CHAPTER 4. ELECTROCHEMICAL REACTIONS 4.5
Step 2 : Balance the number ofatoms on both sides ofthe equation.
You are allowed to addhydrogen ions (H+) and water molecules
if the reaction takes place in an acid medium. If the reaction
takes place in a basic medium, you can add either hydroxide ions
(OH−) or water molecules. Inthis case, there is one magnesium
atom on the left and oneon the right, so no additional atoms need
to be added.
Step 3 : Once the atoms are balanced, check that the charges balance.
Charges can be balanced by adding electrons toeither side. The
charge on the left of theequation is 0, but the charge on the right
is +2. Therefore, two electrons must be addedto the right hand
side so that the chargesbalance. The half reaction is now:
Mg→ Mg2++ 2e−
Step 4 : Repeat the above steps, but this time using the reduction half
reaction.
The reduction half reaction is:
Cu2+→ Cu
The atoms balance but the charges don’t. Two electrons must
be added to the right hand side.
Cu2++ 2e−→ Cu
Step 5 : Multiply each half reaction by a suitable number so that the
number of electrons released in the oxidationhalf reaction is made
equal to the number ofelectrons that are accepted in the reduction half
reaction.
No multiplication is needed because there are two electrons on
either side.
Step 6 : Combine the two halfreactions to get a finalequation for the
overall reaction.
Cancelling the electronsgives:
Mg + Cu2++ 2e−→ Mg2++ Cu + 2e−
Mg + Cu2+→ Mg2++ Cu
Step 7 : Do a final check to make sure that the equation is balanced
We check the number of atoms and the chargesand find that the
equation is balanced.
