Half-reactions are essential for understanding oxidation and reduction in chemical reactions. In the reaction between zinc and hydrochloric acid, the half-reactions can be represented as follows: zinc is oxidized to zinc ions by losing two electrons (Zn = Zn^2+ + 2e-), while hydrochloric acid is reduced, with hydrogen ions gaining electrons to form hydrogen gas (2HCl + 2e- = 2Cl- + H2). The discussion highlights that in redox reactions, the focus should be on the entire compound rather than individual elements, as the whole molecule or ion participates in the reaction. Additionally, oxidation numbers are useful for tracking electron transfer but do not represent measurable properties of atoms. The oxidation state of hydrogen changes during the reaction, raising questions about whether hydrogen or the compound HCl is being reduced. In gas phase reactions, the behavior of HCl can be more straightforward, but in aqueous solutions, chloride ions often act as spectators.