Question

A piece of magnesium ribbon was heated to redness in an atmosphere of nitrogen and on cooling water was added, the gas evolved was (a) ammonia (b) hydrogen (c) nitrogen (d) oxygen

Short answer

The gas evolved is ammonia (a).

Step-by-step solution

Identify Reaction Between Mg and N2

When magnesium is heated in the presence of nitrogen, it reacts to form magnesium nitride (Mg_3N_2). This is the primary reaction taking place: \[3 \text{Mg} + \text{N}_2 \rightarrow \text{Mg}_3\text{N}_2\]

Reaction of Magnesium Nitride with Water

When water is added to magnesium nitride (Mg_3N_2), it reacts to form magnesium hydroxide (Mg(OH)_2) and ammonia gas (NH_3). This reaction is given by: \[\text{Mg}_3\text{N}_2 + 6 \text{H}_2\text{O} \rightarrow 3\text{Mg(OH)}_2 + 2\text{NH}_3\]

Identify Gas Evolved

From the reaction in Step 2, we observe that the gas evolved is ammonia ( NH_3 ). This means that the gas produced when the magnesium nitride reacts with water is ammonia.

Key concepts

Magnesium Nitride Formation

Magnesium, a shiny metallic element, reacts vigorously with nitrogen under the right conditions. When a magnesium ribbon is heated to redness in an atmosphere of nitrogen, it forms a compound known as magnesium nitride. This is a classic chemical reaction, highlighting magnesium's ability to bond with nitrogen. The balanced chemical equation for this process is:

• \[3 \text{Mg} + \text{N}_2 \rightarrow \text{Mg}_3\text{N}_2\]

During this reaction, three magnesium atoms bond with one nitrogen molecule (which consists of two nitrogen atoms) to form a single molecule of magnesium nitride (\(\text{Mg}_3\text{N}_2\)). This compound appears as a yellowish or white powder, in contrast to the greyish color of pure magnesium. Understanding this reaction is key to grasping how metals can react under heat to form new compounds.

Ammonia Production

Ammonia is a significant compound with various industrial applications, from fertilizers to household cleaning products. When water reacts with magnesium nitride, it generates ammonia in a fascinating chemical transformation. This reaction can be expressed by the equation:

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

In this process, magnesium nitride ( \(\text{Mg}_3\text{N}_2\) ) decomposes upon contact with water to form magnesium hydroxide ( \(\text{Mg(OH)}_2\) ) and releases ammonia gas ( \(\text{NH}_3\) ). The ammonia produced is a colorless gas with a pungent odor, and it is lighter than air, enabling it to rise quickly when formed. For students learning chemistry, noticing how ammonia is produced from reactions involving metal nitrides helps in understanding synthesis reactions and the manufacture of useful compounds.

Reactivity with Water

Water is an often underestimated reactive agent in chemistry, especially when interacting with metallic compounds. When magnesium nitride reacts with water, a notable transformation occurs, producing magnesium hydroxide and ammonia gas. This interaction demonstrates the reactivity of water with certain metallic nitrides, detailed in the reaction:

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

During this reaction, each molecule of magnesium nitride requires six molecules of water to fully break down. The magnesium ion bonds with hydroxide ions from water, yielding magnesium hydroxide. This compound is a white solid often used in products like antacids. Meanwhile, the nitrogen from magnesium nitride reacts to produce ammonia gas. These types of reactions highlight the versatility and importance of water in various chemical transformations, making it a pivotal element in both natural and industrial processes.