:

  • breaking back-bonding to central atom (less Lewis acidity)
  • stability of conjugate and delocalisation/resonance to stabilise charge
    • charge density of atom which bears the charge (size)
    • electronegativity of atom which bears charge
  • inductive effect of adjacent atoms that stabilise charge
  • charge on , which polarises O-H bond and repels the proton as it leaves
    • also the size of
    • proton in hydroxyl bond can leave because:
      • the attracts density away from the ligand, pulling electron density away from the hydrogens in
      • thus, the protons on the water are much more easily lost
      • greater charge density → more acidic complex

experimental tests to distinguish acid strength

  • reaction with a reactive metal (e.g. or a carbonate/hydrogen carbonate) (vigorous/mild bubbling to produce )
  • temperature (high/low) when reacting with a strong base
  • conductivity (high/low) for degree of ionisation (more free ions)
  • pH, at same concentration, strong is lower
  • pKa

experimental tests to distinguish base strength

  • reaction with strong/minimal production of
  • temperature (high/low) on reaction with strong acid
  • conductivity (high/low) for degree of ionisation
  • pH, at same concentration, strong is higher

oxo groups:

pauling’s rules: estimates pKa of oxoacids:

X: central atom
: oxo group
: hydroxyl groups

each successive deprotonation increases pKa by 5 units

is an oversimplification due to participation in hydrogen-bonding

  • more likely
  • is surrounded by a cage of in a pentagonal dodecahedral arrangement

since concentration of water is assumed to be constant

therefore, when the concentration of water is not constant (i.e. all water is protonated or deprotonated, does not make sense)

“A note on good practice: In precise work, is expressed in terms of the activity of X, , its effective thermodynamic concentration. The acidity constant is based on the assumption that solutions are sufficiently dilute for it to be permissible to write .”

and that the autoprotolysis of water can be neglected

similarly,

autoprotolysis of water (self-ionisation)

experimental value at

  • hence, only a very small fraction of water molecules are present as ions

hence, it is possible to find the acid/base dissociation constants by the other

generally,

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