Problem 157 - A Complete Resource Book in Chemistry for JEE Main

Question

When solid \(\mathrm{NaNO}_{3}\) is heated in a closed vessel, \(\mathrm{O}_{2}\) is liberated and solid \(\mathrm{NaNO}_{2}\) is left behind. At equilibrium (a) Addition of \(\mathrm{NaNO}_{2}\) favours reverse reaction. (b) Addition of \(\mathrm{NaNO}_{3}\) favours forward reaction. (c) Increasing the pressure favours reverse reaction. (d) Increasing the temperature favours forward reaction.

Step-by-Step Solution

Answer

All given options (a, b, c, d) are correct.

Step 1: Understand the Reaction

The given reaction is the decomposition of sodium nitrate \( \mathrm{NaNO}_{3} \) to sodium nitrite \( \mathrm{NaNO}_{2} \) and oxygen \( \mathrm{O}_{2} \). It can be written as: \[ \mathrm{2NaNO}_{3(s)} \rightarrow \mathrm{2NaNO}_{2(s)} + \mathrm{O}_{2(g)} \]. The forward reaction produces oxygen gas and the reverse reaction consumes it.

Step 2: Analyze Addition of Substances

According to Le Chatelier's principle, adding more \( \mathrm{NaNO}_{2} \) will shift the equilibrium toward the reverse reaction to consume the excess product, while adding more \( \mathrm{NaNO}_{3} \) will shift the equilibrium toward the forward reaction to reduce the excess reactant.

Step 3: Effect of Pressure Changes

The equilibrium shifts depending on the number of gas molecules. Increasing the pressure will shift the equilibrium towards the side with fewer gas moles. Since the forward reaction produces gas and the reverse reaction consumes it, increasing pressure favors the reverse reaction.

Step 4: Effect of Temperature Changes

For decomposition reactions, they tend to be endothermic, meaning they absorb heat. Therefore, increasing the temperature tends to favor the forward reaction, as it increases the rate of endothermic processes.

Step 5: Evaluate Each Option

- Option (a) is true because adding \( \mathrm{NaNO}_{2} \) favors the reverse reaction. - Option (b) is true because adding \( \mathrm{NaNO}_{3} \) favors the forward reaction. - Option (c) is true because increasing pressure favors the reverse reaction by reducing gas volume. - Option (d) is true because increasing temperature favors the endothermic forward reaction.

Key Concepts

Le Chatelier's Principle

Le Chatelier's Principle is a fundamental concept in chemistry that helps predict how an equilibrium reaction responds to external changes. When a system at equilibrium experiences a change in concentration, temperature, or pressure, it will adjust to counteract that change and restore balance. In the context of chemical reactions, this principle helps us understand how adding reactants or products affects the position of equilibrium:

• Adding more reactants will shift the balance towards producing more products, often favoring the forward reaction.
• Similarly, adding more products shifts the equilibrium towards reactants, favoring the reverse reaction.

This principle offers valuable insights into the behavior of reactions under different conditions, making it essential for evaluating chemical equilibria.

Decomposition Reactions

Decomposition reactions involve a single compound breaking down into two or more simpler substances. For example, in this exercise, sodium nitrate (\( \mathrm{NaNO}_{3} \)) decomposes into sodium nitrite \( \mathrm{NaNO}_{2} \) and oxygen gas \( \mathrm{O}_{2} \):\[ \mathrm{2NaNO}_{3(s)} \rightarrow \mathrm{2NaNO}_{2(s)} + \mathrm{O}_{2(g)} \]Decomposition reactions are crucial because they transform compounds into potentially more useful or reactive forms. These reactions are typically initiated by external energy sources like heat, electricity, or light. By understanding the nature of decomposition, it's easier to manipulate conditions to either promote or inhibit these reactions in practical applications.

Endothermic Reactions

Endothermic reactions absorb energy, typically in the form of heat, from their surroundings to proceed. In the context of the given decomposition reaction of sodium nitrate, heat is absorbed during the formation of sodium nitrite and oxygen. This energy absorption explains why increasing temperature tends to favor forward endothermic reactions:

• Higher temperatures provide more energy, facilitating the decomposition process.
• This shifts the equilibrium toward the product side, as seen in the formation of oxygen gas.

Understanding the endothermic nature of these reactions helps in designing conditions for optimal reaction rates, particularly when temperature control is involved.

Effect of Pressure on Equilibrium

The effect of pressure changes on equilibrium is substantial, particularly when gases are involved in the reaction. For reactions with gases, like the decomposition of \( \mathrm{NaNO}_{3} \), altering pressure can significantly shift equilibrium:

• Increasing pressure will shift equilibrium toward the side with fewer gas moles.
• In this specific reaction, the reverse reaction is favored under increased pressure because it consumes \( \mathrm{O}_{2} \), reducing the number of gas molecules.

This knowledge is essential when manipulating reactions that involve gas production or consumption, allowing better control over industrial chemical processes or even in laboratory settings where gas reactions are common.