Question

Which of the following reactions cannot be used to produce hydrogen gas?
A.\[Zn + dil.{H_2}S{O_4}\]
B.\[Zn + dil.HN{O_3}\]
C.\[Zn + conc.HCl\]
D.\[Mg + {H_2}O(hot)\]

Answer 1

Hint: There are several methods for the production of hydrogen in the laboratories. Though hydrogen enters the environment through fossil fuels by steam reforming of natural gas and by coal gasification.

Complete answer:
 Let's solve these equations one by one and find out if that particular reaction liberates hydrogen gas or not.
A.Looking forward to our first reaction :
\[Zn + dil.{H_2}S{O_4} \to ZnS{O_4} + {H_2}\]
So, when zinc metal reacts with dilute sulphuric acid, the zinc sulphate and hydrogen gas is produced.
B.\[4Zn + dil.HN{O_3} \to 4Zn{(N{O_3})_2} + 5{H_2}O + {N_2}O\]
When zinc reacts with dilute nitric acid, it yields zinc nitrate, water and nitrogen oxide\[({N_2}O)\].
C.When we solve option C , we will get :
\[Zn + conc.2HCl \to ZnC{l_2} + {H_2}\]
Thus, when zinc reacts with concentrated hydrochloric acid, zinc chloride and hydrogen gas is produced as the products.
D.On solving the last option , we get :
\[Mg + {H_2}O(hot) \to MgO + {H_2}\]
So, when magnesium reacts with hot water, we will get magnesium oxide and hydrogen gas will be liberated out.
Thus, on solving all the equations we can say our correct option is B i.e. \[Zn + dil.HN{O_3}\].
Zinc on reacting with dilute nitric acid does not liberate hydrogen gas.

Note:
Zinc on reacting with dilute nitric acid does not liberate hydrogen gas because nitric acid is a very strong oxidising agent. It immediately converts the formed hydrogen into water and reduces itself into the oxides of nitrogen, so this is the reason why nitric acid in the dilute form is not able to produce hydrogen gas.