Exemplar Problems

Problem 2 : Balance the following redox equation occurring in basic solution:

$$[NO_2^- + Al \rightarrow NH_3 + Al(OH)_4^-]$$

Solution :

To balance this equation in basic solution, follow these steps:

Step 1: Assign oxidation states to each element: $$[NO_2^- : N^{+},]$$ $$[Al : Al^{3+},]$$ $$[NH_3 : N^{-3},]$$ $$[Al(OH)_4^- : Al^{3+}.]$$

Step 2: Write down the unbalanced equation: $$[NO_2^- + Al \rightarrow NH_3 + Al(OH)_4^- + H_2O.]$$

Step 3: Break the reaction into half-reactions for oxidation and reduction: $$[Oxidation: Al \rightarrow Al(OH)_4^-,]$$ $$[Reduction: NO_2^- \rightarrow NH_3^-]$$

Step 4: Balance each half-reaction: Oxidation: $$[Al \rightarrow Al(OH)_4^-]$$ Add 3 electrons (e⁻) to the left side to balance the charge.

Reduction: $$[NO_2^- \rightarrow N_2 + H_2O]$$ Add 4 electrons (e⁻) to the right side to balance the charge.

Step 5: Multiply the half-reactions by coefficients to balance the number of electrons: $$[4(Al \rightarrow Al(OH)_4^-)]$$ $$[3(NO_2^- \rightarrow NH_3)]$$

Step 6: Add the balanced half-reactions to get the overall balanced equation: $$[4Al + 12OH^- + 3NO_2^- \rightarrow 4Al(OH)_4^- + 3NH_3.]$$