Why does a mixture of copper turnings and concentrated nitric acid produce a dark brown gas rather than hydrogen gas?
Answer and explanation
Correct answer: C. Nitric acid is a strong oxidizing acid reduced to nitrogen dioxide
Copper has a positive reduction potential (E° = +0.34 V) and cannot reduce H+ to H2 gas. Instead, concentrated HNO3 acts as a strong oxidizing agent, reducing nitrate to reddish-brown nitrogen dioxide gas: Cu + 4 HNO3 -> Cu(NO3)2 + 2 NO2 + 2 H2O.
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