Bronsted Acids and Bases

[melissa_dis4K]

How do you know when the molecule is best accepting or donating its hydrogen?

[Chem_Mod]

It's helpful to draw the lewis structures to see if the structure is stable enough to either donate or accept a hydrogen.

For example, NH4+ is a base because it can only donate a hydrogen. If it accepted a hydrogen, Nitrogen would have 5 bonds, which we know is not possible. Another example is OH-. Drawing the structure, we see that if oxygen gained a hydrogen, it could use a lone pair to make a bond with H, making the overall structure more stable since the formal charges are 0.

[nolansheow3G]

Another quick (but much less reliable way) would be to think about common compounds that have just one more or one less proton.

Ex: Given NH4+, try to recall a compound you've seen before (NH3)

Same with OH- and H20, H30+ and H2O, HNO3 and NO3-, etc.

I would only recommend using this method if you are familiar with all the strong acids and bases, which would be helpful to memorize. If you memorize these acids and bases, then you will know their conjugates off the top of your head.

[melissa_dis4K]

It's helpful to draw the lewis structures to see if the structure is stable enough to either donate or accept a hydrogen.

For example, NH4+ is a base because it can only donate a hydrogen. If it accepted a hydrogen, Nitrogen would have 5 bonds, which we know is not possible. Another example is OH-. Drawing the structure, we see that if oxygen gained a hydrogen, it could use a lone pair to make a bond with H, making the overall structure more stable since the formal charges are 0.

I understand the OH- example, thank you. But I am still a little confused on the NH4+ would it be a bronsted acid because it can't accept anymore hydrogens so it donates protons? Or are you saying it is a Lewis base, in which case it donates its proton? Thank you!

[Chem_Mod]

NH4+ will act as an acid, since it will donate protons. It will not act as a base. It is impossible for it to accept any more protons, since that would make NH5 2+. Nitrogen cannot have more than an octet.