Problem 98 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following equations is not correct? (1) \(2 \mathrm{Na}+2 \mathrm{NH}_{3} \longrightarrow{\Delta}{\longrightarrow} 2 \mathrm{NaNH}_{2}+\mathrm{H}_{2}\) (2) \(4 \mathrm{NH}_{3}+2 \mathrm{NaOCl} \longrightarrow 2 \mathrm{NH}_{4} \mathrm{Cl}+2 \mathrm{NaNH}_{2}\) (3) \(\mathrm{NH}_{3}+3 \mathrm{Cl}_{2} \stackrel{\text { Excess }}{\text { Excess }}+\mathrm{O}_{2}^{4}\) (4) \(\mathrm{NH}_{3}+3 \mathrm{Cl}_{2}^{2} \stackrel{\text { Excess }}{\longrightarrow} \mathrm{NCl}_{3}+3 \mathrm{HCl}\) (a) 2 and 3 only (b) 1 and 4 only (c) 3 only (d) 2 only

Step-by-Step Solution

Answer

The incorrect equations are 2 and 3; answer is (a) 2 and 3 only.

Step 1: Analyze Equation 1

Equation 1 is given as \(2 \mathrm{Na}+2 \mathrm{NH}_{3} \longrightarrow \Delta \longrightarrow 2 \mathrm{NaNH}_{2}+\mathrm{H}_{2}\). This represents the reaction of sodium with ammonia to form sodium amide and hydrogen gas. This is a known and correct reaction.

Step 2: Analyze Equation 2

Equation 2 is \(4 \mathrm{NH}_{3}+2 \mathrm{NaOCl} \longrightarrow 2 \mathrm{NH}_{4} \mathrm{Cl}+2 \mathrm{NaNH}_{2}\). Ammonia reacting with sodium hypochlorite should produce chloramine, not ammonium chloride or sodium amide. Hence, Equation 2 is incorrect.

Step 3: Analyze Equation 3

Equation 3 is \(\mathrm{NH}_{3}+3 \mathrm{Cl}_{2} \stackrel{\text { Excess }}{\text{ Excess }}+\mathrm{O}_{2}^{4}\). This statement is incomplete and doesn't follow any recognized chemical reaction pattern. Thus, it is incorrect.

Step 4: Analyze Equation 4

Equation 4 is \(\mathrm{NH}_{3}+3 \mathrm{Cl}_{2}^{2} \stackrel{\text { Excess}}{\longrightarrow} \mathrm{NCl}_{3}+3 \mathrm{HCl}\). This correctly represents the reaction where excess chlorine reacts with ammonia to form nitrogen trichloride and hydrochloric acid, a known reaction.

Step 5: Decide which equations are incorrect

Based on the analysis, Equation 2 does not result in the stated products and Equation 3 is incomplete and nonsensical, making both of these incorrect.

Key Concepts

Balanced Chemical Equation

A balanced chemical equation is essential for accurately representing a chemical reaction. It ensures that the number of atoms for each element is the same on both sides of the equation. This follows the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction.
To balance an equation, adjust the coefficients (numbers in front of molecules) so that the number of atoms of each element is equal on both the reactant and product sides.

• For instance, in the equation from Exercise 1: \(2 \mathrm{Na}+2 \mathrm{NH}_{3} \longrightarrow 2 \mathrm{NaNH}_{2}+\mathrm{H}_{2}\), sodium (Na) and nitrogen (N) are balanced, each having two on both sides.
• Always verify that compounds are correctly balanced with the respective stoichiometry to ensure a valid representation of the reaction.

Balancing not only reflects reality but helps in understanding how much reactant is needed or how much product can be expected.

Reagents and Products

In any chemical reaction, the substances you start with are called reagents (or reactants), and what you end up with are products. Properly identifying and understanding these can elucidate the nature of a reaction.
For example, in the agenda exercise:

• Equation 1 involves sodium (Na) and ammonia (\(\mathrm{NH}_{3}\)) as reagents. They react to form sodium amide (\(\mathrm{NaNH}_{2}\)) and hydrogen gas (\(\mathrm{H}_{2}\)). Here, \(\mathrm{Na}\) and \(\mathrm{NH}_{3}\) are reagents, while \(\mathrm{NaNH}_{2}\) and \(\mathrm{H}_{2}\) are products.
• Equation 2 incorrectly identifies products. It states ammonium chloride (\(\mathrm{NH}_{4}\mathrm{Cl}\)) and sodium amide should be formed from ammonia and sodium hypochlorite (\(\mathrm{NaOCl}\)), which is incorrect.

Understanding reagents and products lets you predict the results and calculate the yield of chemical reactions.

Chemical Reaction Analysis

Analyzing a chemical reaction involves understanding its components and ensuring coherence. This includes evaluating whether the reactants can indeed form the proposed products and if all elements balance.
Take Equation 3 from the exercise, which says \(\mathrm{NH}_{3}+3 \mathrm{Cl}_{2} \stackrel{\text { Excess }}{\text{ Excess }}+\mathrm{O}_{2}^{4}\). It is deemed incorrect because it doesn't define a valid reaction process or products. The role of oxygen is vague and doesn't align with typical reaction formulas.

• Consider the necessary conditions under which reactions occur, like the need for a catalyst or specific temperature.
• Verify that all compounds can logically combine, considering their chemical properties and typical reaction pathways.

Meticulous reaction analysis helps prevent errors in experimentation and ensures those studying the reaction have an accurate understanding of the chemical processes involved.