H20 to OH- or H3O+

[stella]

In a reaction, how do we know when water will turn into OH- on the product side of H3O+?

For example: NH3 + H2O <> NH4+ + OH-
How do we know the NH3 will gain the hydrogen from H2O and become NH4+ and not lose it and become NH2-

or also in, HCO3- + H2O <> CO3^2- + H3O+
how do we know whether the reactant will gain the H or lose the H when forming the product?

[Minh-Anh Pham 1B]

You kinda have to just know what is the other thing that is on the same side as water. If the reactant is an acid, then the acid will lose a proton and water will become H3O+, and if the reactant is a base (often a conjugate base), then the base will gain a proton and water will become OH-. In your example, ammonia (NH3) is the conjugate base of NH4+, thus it will gain a proton. In the second equation, things with H in front of them are usually acids, so you can predict that it will lose its proton, and water will become the hydronium ion H30+. Hope this helps!