Different definitions of acids/bases

[member 392791]

I am having some difficulty in understanding the reason for the various types of acids/bases, of which i refer to bronsted-lowry, arrehenius, and lewis acids/bases. A bronsted acid donates an H+ and a bronsted base accepts a H+. However, a lewis accept acceps an e- pair and a lewis base donates an e- pair. This seems contradictory, is a lewis acid a bronsted base and lewis base a bronsted acid? What is the purpose of this, why isn't there a single definition of an acid and base.

 

[Jorriss]

Generally speaking, something is an acid if you add it to water and it lowers the pH. Clearly, an arrhenius or bronsted-lowry acid does this - they simply donate a proton.A lewis acid also does the same thing through the reactions;

X + H2O -> X-OH2 -> X-OH + H+

So we see a lewis acid also produces a hydrogen ion.

 

[member 392791]

I am still not clear on how lewis bases/acids work, or why we need them. It seems that bronsted works just fine

 

[Jorriss]

Well, if you add a lewis acid to water, the pH goes down. Bronsted Lowry theory does not account for that. So SOMETHING new was needed.

Lewis acid/base theory is also more generally useful as you'll (possibly) find out in organic and inorganic chem.

 

[ChiralWaltz]

"This seems contradictory, is a lewis acid a bronsted base and lewis base a bronsted acid?"

Not quite. A Lewis base is also a BL base but Lewis acids don't have to be BL acids. In Lewis theory, we focus on the electron pair. In BL theory, we focus on the hydrogen proton (H+).

I am still not clear on how lewis bases/acids work, or why we need them. It seems that bronsted works just fine

In organic chemistry, we generally are trying to find where the electron/s wants to go. What Lewis theory allows us to do is expand our definitions beyond transfer of just H+. So now we can treat reactions that do not involve proton transfer (BL theory) as acid-base reactions with Lewis theory. Thus we have increased the number of acid species available to work with based on the definition provided by Lewis Theory.

I do agree with you on how the definitions can be tedious or seem whimsical at times. It's important not to get slowed down by them though.