Final answer:
The oxidized reactant is the species that experiences an increase in oxidation number, and the reducing agent is the same species as it loses electrons. For instance, in the reaction of rubidium with magnesium chloride, rubidium is the reducing agent as it is oxidized from 0 to +1.
Step-by-step explanation:
To identify the oxidized reactants and reducing agents in a redox reaction, we must examine the changes in oxidation numbers of the reactants. The oxidized reactant is the species that undergoes an increase in oxidation number, meaning it loses electrons. The reducing agent is the substance that causes another substance to be reduced and is itself oxidized; therefore, it is the same as the oxidized species.
For example, in the reaction 2Rb + MgCl2 -> 2RbCl + Mg, Rb goes from an oxidation state of 0 in elemental form to +1 in RbCl, indicating that it has lost electrons and has been oxidized. Therefore, Rb is the reducing agent. On the other hand, Mg in MgCl2 goes from a +2 oxidation state to 0 in elemental Mg, indicating that Mg has been reduced. Consequently, MgCl2 is the oxidizing agent.