Acid vs. Base

[Vivian Leung 1C]

Hi,

I am a little confused as to how to tell if a compound is an acid or a base just by looking at it.

From my general understanding, you can use either the Bronsted or the Lewis definition of an acid or base:

Bronsted: Acids are proton donors while Bases are proton acceptors
Lewis: Acids are electron acceptors and Bases are electron donors

But for a molecule like NH3, I know it is a weak base but conceptually, how would you tell?

In the lewis structure of NH3, there is a lone pair of electrons on the N atom bounded by the 3 H atoms.

How would you know that NH3 would accept protons rather than lose protons? For instance, the lone pair of electrons on N would attract the partially-positively charged H molecule from water right? But couldn't the partially-negatively charged Oxygen on water take a H atom on NH3 away from NH3 (like break the bond between the N-H on NH3)?

[Katie Lam 2J]

I believe NH3 is a weak base and would rather accept a proton because NH4+ is a polyatomic ion. Because NH4+ is a polyatomic ion, I think it is a more favorable structure than NH2-. Please correct me if I am wrong!

[JoshMoore2B]

Hi,

I am a little confused as to how to tell if a compound is an acid or a base just by looking at it.

From my general understanding, you can use either the Bronsted or the Lewis definition of an acid or base:

Bronsted: Acids are proton donors while Bases are proton acceptors
Lewis: Acids are electron acceptors and Bases are electron donors

But for a molecule like NH3, I know it is a weak base but conceptually, how would you tell?

In the lewis structure of NH3, there is a lone pair of electrons on the N atom bounded by the 3 H atoms.

How would you know that NH3 would accept protons rather than lose protons? For instance, the lone pair of electrons on N would attract the partially-positively charged H molecule from water right? But couldn't the partially-negatively charged Oxygen on water take a H atom on NH3 away from NH3 (like break the bond between the N-H on NH3)?

For this example, NH3 acts like a Lewis base because it has an extra pair of electrons (which can be seen by making its Lewis structure). This extra pair can be donated to an H, creating NH4+.

By definition, a Lewis base gives away lone pairs, so NH3 would be basic.

[Vivian Leung 1C]

I see. Thank you!