How to identify acids and bases?

[Zoe FC 1C]

Hi,
I'm sort of confused on how to identify acids and bases. I thought that all acids had a hydrogen, and all bases had an OH, but ammonia (NH3) is considered a weak base even though it doesn't have OH. Can someone clarify why this is?

[Sydney Lam_2I]

Hi! So the best way to identify if the molecule is an acid or base is to see if the reaction will produce OH- molecules or H3O+ molecules. In your example, NH3 would combine with water and the proton from the water would attach to the NH3 to form NH4+ and OH-. Since OH- is produced. the solution would be basic.

[Justin Nguyen 3E]

To add on to what Sydney said, acids can donate their protons (in the form of H+), whereas bases accept those protons.

[Khoa Vu 3l]

You can also use different definitions of acids and bases to identify which molecules are an acid or which molecules are a base. Those before me have stated the Arrhenius definition of an acid vs a base (H+ creator in H2O vs OH- creator in H2O) and the Bronsted-Lowry definition of an acid vs a base (H+ donor vs H+ acceptor). However, for your example, I believe it best to use the Lewis definition for an acid vs a base (Electron-pair acceptor vs an electron-pair donor). NH3 has a lone pair of electrons that it can donate to form a bond with hydrogen, making NH4. Because it is an electron pair donor, it can be called a lewis base.

[Tam Nguyen 2B]

There are multiple definitions to acids and bases because they have different properties. As long as you remember the Arrhenius, Bronsted-Lowry, and Lewis definitions in any sort of combination, you'll be able to distinguish acids and bases.

[Vince Li 2A]

An acid is a compound that either gives off a H+ proton (Bronsted Acid) or is an electron pair acceptor (Lewis Acid), due to how the H+ ion leaves to form a coordinate covalent bond with the base, and the original bond becomes a lone pair on the acid. A base is a compound that accepts a H+ proton (Bronsted Base) or is an electron pair donor(Lewis Base), due to how the base forms a coordinate covalent bond with the H+ ion, thus donating an electron pair. Regarding NH3, in the presence of water, the NH3 will act as a base, due to how the H+ proton on water leaves to form NH4+. However, it is important to note that this does not happen often, which explains the very low Kb value (weak base).

[Rahul Sobti 1E]

If it is able to donate an H it as an acid or if it can accept an H it is a base. Even for salts, LiF is a weak base because once the Li and F dissociate in water, F- wants to pick up a proton, just as a base would.

[Simi Kapila_3E]

Acids are typically substances that release and proton (H+), while bases typically want to gain a proton. In your example NH3, ammonia, would be more stable as NH4+, ammonium, than it would be as NH2-, so it would be more inclined to take on a proton.

[Akshat Katoch 2K]

It depends on what definitions for acids and bases you are using.

Lewis Acid/Base:
Lewis Acid: electron pair acceptor
Lewis Base: electron-pair donor

Bronsted-Lowry Acid/Base:
Bronsted acid: proton donor
Bronsted base: proton acceptor

Arrhenius Acid/Base:
Acid: supplies H+
Base: supplies OH-

Once you figure out what is being accepted and donated you can figure out what definition you are using and then figure out the acid and bases accordingly.