What is the oxidation half reaction for #Mg(s) + ZnCl_2(ag) -> MgCl_2(ag) + Zn(s)#?
The oxidation half-reaction is
If you write what is known as the net ionic equation, it is a simpler matter to identify the oxidation (and the reduction form that matter).
First, write the two salts in aqueous ion form:
Since no change occurs in the chloride ion, these are "spectators" and can be omitted:
Now, we have a more clear description of what has taken place.
Anode: Mg →
From the equation we see that Mg metal (oxidation state 0) changes to the ion
Zn is changed from it’s ionic state,
By convention, all half-cell emf's are compared to the emf of the standard hydrogen electrode. The emf of a half-cell, with respect to the standard hydrogen electrode, is called the reduction potential.
In an electrochemical cell, the general equation is:
The emf for the Mg-Zn cell described would be:
The two “standard cell” cathode (reduction) half reactions are:
Put in the general equation for the actual cell, the full reaction is:
-0.76 - (-2.38) = 1.62V