Redox Analysis via Oxidation States

Steps:

• Recognize the given equation is unbalanced (4 O atoms left, 7 right), likely intending (NH4)2Cr2O7 → Cr2O3 + N2 + 4H2O for ammonium dichromate decomposition.
• Compute N oxidation state: -3 in NH4+ to 0 in N2 (increases, so oxidized).
• Compute Cr oxidation state: +6 in Cr2O7^2- to +3 in Cr2O3 (decreases, so reduced).
• Verify H (+1 to +1) and O (-2 both sides) show no change.

Why B is correct:

• Nitrogen oxidized (-3 to 0, loss of electrons) and chromium reduced (+6 to +3, gain of electrons), per oxidation number rules.

Why the others are wrong:

• A: Chromium reduced (ox state decreases), not oxidized; nitrogen oxidized but pairing reversed.
• C: Hydrogen unchanged (+1 both sides), not oxidized; chromium reduced but incorrect pairing.
• D: Hydrogen unchanged, not reduced; nitrogen oxidized but wrong pairing.

Final answer: B