The oxidation number of iron in the compound formed during brown ring test for NO₃⁻ iron is ________.

Solution

Chemistry ma’am · 4 steps

In the brown ring complex, NO sits as NO⁺, so iron is pushed down to +1.

1. Nitrate is reduced. In acid, Fe²⁺ reduces nitrate to nitric oxide: NO₃⁻ + 3Fe²⁺ + 4H⁺ → NO + 3Fe³⁺ + 2H₂O. The extra Fe²⁺ then takes up this NO.
2. Brown complex forms. NO replaces one water ligand: [Fe(H₂O)₆]²⁺ + NO → [Fe(H₂O)₅(NO)]²⁺ + H₂O. This complex is the brown ring.
3. Charge on NO. Inside the complex, NO gives one electron to iron and binds as the nitrosonium ion NO⁺. Water is neutral, so it adds nothing to the charge.

   x + (+1) + 5(0) = +2 ⇒ x = +1

4. So. Iron in [Fe(H₂O)₅(NO)]²⁺ has an oxidation number of +1.

The common slip: 2. Treating NO as a neutral ligand makes iron carry the whole +2 charge, which gives 2, not 1.

The idea to keep. For any nitrosyl complex, first decide whether NO is NO⁺, NO or NO⁻. In the brown ring complex it is NO⁺, which is why iron ends up in the unusual +1 state.

Answer from the NTA final answer key. Solution by PrepWiser.