Question

Which of the following reactions will not produce any hydrogen gas?

• A. Reaction between $Fe$ and dil. $HCl$.
• B. Reaction between $Zn$ and conc. $H_{2}S{O}_4$.
• C. Reaction between $Zn$ and $NaOH$.
• D. Electrolysis of $NaCl$ in Nelson's cell.

Answer 1

The correct option is B Reaction between $Zn$ and conc. $H_{2}S{O}_4$.

(A) Iron reacts with dil. $HCl$ to produce ferrous chloride and hydrogen.

$Fe+2HCl\to FeC{l}_2+H_2$

(B) Zinc reacts with dil sulphuric acid to produce zinc sulphate and hydrogen (lab method).

$Zn+\text{dil.}{H}_{2}S{O}_4\to ZnS{O}_4+H_2$.

However conc. sulphuric acid cannot be used because $S{O}_2$ and $H_{2}O$ is evolved.

$H_{2}S{O}_4+H_2\to 2H_{2}O+S{O}_2$.

(C) Zinc reacts with NaOH to form sodium zincate and hydrogen.
$Zn+2NaOH\to \underset{\text{sodium zincate}}{N{a}_{2}Zn{O}_2}+H_2$
(D) During electrolysis of aqueous $NaCl$ in Nelson's cell, hydrogen gas is obtained. Water dissociates to small extent:

$H_{2}O\rightleftharpoons H^{+}+O{H}^{-}$.

Hydrogen ions are reduced to hydrogen.

$2H^{+}+2e^{-}\to H_2$.