A reduction reaction is one in which a reactant in a chemical reaction gains one or more electrons. Reduction reactions always occur in conjunction with oxidation reactions, in which a reactant loses one or more electrons. Reduction-oxidation reactions are often called redox equations. A reduction reaction is only one half of a redox reaction. The other half is the oxidation reaction.
Consider the single replacement (displacement) reaction between zinc metal and hydrochloric acid:
#Zn(s) + 2HCl(aq) → ZnCl_"2"(aq) + H_2(g)#
The ionic equation for this reaction:. #Zn(s) + 2H^"+"(aq) + 2 Cl^"-"(aq) → Zn^"2+"(aq) + 2Cl^"-"(aq) + 2H_2(g)#
Now we can break this reaction into two parts; reduction and oxidation. Lets start with the zinc atom for the first half of the reaction. At the beginning of the reaction, the zinc atom was neutral, but during the reaction, the it lost two electrons, forming a #Zn^"2+"# ion.
#Zn(s) → Zn^"2+"(aq) + 2 e-#
Since the zinc atom lost electrons, it is an oxidation reaction.
The other half of the equation involves the hydrogen ions (initially bonded to the chloride ion in the hydrochloric acid). The hydrogen ions gain the electrons lost by the zinc atom, and bond together to form hydrogen gas.
#2H^+(aq) + 2e- → H_2(g)#
Since the hydrogen ions gain electrons, it is a reduction reaction.
Notice that the chloride ion #(Cl^-)#, was not involved in either the oxidation or reduction. This is because it neither gained nor lost electrons during the reaction.
Another example of a redox reaction is this combination (synthesis) reaction between magnesium and chlorine.
#Mg(s) + Cl_2(g) → MgCl_2(s)#
The ionic equation is #Mg(s) + Cl_2(g) → Mg^"2+"(s) + 2Cl^"-"(s)#
The first half of the equation is
#Mg(s) → Mg^"2+"(s) + 2e-#
#Mg# lost electrons, so this is an oxidation reaction.
The second half of the equation is
#Cl_2(g) + 2e+ → 2Cl^"-"(s)#
#Cl_2# gained two electrons, so this is a reduction reaction.