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Conjugate acid/base pairs

Submitted by Chemkidfan on Sun, 05/02/2010 - 01:42

Hello

why do strong acids have weak conjugate bases and strong bases have weak conjugate acids.

Also howcome weak acids dont have strong conjugate bases and weak bases dont have strong conjugate acids..

Thank you  :) for any help


definitions are fixed but you should always use examples to illustrate the definition, after all what you see might explain better what you used lines after lines to explain.

so take HCl, for example, you(based on chemical knowledge knows it is  a strong acid),
so
HCl ---> H+ + Cl-
Cl- is its conjugate base, ie as a base, it is supposed to protonate, because HCl is a strong acid(relative to say acetic acid, a weak acid), its conjugate base is less likely to protonate compared to the conjugate base of acetic acid (based on reverse arguments)

strong base, lets take NaOH(a strong base) vs NH3(a weak base), based on their pH
NaOH ----> Na+ + OH-
NH3 + H2O ---> NH4+ + OH-

Strong base => weak conjugate acid(which would be Na+, as it'd be well solvated in aqueous solutions, hence more OH- remains in such solutions, making it more basic than NH3)

Weak base(ie NH3 which exists as gas molecules in its standard state), gives NH4+ as conjugate acid, which is less "ionic" than Na+, less solvated, more available for protonating OH- , hence reforming NH3, making it a weak base, so NH4+ is said to be a better proton donor than Na+(or alternatively in lewis acidity term, better electron acceptor ability) than Na+.


You might have given a table for relative strength of acids and bases that has been developed by comparig various acid-base reaction.Acid strength decreases down the table and base strength increases down the table.And in this u can that a definite relationship exists between acids and base strenghts,when u say an acid loses its proton very readily you can also say that its conjugate base does not hold proton very tightly.
For example:  
Lets see HCl and its conjugate base Cl- where they both exist in that table HCL is at the top of the table which shows that it is a strong acid as acid strenght decreses down the group while its conjugate base Cl- also at the same postion which indicates it is a weak base as base strength decrease from bottom to top.
Also the direction for an acid base reaction always favours the weaker acid and base that is direction of reaction is from stronger acid and base to weaker acid  and base