JEE Main 2020 · 7 January · evening · Single correct · +4 / −1
The redox reaction among the following is :
- (a) reaction of H₂SO₄ with NaOH.
- (b) formation of ozone form atmosphere oxygen in the presence of sunlight.
- (c) combination of dinitrogen with dioxygen at 2000 K
- (d) reaction of [Co(H₂O)₆]Cl₃ With AgNO₃
Answer and solution
Answer (c) combination of dinitrogen with dioxygen at 2000 K
Solution
Chemistry ma’am · 3 steps
Track oxidation numbers: a redox reaction needs at least one element whose number actually changes.
- What to check. A reaction is redox only if some element changes its oxidation number. Write the oxidation number of each element on both sides for every option.
Option Reaction Oxidation numbers Redox? (a) H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O H stays +1, S stays +6, O stays −2, Na stays +1 No, acid–base neutralisation (b) 3O₂ → 2O₃ O is 0 in both No, only the allotrope changes (c) N₂ + O₂ → 2NO N: 0 to +2, O: 0 to −2 Yes (d) [Co(H₂O)₆]Cl₃ + 3AgNO₃ → [Co(H₂O)₆](NO₃)₃ + 3AgCl Co stays +3, Ag stays +1, Cl stays −1 No, precipitation only - Why (c) works. In NO, oxygen is −2, so nitrogen must be +2. Nitrogen goes from 0 to +2, so it is oxidised. Oxygen goes from 0 to −2, so it is reduced.
- So. Only the combination of N₂ and O₂ at 2000 K is a redox reaction, which is option (c).
The idea to keep. Neutralisation, precipitation and allotrope changes do not change any oxidation number. When two free elements combine into a compound, their oxidation numbers move away from zero, so that reaction is always redox.
Answer from the NTA final answer key. Solution by PrepWiser.