Question

Which of the following will not produce hydrogen gas?
(A) Reaction between Fe and dil. HCl
(B) Reaction between zinc and conc. ${H_2}S{O_4}$
(C) Reaction between Zn and NaOH
(D) Electrolysis of NaCl in Nelson’s Cell

Answer 1

The above question is based on the reactivity series of metals. The reactions taking place as displacement reactions which means which the more reactive metal replaces the less reactive metal while reacting and which forms the compounds.

Complete step by step answer:
First of all, we take the first reaction that is between Fe and dilute HCl. On the reaction of iron with hydrochloric acid, it forms ferrous chloride and hydrogen gas is evolved. The given reaction is
$Fe + 2HCl \to FeC{l_2} + {H_2}$
So we can see that hydrogen gas is evolved in this reaction.

The second reaction that is between zinc and concentrated sulfuric acid, it does not evolve hydrogen gas because Concentrated sulphuric acid and zinc react to form Zinc sulfate, water, and sulfur dioxide. The concentrated sulphuric acid acts as an oxidizing agent.
$2{H_2}S{O_4} + Zn \to ZnS{O_4} + 2{H_2}O + S{O_2}$
There is no evolution of hydrogen gas in the above reaction.

The third reaction which is between zinc and sodium hydroxide forms sodium zincate and it evolves hydrogen gas. The below reaction explains the above situation.
$Zn + NaOH \to N{a_2}Zn{O_2} + {H_2}$
So hydrogen gas is evolved in this reaction.

The last reaction is the electrolysis of sodium chloride in Nelson's cell. Hydrogen gas is obtained. The below reaction depicts the reaction conditions of electrolysis of sodium chloride in Nelson's cell

$2NaCl + 2{H_2}O \to {H_2} + C{l_2} + 2NaOH$
Here hydrogen gas is evolved in the reaction.
Therefore in the reaction between zinc and concentrated sulfuric acid hydrogen gas is not evolved and in other reactions, hydrogen gas is evolved.

So, the correct answer is Option B.

Note: One thing to be noted is that in the reaction between zinc and concentrated sulfuric acid hydrogen gas is not evolved as it readily decomposes into water and sulfur dioxide. If the reaction is carried out in the presence of dilute sulphuric acid, hydrogen will be evolved because it does not act as an oxidizing agent.