Problem 36 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Element \(A\) burns in nitrogen to give an ionic compound \(B\). Compound \(B\) reacts with water to give \(C\) ' and \(D\). A solution of \(C\) becomes ' milky' on bubbling carbon dioxide. Identify \(A, B, C\) and \(D\).

Step-by-Step Solution

Answer

A is Li, B is Li_3N, C is LiOH, and D is NH_3.

Step 1: Identify Element A

Element \(A\) burns in nitrogen to give an ionic compound. A common element known to do this is lithium (\(Li\)). When lithium burns in nitrogen, it produces lithium nitride (\(Li_3N\)). Therefore, \(A\) is \(Li\).

Step 2: Determine Compound B

Since \(A\) is lithium and it forms an ionic compound \(B\) with nitrogen, \(B\) must be lithium nitride, \(Li_3N\). This compound is formed from the reaction of lithium with nitrogen: \(6Li + N_2 \rightarrow 2Li_3N\).

Step 3: React Compound B with Water to Find C and D

Lithium nitride \((Li_3N)\) reacts with water to give lithium hydroxide \((LiOH)\) and ammonia \((NH_3)\). The balanced chemical reaction is: \(Li_3N + 3H_2O \rightarrow 3LiOH + NH_3\). Thus, \(C\) is \(LiOH\) and \(D\) is \(NH_3\).

Step 4: Verify C with Carbon Dioxide Interaction

The solution of \(C\) is \(LiOH\), which reacts with carbon dioxide \((CO_2)\) to form lithium carbonate \((Li_2CO_3)\). The equation is: \(2LiOH + CO_2 \rightarrow Li_2CO_3 + H_2O\). \(Li_2CO_3\) is insoluble and produces a milky solution, confirming \(C\) as \(LiOH\).

Key Concepts

Element Identification

In chemical reactions, the first crucial step is to identify the elements involved. For the problem at hand, Element \(A\) is a substance that burns in nitrogen to form an ionic compound. To crack this mystery, we consider elements known for reacting with nitrogen. Lithium \(Li\) is a well-known candidate that reacts in such a manner. When lithium burns in the presence of nitrogen gas \((N_2)\), it forms lithium nitride \((Li_3N)\). This reaction can be confirmed through the equation: - \(6Li + N_2 \rightarrow 2Li_3N\).This clearly identifies that Element \(A\) is lithium \(Li\), a metal alkali that uniquely reacts under these conditions. Identifying elements in reactions helps us predict both the process and the products accurately.

Ionic Compounds

The term "ionic compound" refers to a chemical compound comprising ions held together by electrostatic forces. These are often formed between metals and non-metals. In our scenario, compound \(B\) results from the combination of lithium \(Li\) and nitrogen \(N_2\), creating lithium nitride \(Li_3N\).Ionic compounds like lithium nitride are characterized by the transfer of electrons between atoms, resulting in positively charged ions (cations) and negatively charged ions (anions). Here:

• Lithium loses electrons to form \(Li^+\) ions.
• The nitrogen gains electrons to form \(N^{3-}\) ions.

Thus, ionic compounds are crucial in various chemical reactions and often tend to have high melting and boiling points due to the strong forces holding the ions together.

Chemical Equations

Chemical equations are symbolic representations of chemical reactions. They describe the starting substances, called reactants, and the substances formed, known as products.Consider the transformation of compound \(B\), lithium nitride \(Li_3N\), reacting with water \(H_2O\) to yield products \(C\) and \(D\):- The balanced equation for this reaction is: \[ Li_3N + 3H_2O \rightarrow 3LiOH + NH_3 \]Specifically:

• 3 moles of \(LiOH\) (lithium hydroxide) are formed as \(C\).
• 1 mole of \(NH_3\) (ammonia) is formed as \(D\).

This illustrates not only the stoichiometry of the reaction but also confirms the identities of \(C\) and \(D\). Chemical equations are essential tools for understanding and predicting chemical transformations in reactions.

Reaction Products

Reaction products are substances that form as a result of a chemical reaction. In the given exercise, the focus is on identifying \(C\) and \(D\) once lithium nitride reacts with water.Lithium hydroxide \(LiOH\) is one such product, known to react further with carbon dioxide \(CO_2\):- The reaction upon exposure to \(CO_2\) is: \[ 2LiOH + CO_2 \rightarrow Li_2CO_3 + H_2O \]This results in the formation of lithium carbonate \(Li_2CO_3\), which is poorly soluble in water, causing a milky suspension. Thus: - \(C\) is confirmed as lithium hydroxide \(LiOH\).- \(D\) is ammonia \(NH_3\), a gaseous product frequently released in reactions involving nitrogen compounds.Understanding reaction products allows chemists to predict the results of reactions and to design chemical processes accordingly.