# How to know if a reaction forms H3O+ ions or OH- ions? [closed]

How would you know when an acid or base is paired with $$\ce{H2O}$$ that it will form an $$\ce{OH-}$$ ion or a $$\ce{H3O+}$$ ion? I just started the acid and base equilibrium unit, and I'm just confused on what conditions $$\ce{OH-}$$ or $$\ce{H3O+}$$ ions form.

• Why, that's simple: acids give H3O+, bases give OH-. Mar 21, 2021 at 19:56

Acids are hydrogen ion donors. When the react with water, they can give a hydrogen ion to form $$\ce{H3O+}$$.
For example:

$$\ce{HCl(aq) + H2O(l) \rightarrow Cl-(aq) + H3O+(aq)}$$

Simple acids, such as $$\ce{HCl}$$ or $$\ce{H2SO4}$$, can be recognized as acids by the H at the start of the formula. Other more complex acids may be written with $$\ce{COOH}$$ at the end of the formula, which denotes a carboxylic acid. An example of this is acetic acid: $$\ce{CH3COOH}$$. Otherwise, the name of the compound should give it away.

Bases are hydrogen ion acceptors, and generate $$\ce{OH-}$$ when they react with water. Strong bases are generally soluble metal hydroxides, such as $$\ce{NaOH}$$. You should be able to recognize them from their formulas.

Simple weak bases often contain nitrogen, like ammonia: $$\ce{NH3}$$.

$$\ce{NH3(aq) + H2O(l) \rightleftharpoons NH4+(aq) + OH-(aq) }$$

H3O+ is just the combination of a a $$\ce{H+}$$ ion, which we know to be released from the dissociation of an acid in an aqueous solution, with a water molecule. It happens because the $$\ce{H+}$$ is so, so positive (and therefore so reactive) that the water molecules, with its lone pairs (which are locally negatively charged), are willing to form a dative covalent bond with the $$\ce{H+}$$ ion.

So like Ivan said, whenever you see an acid in an aqueous solution you can consider $$\ce{H+ \equiv H3O+}$$. The key there though, is that it must be in aqueous solution so that it can dissociate (and the $$\ce{H3O+}$$ form in the first place). That is the condition that is needed to form it. This is the same for $$\ce{OH-}$$, which requires an aqueous solution to form and thus release the ions in say NaOH, forming $$\ce{Na+}$$ and $$\ce{OH-}$$.