Question

On reaction with Mg, very dilute nitric acid produces
A. NH₃
B. Nitrous oxide
C. Nitric oxide
D. Hydrogen

Answer 1

Hint: Nitric acid acts as a good oxidising agent when it reacts with metals it oxidises the metal to and produces metal nitrates. It also produces a gas which on burning gives a pale blue colour flame.

Complete Step by Step Answer:
Magnesium is a highly reactive metal and it mostly gets oxidised to +2 oxidation state so when Mg reacts with nitric acid it will get oxidised to Mg²⁺. Mg is a good reducing agent and Mg is an element of the second group; the maximum oxidation of state of the second group element is +2.

The reaction will be:
$Mg+2HNO_{3\ }\rightarrow\ Mg\left(NO_3\right)_2+H_2$
Thus, Magnesium reacts with Nitric acid and gives magnesium nitrate and hydrogen gas because the nitric acid used in the reaction was dilute nitric acid. If the nitric acid was concentrated it would have produced nitrogen dioxide and water.

The mechanism of the reaction is as follows:
$Mg\rightarrow{\rm Mg}^{2+}+2e^-$
$HNO_3\rightarrow\ H^+\ +{NO_3}^-\ $
${\rm Mg}^{2+}+\ 2{NO_3}^-\rightarrow Mg{(NO_3)}_2$
$2H^++2e^-\ \rightarrow\ H_2$
So; the gas produced when magnesium reacts with dilute nitric acid is hydrogen gas. It is a light and highly flammable gas.
Thus, Option (D) is correct

Note: In the given reaction nitric acid acted as a weak oxidising agent because it was present in the diluted form. So, the formation of water which is the oxidation of hydrogen was not favourable. That's why this reaction does not produce water and nitrogen dioxide. It is an acid-base reaction where Magnesium acts as base and Nitric acid as acid which produces salt; Magnesium nitrate is an inorganic salt.