In the reaction #Mg(s) + 2HCl(aq) -> MgCl_2(aq) + H_2(g)#, what role does magnesium play?
Magnesium acts as a reducing agent.
In the chemical equation given to you
#"Mg"_ ((s)) + 2"HCl"_ ((aq)) -> "MgCl"_ (2(aq)) + "H"_ (2(g))#
magnesium is a reactant, which is why you see it added to the left of the reaction arrow.
Now, this chemical equation describes a redox reaction. Here magnesium metal is being oxidized to magnesium cations,
On the other hand, hydrogen is being reduced from hydrogen ions,
#stackrel(color(blue)(0))("Mg") _ ((s)) + 2stackrel(color(blue)(+1))("H") stackrel(color(blue)(-1)) ("Cl")_ ((aq)) -> stackrel(color(blue)(+2))("Mg") stackrel(color(blue)(-1)) ("Cl"_ 2) ""_ ((aq)) + stackrel(color(blue)(0))("H") _(2(g))#
Notice that magnesium goes from on oxidation state of
Hydrogen goes from an oxidation state of
Therefore, you can say that magnesium is acting a reducing agent because it reduces hydrogen to hydrogen gas while being oxidized in the process.
Hydrochloric acid, the compound that delivers the hydrogen ions to the reaction, is acting as an oxidation agent because it oxidizes magnesium to magnesium cations while being reduced in the process.