Question

The products formed when heavy water is reacted with magnesium nitride, is / are_____.
A. $N{H_3},Mg{(OH)_2}$
B. $N{H_3},Mg{(OD)_2}$
C. $N{D_3},Mg{(OH)_2}$
D. $N{D_3},Mg{(OD)_2}$

Answer 1

The reaction between heavy water and magnesium nitride is a double decomposition reaction in which the anion of one reactant is exchanged with the anion of another reactant, and new products are formed. Or, it can also be said that the cation of one reactant is exchanged with the cation of another reactant to form new products.

Complete step by step answer:
To proceed with the question, let us first understand what heavy water is, and how is it different from normal water.
Normal water is the molecule of water containing two atoms of hydrogen, and one atom of oxygen, represented by the formula ${H_2}O$, while heavy water is the water containing two atoms of deuterium, which is an isotope of hydrogen, having the formula $_1^2D$, and one atom of oxygen.
Therefore, the chemical formula for heavy water is ${D_2}O$.

Now let us talk about the other compound mentioned in the question, which is magnesium nitride. The chemical formula for magnesium nitride is $M{g_3}{N_2}$.

We know that magnesium has a valency of $+ 2$, so its chemical symbol is $M{g^{2 + }}$.

Nitride is an anion of nitrogen, having a valency of $- 3$. So, its chemical symbol is ${N^{3 - }}$. Now, exchanging the valency of two, we can establish that the chemical formula for magnesium nitride is $M{g_3}{N_2}$.

Now, let us get back to the reaction that is mentioned in the question. It has been said that heavy water reacts with magnesium nitride. So, we will write both the species as reactants on the left side of the reaction arrow as follows:
${D_2}O + M{g_3}{N_2} \to ?$
Now, since this is a double decomposition reaction, it simply means that both the reactants will decompose into their respective ions, and exchange them with each other.
Therefore, the reaction between heavy water and magnesium nitride can be simply represented by the following equation:
$6{D_2}O + M{g_3}{N_2} \to 2N{D_3} + 3Mg{(OD)_2}$
Therefore, as we can see that the products formed on reacting heavy water with magnesium nitride are $N{D_3},Mg{(OD)_2}$.

So, the correct answer is Option D.

Note: Magnesium is positively charged with a valency of $+ 2$, and nitride is negatively charged with a valency of $- 3$. Similarly, heavy water breaks into ${D^ + }$ and $O{D^ - }$ ions in the reaction. A positive ion will always combine with a negative ion, and a negative ion will always combine with a positive ion.So, $M{g^{2 + }}$ ion reacts with the $O{D^ - }$ ion of heavy water, and ${N^{3 - }}$ ion reacts with the ${D^ + }$ ion of heavy water, and a double decomposition reaction takes place.