Question

Zn gives $H_2$ gas with $H_2SO_4$ and HCI but not with $HNO_3$ because :-

• A. $Zn$ act as oxidising agent when react with $ HNO_3$
• B. $HBO_3$ is weaker acid than $H_2SO_4$ & HCI
• C. In electrochemical series Zn is placed above hydrogen
• D. $NO^{?}_{3}$ is reduced in preference to hydronium ion

Answer 1

The Correct Option is D

To understand why zinc (Zn) gives hydrogen gas with sulfuric acid (H_2SO_4) and hydrochloric acid (HCl) but not with nitric acid (HNO_3), we need to consider the chemical reactions involved and the role of each reactant.

1. Reactions with H_2SO_4 and HCl:

   Zinc reacts with sulfuric acid and hydrochloric acid to produce hydrogen gas (H_2) and the respective zinc salts. This is because both H_2SO_4 and HCl are strong acids that readily release hydronium ions (H^+), which are reduced to form hydrogen gas:

   • With H_2SO_4: Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\uparrow
   • With HCl: Zn + 2HCl \rightarrow ZnCl_2 + H_2\uparrow
2. Reaction with HNO_3:

   Zinc does not produce hydrogen gas when reacting with nitric acid because HNO_3 is a strong oxidizing agent. Instead of forming hydrogen gas, nitric acid oxidizes zinc to Zn²⁺ and itself gets reduced, often forming nitrogen oxides such as NO or NO₂. The nitrate ion (NO_3^-\) is reduced in preference to the hydronium ion:

   • The reaction might proceed as: 4Zn + 10HNO_3 \rightarrow 4Zn(NO_3)_2 + 5H_2O + N_2O\uparrow

     This variation involves the reduction of nitric acid to nitrogen oxides, rather than hydrogen gas formation.

3. Reasoning:

   The correct alternative is that NO_3^-\ is reduced in preference to hydronium ion. Nitric acid's strong oxidizing nature prevents the liberation of hydrogen gas, unlike sulfuric acid and hydrochloric acid.

Therefore, the answer is: NO^{?}_{3} is reduced in preference to hydronium ion, due to the oxidizing properties of nitric acid that favor the reduction of nitrate ions over the reduction of hydronium ions.