Rank the following species in order of increasing acidity. Explain your reasons for ordering them as you do?

get rank
alyssia_o6 asked:

NH3, H2SO4, CH3OH, CH3COOH

Please help me to understand why you are ranking them the way that you do. There are so many rules and exceptions it seems for acids and bases that it is hard for me to rank a group of more than two.



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2 Responses to 'Rank the following species in order of increasing acidity. Explain your reasons for ordering them as you do?'

  1. steve_geo1 - May 19th, 2010 at 9:45 am

    NH3 < CH3OH < CH3COOH < H2SO4

  2. Steve M - May 22nd, 2010 at 1:40 am

    Order: NH3 < CH3OH < CH3COOH < H2SO4

    Logic:

    (1) Inorganic/mineral acids are always VERY strong acids, stronger than most (if not all) organic acids - hence H2SO4

    (2) The amines/amino compounds are basic, not very acidic, hence this NH3 is the weakest of all these acids.

    This leaves CH3OH and CH3COOH. To compare acidities it is usually helpful to compare the stabilities of the conjugate bases (deprotonated form):

    CH3OH --> CH3O(-) + H(+)
    CH3COOH –> CH3COO(-) + H(+)

    The most stable conjugate base has the most acidic parent molecule. In this case, CH3COO(-) is more stable than the CH3O(-) because the is resonance stabilization in the CO2(-) group. As this conjugate base is more stable, the CH3CO2H molecule is more acidic than the CH3OH molecule.

    This leave us with the ordering:
    NH3 < CH3OH < CH3COOH < H2SO4

    Hope this helps!


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