Question

6. Write balanced redox half reactions (assuming acidic conditions) for the conversion of:

a. ammonium to nitrate

b. nitrite to nitrogen gas.

Answer 1

Balancing the given half reactions in acidic medium include following steps:

a. Ammonium to nitrate

NH₄⁺ + O₂ -----> NO₃^-

Step I - First you have to balance elements other than hydrogen and oxygen

Here nitrogen is the other element present which is balanced.

Step II- Next step is to balance oxygen by adding H₂O on another side.

Here you have 2 O on left side and 3 O on right side so need to add 1 more O on left side so add 1 H₂O.

NH₄⁺ + O₂ + H₂O -----> NO₃^-

Step III - Now you have to balance hydrogen by adding H⁺

Here you have six H on left side hence need to add 6H⁺ on right side of the reaction.

NH₄⁺ + O₂ + H₂O  -----> NO₃^- + 6H⁺

Step IV - Last step is to balance charges on both side.

You have 1+ charge on left side and 6+, 1- i.e. 5+ charge on right side so need to add 4e^-on right side to balance 1+ charge on both side.

NH₄⁺ + O₂ + H₂O  -----> NO₃^- + 6H⁺ + 4e^-

This is the balanced half reaction,

You have given half reaction only so no need to add which is the normally last step in balancing the redox reaction.

b. Nitrite to nitrogen gas.

NO₂^- ------> N₂

Step I - First you have to balance elements other than hydrogen and oxygen

Here nitrogen is the other element present, you have two N on right side so balanced it on left side.

2NO₂^- ------> N₂

Step II- Next step is to balance oxygen by adding H₂O on another side.

Here you have 4 O on left side so need to add 4 O on left side for that add 4 H₂O on right side.

2NO₂^- ------> N₂ + 4 H₂O

Step III - Now you have to balance hydrogen by adding H⁺

Here you have 8 H on right side hence need to add 8H⁺ on left side of the reaction.

2NO₂^- + 8 H⁺------> N₂ + 4 H₂O

Step IV - Last step is to balance charges on both side.

Here you have 2- and 8+ i;e 6+ charge on the left side of reaction so need to add 6e^- on left side of the reaction to balance both side neutral.

2NO₂^- + 8 H⁺ + 6e^-------> N₂ + 4 H₂O

This is the balanced half reaction,

You have given half reaction only so no need to add which is the normally last step in balancing the redox reaction.

I hope this will help you.