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 statements are correct.

Step 1: Equation Setup

First, identify the chemical reaction involved. When solid \( \mathrm{NaNO}_{3} \) is heated, it decomposes into \( \mathrm{NaNO}_{2} \) and \( \mathrm{O}_{2} \): \[ \mathrm{2 \ NaNO}_{3(s)} \rightarrow \mathrm{2 \ NaNO}_{2(s)} + \mathrm{O}_{2(g)} \]. This reaction reaches equilibrium in the closed system when the rate of the forward reaction equals the rate of the reverse reaction.

Step 2: Effect of Solid Sodium Nitrite Addition

According to Le Chatelier's Principle, adding more \( \mathrm{NaNO}_{2} \), a product, will shift the equilibrium towards the reactants to counteract the change. This means the reverse reaction is favored. Therefore, (a) Addition of \( \mathrm{NaNO}_{2} \) favors the reverse reaction.

Step 3: Effect of Solid Sodium Nitrate Addition

Adding \( \mathrm{NaNO}_{3} \), a reactant, to the system will shift the equilibrium towards the products to use up the added reactant. Thus, the forward reaction is favored, supporting statement (b): Addition of \( \mathrm{NaNO}_{3} \) favors the forward reaction.

Step 4: Effect of Pressure Increase

Since \( \mathrm{O}_{2} \) is the only gas, increasing the pressure will favor the side with fewer moles of gas. In this case, the reactant side (with no gaseous moles) is favored, thus supporting statement (c): Increasing the pressure favors the reverse reaction.

Step 5: Effect of Temperature Increase

As this decomposition is endothermic (it absorbs heat), increasing the temperature will shift the equilibrium towards the products to absorb the added heat. Therefore, the forward reaction is favored, confirming statement (d): Increasing the temperature favors the forward reaction.

Key Concepts

Le Chatelier's Principle

Le Chatelier's Principle is a fundamental concept in chemistry that helps explain how equilibrium systems respond to external changes. If a system at equilibrium experiences a change in concentration, temperature, or pressure, Le Chatelier's principle states that the system will adjust to counteract this change and re-establish equilibrium.

This principle implies that a system will shift its equilibrium position:

• Toward the side with fewer molecules if the pressure is increased.
• Away from an added material to decrease its effect.
• In the endothermic direction with an increase in temperature and the exothermic direction with a decrease in temperature.

Understanding this principle allows chemists to predict the effect of external changes on the equilibrium position of a reaction. It provides a way to control the conditions to achieve the desired outcome from a chemical process.

Decomposition Reaction

A decomposition reaction is a type of chemical reaction where a single compound breaks down into two or more simpler substances. This process typically requires energy input, such as heat, to break the chemical bonds in the compound.

In the case of sodium nitrate ( \( \mathrm{NaNO}_{3} \)), heating causes it to decompose into solid sodium nitrite (\( \mathrm{NaNO}_{2} \)) and oxygen gas (\( \mathrm{O}_{2} \)). This specific reaction can be seen as:

• \( \mathrm{2 \, NaNO}_{3(s)} \rightarrow \mathrm{2 \, NaNO}_{2(s)} + \mathrm{O}_{2(g)} \)

The reaction releases oxygen gas, making it useful in situations where oxygen needs to be generated or consumed. Like many decomposition reactions, this process is sensitive to external conditions such as pressure and temperature, which help define the direction and extent of the reaction's progress.

Effect of Pressure on Equilibrium

The effect of pressure on equilibrium is an important consideration, particularly for reactions involving gases. Changes in pressure affect only those parts of a system where gases are involved. According to Le Chatelier's Principle, an increase in pressure will shift the equilibrium towards the side with fewer moles of gas.

For the decomposition of sodium nitrate, this principle applies because only oxygen gas is released as a product:

• Increasing pressure favors the reverse reaction where no gaseous products are formed, which means more sodium nitrate remains unreacted.
• If the pressure is decreased, the reaction might shift towards producing more oxygen gas.

Understanding the pressure effect is crucial for optimizing industrial processes that involve gases, as it influences the yields and efficiency of these reactions.

Effect of Temperature on Equilibrium

Temperature changes have a profound impact on the direction of chemical reactions, particularly those that are endothermic or exothermic. When a reaction is endothermic, like the decomposition of sodium nitrate, increasing the temperature provides the energy needed for the reaction to proceed.

• An increase in temperature moves the equilibrium toward the formation of products (oxygen gas and sodium nitrite), favoring the forward reaction.
• If the reaction were exothermic, an increase in temperature would favor the reverse reaction to dissipate the excess heat.

Adjusting temperature is a common method used by chemists to direct the outcome of a reaction. It helps in various applications, including synthetic chemical production, where specific products are desirable.