Everything Science Grade 12

CHAPTER 4. ELECTROCHEMICAL REACTIONS 4.5

Step 7 : Do a final check to make sure that the equation is balanced

The equation is balanced.

Example 9: Balancing redox reactions in an acid medium

QUESTION

The following reaction takes place in an acid medium:

Cr 2 O^27 −+ H 2 S→ Cr3++ S

Write a balanced equation for this reaction.

SOLUTION

Step 1 : Write down the oxidation half reaction.

Cr 2 O^27 −→ Cr3+

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

We need to multiply the right side by two sothat the number

of Cr atoms will balance. To balance the oxygen atoms, we will

need to add water molecules to the right hand side.

Cr 2 O^27 −→ 2Cr3++ 7H 2 O

Now the oxygen atoms balance but the hydrogens don’t. Because

the reaction takes placein an acid medium, we can add hydrogen

ions to the left side.

Cr 2 O^27 −+ 14H+→ 2Cr3++ 7H 2 O

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

The charge on the left of the equation is (-2+14) = +12, but the

charge on the right is +6. Therefore, six electrons must be added

to the left hand side so that the charges balance.The half reaction

is now:

Cr 2 O^27 −+ 14H++ 6e−→ 2Cr3++ 7H 2 O

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

reaction.

The reduction half reaction after the charges have been balanced

is:

S^2 −→ S + 2e−