NDA (II) 2018 General Ability Test · Question 79 of 146

Correct — A, copper with dilute nitric acid. The balanced equation is 3Cu + 8HNO₃ (dilute) → 3Cu(NO₃)₂ + 2NO + 4H₂O, and the printed coefficients 3 and 8 are the giveaway: they only balance if two molecules of nitric oxide come out. Nitric acid is an oxidising acid, so a metal reacting with it does not release hydrogen; instead the nitrate ion itself is reduced, and how far it is reduced depends on the concentration of the acid and on the reactivity of the metal. With the dilute acid the reduction stops at nitrogen in the +2 state, which is NO, colourless nitric oxide that turns brown in air as it picks up oxygen to become NO₂.

• (b)Cu + 4HNO₃ (conc.)→ — Concentrated nitric acid is the stronger oxidising agent, so the nitrogen is reduced only as far as the +4 state: Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O. The product is nitrogen dioxide, the choking reddish-brown gas, not nitric oxide. Again the coefficients tell the story — one copper needing four acid molecules means two of them leave as NO₂.
• (c)4Zn + 10HNO₃ (dilute)→ — Zinc is far more reactive than copper, so with dilute nitric acid it drives the reduction all the way down to nitrogen in the −3 state: 4Zn + 10HNO₃ → 4Zn(NO₃)₂ + NH₄NO₃ + 3H₂O. The nitrogen ends up in ammonium nitrate, which stays in solution — no gaseous oxide of nitrogen is released at all.
• (d)Zn + 4HNO₃ (conc.)→ — With the concentrated acid zinc behaves like copper does: Zn + 4HNO₃ → Zn(NO₃)₂ + 2NO₂ + 2H₂O. The gas is nitrogen dioxide, not nitric oxide. The rule of thumb holds across the pair — concentrated nitric acid gives NO₂ whichever of these two metals you use.

Nitric acid is unusual among common acids because the anion, not the hydrogen ion, does the reacting when a metal is added. In hydrochloric or dilute sulphuric acid a metal above hydrogen in the reactivity series displaces hydrogen gas; in nitric acid the nitrate ion is reduced instead, so hydrogen is almost never seen. How far the nitrogen falls from its +5 oxidation state depends on two things — the concentration of the acid and the reducing power of the metal. Concentrated acid, being a strong oxidant that is quickly used up at the metal surface, gives NO₂; dilute acid gives NO with a moderately reactive metal such as copper, and can go as far as ammonium nitrate with a reactive metal such as zinc.

Two levers, two answers. Change the concentration and you change the product; change the metal and you change it again. Copper plus dilute acid is the single combination in this list that lands on NO, and it is worth memorising as the standard laboratory preparation of nitric oxide. A useful check that costs no chemistry at all is to look at the coefficients the paper has printed, since they are the balanced ones: 3Cu with 8HNO₃ can only balance with 2NO, while 1Cu with 4HNO₃ can only balance with 2NO₂. Also remember the visual difference — nitric oxide is colourless when it leaves the flask and browns as soon as it meets air, whereas nitrogen dioxide is brown from the start.

• 3Cu + 8HNO₃ (dilute) → 3Cu(NO₃)₂ + 2NO + 4H₂O is the standard laboratory route to nitric oxide.
• Cu + 4HNO₃ (concentrated) → Cu(NO₃)₂ + 2NO₂ + 2H₂O gives brown nitrogen dioxide instead.
• Very dilute nitric acid with zinc gives ammonium nitrate: 4Zn + 10HNO₃ → 4Zn(NO₃)₂ + NH₄NO₃ + 3H₂O.
• Metals generally do not liberate hydrogen from nitric acid, because the nitrate ion is reduced in preference to the hydrogen ion.

• Expecting hydrogen gas from a metal and an acid. With nitric acid it is the nitrate that is reduced, so an oxide of nitrogen or an ammonium salt appears instead.
• Pairing 'dilute' with 'less drastic reduction'. It is the other way round — dilute acid lets the nitrogen fall further, all the way to ammonium with a reactive metal.
• Treating NO and NO₂ as interchangeable. Nitric oxide is colourless and browns in air; nitrogen dioxide is brown at once.

NDA asks which gas a named metal gives with nitric acid at a stated concentration, or presents four balanced skeletons and asks which yields a particular product.

• practice — not a real PYQ

  Copper reacting with concentrated nitric acid gives which gas as a product?

  • (a)Hydrogen
  • (b)Nitric oxide
  • (c)Nitrogen dioxide
  • (d)Nitrous oxide

  Answer(c) Nitrogen dioxide — concentrated nitric acid reduces only to the +4 state, giving brown NO₂.

• practice — not a real PYQ

  Metals generally do not liberate hydrogen gas when treated with nitric acid because

  • (a)nitric acid is a weak acid
  • (b)the nitrate ion is reduced in preference to the hydrogen ion
  • (c)hydrogen is insoluble in nitric acid
  • (d)nitric acid does not ionise in water

  Answer(b) the nitrate ion is reduced in preference to the hydrogen ion — nitric acid acts as an oxidising agent rather than as a source of hydrogen.