How to balance simple oxidation-reduction reactions by the half-reaction method.
Mg (s) + N2 (g)= Mg3N2 (s)
To balance the given chemical equation using the half-reaction method, you need to follow these steps:
Step 1: Split the equation into two half-reactions.
The oxidation half-reaction will involve the loss of electrons, while the reduction half-reaction will involve the gain of electrons.
Oxidation half-reaction:
Mg (s) → Mg2+ (aq)
Reduction half-reaction:
N2 (g) → 3N2- (aq)
Step 2: Balance the atoms involved in each half-reaction, excluding oxygen and hydrogen.
In the oxidation half-reaction, there is only one magnesium atom on both sides, so it is already balanced.
In the reduction half-reaction, there are two nitrogen atoms on the left side (N2) and six nitrogen atoms on the right side (3N2-). To balance the nitrogen atoms, you need to multiply the reduction half-reaction by 3:
3N2 (g) → 9N2- (aq)
Step 3: Balance the oxygen atoms by adding water (H2O) molecules.
In this reaction, there are no oxygen atoms present, so this step is not required.
Step 4: Balance the hydrogen atoms by adding hydrogen ions (H+).
Again, in this reaction, there are no hydrogen atoms present, so this step is not required.
Step 5: Balance the charges by adding electrons (e-).
In the oxidation half-reaction, there is no charge on either side, so no electrons are needed.
In the reduction half-reaction, there are 9N2- ions on the right side, each with a charge of -2. To balance the charges, you need to add 18 electrons (9 × -2) to the left side of the reduction half-reaction:
3N2 (g) + 18e- → 9N2- (aq)
Step 6: Equalize the electrons transferred in both half-reactions.
To equalize the electrons transferred, you need to multiply the oxidation half-reaction by 18:
18Mg (s) → 18Mg2+ (aq)
Step 7: Combine the two half-reactions.
Now that both half-reactions have the same number of electrons transferred, you can combine them. However, before combining, double-check to ensure that the total number of atoms (excluding oxygen and hydrogen) and the overall charge are the same on both sides of the equation.
Overall balanced equation:
18Mg (s) + 3N2 (g) → Mg3N2 (s)