Problem 37 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

\(\mathrm{Mg}_{3} \mathrm{~N}_{2}\) when reacted with water gives off \(\mathrm{NH}_{3}\) but \(\mathrm{HCl}\) is not obtained from \(\mathrm{MgCl}_{2}\) on reaction with water at room temperature.

Step-by-Step Solution

Answer

\( \mathrm{Mg}_3\mathrm{N}_2 \) reacts with water to produce \( \mathrm{NH}_3 \), but \( \mathrm{MgCl}_2 \) does not release \( \mathrm{HCl} \) when dissolved in water.

Step 1: Analyze the Reactants and their Interactions

First, identify the reactants involved and what is expected upon reaction with water. \( \mathrm{Mg}_{3} \mathrm{~N}_{2} \) (magnesium nitride) will react with water to produce ammonia \( (\mathrm{NH}_3) \). However, when \( \mathrm{MgCl}_{2} \) (magnesium chloride) interacts with water, it remains dissolved as ionic species in the water and does not produce hydrogen chloride \( (\mathrm{HCl}) \) gas under standard conditions.

Step 2: Write the Chemical Equation for Magnesium Nitride with Water

For the reaction of \( \mathrm{Mg}_{3} \mathrm{~N}_{2} \) with water, the chemical reaction can be written as:\[\mathrm{Mg}_{3} \mathrm{~N}_{2} + 6 \mathrm{H}_2\mathrm{O} \rightarrow 3 \mathrm{Mg}(\mathrm{OH})_2 + 2 \mathrm{NH}_3\]This shows that the interaction yields magnesium hydroxide and ammonia.

Step 3: Explain Why MgCl₂ Does Not Release HCl with Water

When \( \mathrm{MgCl}_2 \) dissolves in water, it dissociates into \( \mathrm{Mg}^{2+} \) and \( \mathrm{Cl}^- \) ions; it doesn't release \( \mathrm{HCl} \). Thus, the reaction with water remains as:\[\mathrm{MgCl}_2 (aq) \rightarrow \mathrm{Mg}^{2+} (aq) + 2\mathrm{Cl}^{-} (aq)\]Hydrochloric acid is not formed because there is no source of \( \mathrm{H}^+ \) ions being released from the \( \mathrm{MgCl}_2 \) solution.

Key Concepts

Magnesium Nitride

Magnesium nitride, represented chemically as \( \mathrm{Mg}_3 \mathrm{N}_2 \), is an inorganic compound that plays a significant role in chemical reactions involving water. This compound is composed of three magnesium atoms and two nitrogen atoms. It appears as a yellowish-white crystalline solid.
When magnesium nitride comes into contact with water, it undergoes a chemical change. This reaction can be described by the equation:

• \( \mathrm{Mg}_3 \mathrm{N}_2 + 6 \mathrm{H}_2\mathrm{O} \rightarrow 3 \mathrm{Mg} (\mathrm{OH})_2 + 2 \mathrm{NH}_3 \)

This reaction results in the formation of magnesium hydroxide \( \mathrm{Mg} (\mathrm{OH})_2 \) and ammonia gas \( \mathrm{NH}_3 \). This specific ability of magnesium nitride to produce ammonia during its interaction with water is useful in various industrial applications, particularly in the generation of ammonia in situ.
Understanding this reaction helps in identifying the different pathways and processes that elements and compounds undertake during chemical interactions.

Ammonia Production

Ammonia, a compound of nitrogen and hydrogen with the formula \( \mathrm{NH}_3 \), is a colorless gas with a pungent odor. It is an important building block for the synthesis of numerous compounds used in agricultural, industrial, and chemical sectors. The production of ammonia through the reaction of magnesium nitride with water is one of the less common but effective methods.
In the reaction involving magnesium nitride, water molecules act as a reactant to liberate ammonia from the nitride. The resulting reaction,

• \( \mathrm{Mg}_3 \mathrm{N}_2 + 6 \mathrm{H}_2\mathrm{O} \rightarrow 3 \mathrm{Mg} (\mathrm{OH})_2 + 2 \mathrm{NH}_3 \),

illustrates ammonia's emergence as a gaseous product.
This method of ammonia production can be effective in controlled environments where the direct sourcing of ammonia is desired without utilizing conventional methods like the Haber process. Understanding this reaction pathway is crucial for designing efficient processes in contexts where ammonia needs to be produced in smaller, localized setups and provides insights into alternative synthesis routes.

Ionic Dissociation

Ionic dissociation is a key concept in chemistry, particularly when understanding reactions involving ionic compounds in aqueous solutions. When a compound like magnesium chloride \( \mathrm{MgCl}_2 \) is dissolved in water, it undergoes dissociation rather than a chemical reaction that produces a new compound. During dissociation, the compound splits into its positive and negative ions:

• \( \mathrm{MgCl}_2 (\mathrm{aq}) \rightarrow \mathrm{Mg}^{2+} (\mathrm{aq}) + 2 \mathrm{Cl}^{-} (\mathrm{aq}) \)

In this case, magnesium chloride dissociates into magnesium ions \( \mathrm{Mg}^{2+} \) and chloride ions \( \mathrm{Cl}^{-} \). This behavior explains why \( \mathrm{MgCl}_2 \) does not release hydrochloric acid \( \mathrm{HCl} \) when mixed with water; there isn't a source of hydrogen ions \( \mathrm{H}^{+} \) needed to form \( \mathrm{HCl} \).
The distinction between dissociation and chemical reactions, like the one seen with magnesium nitride and water, is vital to comprehend how different substances interact in aqueous environments. Recognizing ionic dissociation helps predict the behavior of compounds in solution and their resulting impacts on reactions and processes they are involved in.