Final answer:
The reagent H3O+ is known as a hydronium ion, which forms when a water molecule gains an additional proton, typically when acids dissolve in water. It is used to represent a hydrogen ion in an aqueous solution.
Step-by-step explanation:
The reagent H3O+ represents a hydronium ion, which is a water molecule (H2O) that has accepted an additional proton, hence forming H3O+. This often occurs when acids are dissolved in water. For example, when hydrofluoric acid (HF) is dissolved in water, H3O+ and fluoride ions (F-) are formed; H3O+ being the acid, while F- is the base. In a pure water reaction, which autoionizes, one water molecule donates a proton and becomes a hydroxide ion (OH-), while another accepts a proton, forming the hydronium ion (H3O+).
It's important to note that in many reactions H+ and H3O+ are treated equivalently, though H3O+ more accurately describes the form of a hydrogen ion in an aqueous solution. As a proton (H+) does not exist freely in solution, it attaches to water molecules, resulting in the formation of hydronium ions.