[joelhall]

ok ok stupid question (especially for a first post!), but i always have trouble with acids and bases (not studied for years and recently restarted) so ill start simple...

this is from mcmurray...

'amide ion, H2N-, is a much stronger base than hydroxide ion, HO-.  which would you expect to be a stronger acid, NH3 or H2O?  explain.'

for the life of me ive no idea.  no values of pKa etc are given, so i put it to you all - why are these oxygen compounds stronger acids?  i logically assumed the more electronegative atom/greater polarity would give a stronger base???

any help much appreciated.

joel

[Borek]

It is about acid and conjugate base (or base and conjugate acid).

The stronger the acid, the weaker the conjugate base - and vice versa.

[joelhall]

forgive me! i meant greater electronegativity/polarity = stronger ACID not base in the above post 

i understand the relationship between the reactants and conjugate acid and base strengths, though assuming you have no figures of strength nor measuring equipment, would this rule be generally true, i.e. the molecule with the greatest electronegative atom/polarity is more likely to hold onto its valence e- and therefore it is more likely to donate the proton?

it just seems a pattern between these reactions of basic compounds that the more electronegative atoms cause greater charge separation of the proton from its bonding electron, weakening hydrogens attraction to the molecule and so it is more likely to be able to dissociate.

e.g. HCl + H2O <---> Cl- + H3O+, H2O + H2N- <---> OH- +NH3, CH3COOH + HO- <---> H2O + CH3COO-, etc.

any thoughts?  or is this coincidence?

joel

[gfunk]

Acidity is a function of the stability of the conjugate base.  Therefore, if the resulting conjugate base can stabilise the negative charge well, the acidic species will thus be more acidic.  Thus, electronegative atoms have a higher affinity for electrons/negative charges, and can thus stabilise the negative charge of the conjugate base better.  This leads to stronger acidity.

However, hydrofluoric acid is a funny species!  Hope that helps a bit .

[joelhall]

it certainly does, and only after i posted did i see the huge flaw in my theory  - that being that EN(Cl)<EN(O), so i would have to assume water  to be the stronger acid than HCl and thats a little bit of a problem...

looking up the relative electron affinities and ionization energies was also fruitless, so i guess what i should ask, given that no figures are  given and it is assumed you can solve the problem from whats been told so far:

what characteristics of a chemical formula give an indication of relative strengths of an acid or base, with no other data or  experiment given?  (see the question quoted above).

thanks

joel

[Arctic-Nation]

There are a few general rules concerning the acidity of compounds, but these only give approximations relative to comparable compounds. Even then the real difference in acidity can easily be a few (or more) orders of magnitude.

These are some rules from the top of my head, so don't expect them to be complete, but they're general guidelines.

Negative charges decrease acidity of any remaining acidic hydrogen atoms.

For mineral acids:
1. Halogen acids (HF, HCl, HBr, HI) are all very strong acids (pH -5 and up)
2. Oxoacids (general formula H_mXO_n, for example sulfuric acid) are weak to very strong acids. Acidity increases along with the number of oxygen atoms.

For organic acids (and any organic compound can be made an acid):
1. The presence of electron withdrawing substituents increases acidity, both through inductive and mesomeric effects (not necessarily together). This is a very general rule, and I'm not inclined to go into detail as there's a lot of ground to cover concerning all possible classes of substituents. Again, more electronegative heteroatoms increases acidity.
2. Hybridization of the carbon atom: more s character increases acidity of hydrogen atoms bonded to that carbon.

Remember these pKa values (for the parent compounds, thus only C and H present): Carboxylic acids, 4.50-5. Alcohols, 16-17. Ketones and aldehydes, 20. Acetylene, 25. Ammonia, 33. Alkenes and aromatic rings: 40-44. Alkanes, 50-60.

[joelhall]

thanks guys youve been very helpful