Problem 130 - A Complete Resource Book in Chemistry for JEE Main

Question

When \(\mathrm{NaNO}_{3}(\mathrm{~d}=2.0 \mathrm{~g} / \mathrm{cc})\) is heated in a closed vessel of \(100 \mathrm{ml}\), oxygen is liberated and \(\mathrm{NaNO}_{2}\) \((\mathrm{d}=1.5 \mathrm{~g} / \mathrm{cc})\) is left behind as per the reaction \(2 \mathrm{NaNO}_{3}(\mathrm{~s}) \rightleftharpoons 2 \mathrm{NaNO}_{2}(\mathrm{~s})+\mathrm{O}_{2}(\mathrm{~g}) .\) At equilibrium, the volumes of \(\mathrm{NaNO}_{3}\) left and \(\mathrm{NaNO}_{3}\) left and \(\mathrm{NaNO}_{2}\) produced are very small and can be neglected. Which of the following is a correct statement about this equilibrium? (a) Addition of \(30 \mathrm{~g}\) of \(\mathrm{NaNO}_{3}\) favours reverse reaction. (b) Addition of \(30 \mathrm{~g}\) of \(\mathrm{NaNO}_{2}\) favours forward reaction. (c) Increasing temperature favours reverse reaction. (d) None of these.

Step-by-Step Solution

Answer

(d) None of these.

Step 1: Understand the Reaction

The chemical reaction given is \( 2 \mathrm{NaNO}_3(\mathrm{s}) \rightleftharpoons 2 \mathrm{NaNO}_2(\mathrm{s}) + \mathrm{O}_2(\mathrm{g}) \). This reaction involves the decomposition of sodium nitrate (\( \text{NaNO}_3 \)) into sodium nitrite (\( \text{NaNO}_2 \)) and oxygen gas \((\text{O}_2)\). It is an equilibrium reaction involving solids and gases.

Step 2: Analyzing the Effect of Adding \(\mathrm{NaNO}_3\) or \(\mathrm{NaNO}_2\)

In a closed system at equilibrium, if more reactant \((\text{NaNO}_3)\) is added, according to Le Chatelier's Principle, the system will shift towards the forward reaction to counteract the change, producing more \(\text{NaNO}_2\) and \(\text{O}_2\). Conversely, adding more \(\text{NaNO}_2\) would not favor the forward reaction since \(\text{NaNO}_2\) is a product, and typically it would shift the equilibrium towards the reactants.

Step 3: Effect of Temperature on the Reaction

For this endothermic reaction (since heat is required to decompose \(\text{NaNO}_3\)), increasing temperature will favor the formation of more products according to Le Chatelier's Principle. Thus, increasing the temperature favors the forward reaction where \(\text{NaNO}_3\) decomposes further into \(\text{NaNO}_2\) and \(\text{O}_2\).

Step 4: Analyze Statements Given in the Exercise

(a) "Addition of \(30 \mathrm{~g}\) of \(\text{NaNO}_3\) favors reverse reaction" is incorrect; it favors the forward reaction.(b) "Addition of \(30 \mathrm{~g}\) of \(\text{NaNO}_2\) favors forward reaction" is incorrect; it can drive the reaction backwards.(c) "Increasing temperature favors reverse reaction" is incorrect for an endothermic reaction; it favors the forward direction.(d) "None of these" becomes the right choice as all others are incorrect.

Key Concepts

Le Chatelier's Principle

Le Chatelier's Principle is a key concept in understanding how systems at equilibrium react to changes. It states that if an external change is applied to a system at equilibrium, the system will adjust to counteract the change and restore a new equilibrium. This self-correcting mechanism helps stabilize reactions
To simplify:

• If the concentration of a reactant or product changes, the system shifts to restore balance by consuming or producing more of the reactant or product.
• Temperature changes can affect the direction of the reaction, especially if it is endothermic or exothermic.
• Pressure changes impact reactions involving gases, but not those with only solids or liquids.

In our given reaction, the addition of more sodium nitrate \( ext{(NaNO}_3)\) shifts the equilibrium towards the right, producing more sodium nitrite \( ext{(NaNO}_2)\) and oxygen \(( ext{O}_2)\). This illustrates how Le Chatelier's Principle operates when reactants are added.

Endothermic Reaction

An endothermic reaction absorbs heat from its surroundings. It requires energy to proceed. When you think about endothermic reactions, consider the energy flow:- Energy goes from the environment into the reaction.- To move forward, it needs extra heat, unlike exothermic reactions that release heat.Now in our context, the decomposition of \( ext{NaNO}_3\) is endothermic. This means heating up the system will lead to more products. That's because the system will absorb more heat, which allows more sodium nitrate to break down.
Using heat in an endothermic reaction can resemble turning up the gas on a stove when cooking. More heat speeds up the process and produces more end products.

Equilibrium Shifts

In chemistry, equilibrium shifts relate to how the position of equilibrium moves in response to changes in the system. This is directly connected to Le Chatelier's Principle.Consider the reaction \(2 ext{NaNO}_3( ext{s}) \rightleftharpoons 2 ext{NaNO}_2( ext{s}) + ext{O}_2( ext{g})\). When you alter conditions, the equilibrium position will change accordingly to counteract the applied change.
There are different factors that can cause an equilibrium shift:

• Concentration change: Adding extra \( ext{NaNO}_3\) alters the concentration, causing the system to shift right and create more products.
• Temperature change: As our reaction is endothermic, an increase in temperature shifts equilibrium towards the products.
• Pressure and Volume change: This applies more to gaseous reactions where a pressure increase can shift equilibrium towards fewer gas molecules.

Understanding equilibrium shifts allows chemists to control reaction outcomes by fine-tuning the conditions affecting equilibrium. In practical terms, manipulating temperature and concentration can direct reactions towards desired products, maximizing efficiency.