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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: timchai on March 16, 2009, 07:45:33 AM

Title: F- weaker base than OH- why
Post by: timchai on March 16, 2009, 07:45:33 AM
ans said .A Bronsted-Lowry base is a proton acceptor.
H2O does not self ionize appreciably, while HF does.
OH- can thus accept a proton better than F-, and hence OH- is a
stronger Bronsted-Lowry base than

but i want to ask why . i think the ans is not correct, i think the ans is OH- more stronger than F- , bcoz of electronegativity , so tendency of accept electrons greater
Title: Re: F- weaker base than OH- why
Post by: BluRay on March 16, 2009, 11:06:49 AM
Quote from: timchai on March 16, 2009, 07:45:33 AM
ans said .A Bronsted-Lowry base is a proton acceptor.
H2O does not self ionize appreciably, while HF does.
OH- can thus accept a proton better than F-, and hence OH- is a
stronger Bronsted-Lowry base than

but i want to ask why . i think the ans is not correct, i think the ans is OH- more stronger than F- , bcoz of electronegativity , so tendency of accept electrons greater
In case, it would be the tendency to "give" electrons (to form a covalent bond) which is greater for OH-. However the first explanation is actually better: you have to compare the equilibrium reactions:

OH- + H2O <-->   H2O + OH-

F- + H2O  <-->   HF + OH-

The first reaction has the greater equilibrium constant in water solution and at room T and it's for this reason that OH- is a stronger base than F-
Title: Re: F- weaker base than OH- why
Post by: spirochete on March 18, 2009, 09:15:45 PM
BluRay's equilibrium description is correct but I'll add some qualitative explanation.

Bronsted-Lowry basicity is the tendency of a molecule to accept a proton, not an electron.  It's 100% true that F- is a weaker base than OH- because of electronegativity. 

Having a negative charge is destabilizing, but a more electronegative atom is destabilized less by having a negative charge.  In other words, a more electronegative atom is "happier" with a negative charge.  With neutral bases there is simply an electron pair but the same logic applies.

Acquiring a proton means a base no longer has full possession of an electron pair.  More electronegative atoms will be less likely to do this because they are relatively stable with a lone pair and/or negative charge.
Title: Re: F- weaker base than OH- why
Post by: BluRay on March 19, 2009, 06:23:01 PM
Quote from: spirochete on March 18, 2009, 09:15:45 PM
It's 100% true that F- is a weaker base than OH- because of electronegativity. 
Then why I- (for example) is not a stronger base than F- or than OH- in water solution? Iodine is less electronegative than Fluorine or Oxygen.