Balancing and Half Reactions

Step 4. Equalize the changes in oxidation number by inserting coefficients to get these numbers

Each Zn atom has lost two electrons, and each H atom has gained one electron.

You need 2 atoms of H for every 1 atom of Zn. This gives us total changes of +2 and -2.

1Zn + 2HCl → 1ZnCl2 + 1H2 

Step 5. Finally, double check balance of atoms and charge. 

The balanced equation is

Zn + 2HCl → ZnCl2 + H2 

A half reaction is a reaction that only shows the oxidation or reduction half of the reaction. You balance each half first before combining them into one final equation. 

Electrons are included in the reaction.  

Notice that the equation is far from balanced, as there are no oxygen atoms on the right side. This will be resolved by the balancing method.

Step 2: Write separate half-reactions for the oxidation and the reduction processes. Determine the oxidation numbers first, if necessary.

Step 3: Balance the atoms in the half-reactions other than hydrogen and oxygen. In the oxidation half-reaction above, the iron atoms are already balanced. The reduction half-reaction needs to be balanced with the chromium atoms. 

Step 4: Balance oxygen atoms by adding water molecules to the appropriate side of the equation. For the reduction half-reaction above, seven H2O molecules will be added to the product side. 

Now the hydrogen atoms need to be balanced. In an acidic medium, add hydrogen ions to balance. In this example, fourteen H+ ions will be added to the reactant side. 

Step 5: Balance the charges by adding electrons to each half-reaction. For the oxidation half-reaction, the electrons will need to be added to the product side. For the reduction half-reaction, the electrons will be added to the reactant side. By adding one electron to the product side of the oxidation half-reaction, there is a 2+ total charge on both sides. 

There is a total charge of 12+ on the reactant side of the reduction half-reaction (14−2). The product side has a total charge of 6+ due to the two chromium ions (2×3). To balance the charge, six electrons need to be added to the reactant side. 

Now equalize the electrons by multiplying everything in one or both equations by a coefficient. In this example, the oxidation half-reaction will be multiplied by six.

Step 6: Add the two half-reactions together. The electrons must cancel. Balance any remaining substances by inspection. If necessary, cancel out H2O  or H+  that appear on both sides. 

Step 7: Check the balancing. In the above equation, there are14H, 6Fe, 2Cr, and 7O on both sides. The net charge is 24+ on both sides. The equation is balanced.