Which one of the following reactions will give NO (nitric oxide) gas as one of the products?
- A.3Cu + 8HNO3 (dilute) →
- B.Cu + 4HNO3 (conc.) →
- C.4Zn + 10HNO3 (dilute) →
- D.Zn + 4HNO3 (conc.) →
Correct answer
Explanation
The correct answer is A, 3Cu + 8HNO3 (dilute) →. Copper with dilute nitric acid gives copper nitrate, water and nitric oxide: 3Cu + 8HNO3 → 3Cu(NO3)2 + 2NO + 4H2O.
Nitric acid is a strong oxidising agent, so metals usually do not release hydrogen from it; instead the acid itself is reduced, and the product depends on the metal and on how concentrated the acid is. Copper, a less reactive metal, reduces dilute acid only as far as NO, a colourless gas that turns brown in air as it forms NO2. The more reactive zinc can reduce the acid further. The balancing numbers in each option point to the product.
B, copper with concentrated nitric acid, gives brown nitrogen dioxide: Cu + 4HNO3 → Cu(NO3)2 + 2NO2 + 2H2O.
C, zinc with dilute nitric acid in this ratio, gives nitrous oxide: 4Zn + 10HNO3 → 4Zn(NO3)2 + N2O + 5H2O.
D, zinc with concentrated nitric acid, gives nitrogen dioxide: Zn + 4HNO3 → Zn(NO3)2 + 2NO2 + 2H2O.
Exam tip: Cu + dilute HNO3, NO; Cu or Zn + concentrated HNO3, NO2; Zn + dilute HNO3, N2O.
Practice Questions
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