Everything Science Grade 12

4.5 CHAPTER 4. ELECTROCHEMICAL REACTIONS

Example 8: Balancing redox reactions

QUESTION

Chlorine gas oxidises Fe(II) ions to Fe(III) ions.In the process, chlorineis re-

duced to chloride ions.Write a balanced equation for this reaction.

SOLUTION

Step 1 : Write down the oxidation half reaction.

Fe2+→ Fe3+

Step 2 : Balance the number ofatoms on both sides ofthe equation.

There is one iron atomon the left and one onthe right, so no

additional atoms need tobe added.

Step 3 : Once the atoms are balanced, check that the charges balance.

The charge on the left of the equation is +2, but the charge on

the right is +3. Therefore, one electron must beadded to the right

hand side so that the charges balance. The halfreaction is now:

Fe2+→ Fe3++ e−

Step 4 : Repeat the above steps, but this time using the reduction half

reaction.

The reduction half reaction is:

Cl 2 → Cl−

The atoms don’t balance, so we need to multiply the right hand

side by two to fix this.Two electrons must beadded to the left

hand side to balance thecharges.

Cl 2 + 2e−→ 2Cl−

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.

We need to multiply the oxidation half reaction by two so that

the number of electronson either side are balanced. This gives:

2Fe2+→ 2Fe3++ 2e−

Step 6 : Combine the two halfreactions to get a finalequation for the

overall reaction.

2Fe2++ Cl 2 → 2Fe3++ 2Cl−