Problem 55 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

When \(\mathrm{NaNO}_{3}\) is heated in a closed vessel, oxygen is liberated and \(\mathrm{NaNO}_{2}\) is left behind. At equilibrium. [1986-1 Mark] (a) addition of \(\mathrm{NaNO}_{2}\) favours reverse reaction (b) addition of \(\mathrm{NaNO}_{3}\) favours forward reaction (c) increasing temperature favours forward reaction (d) increasing pressure favours reverse reaction

Step-by-Step Solution

Answer

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

Step 1: Understand the Chemical Reaction

The decomposition reaction for this problem is \(\mathrm{2NaNO}_{3(s)} \rightarrow \mathrm{2NaNO}_{2(s)} + \mathrm{O}_{2(g)}\). In this reaction, solid sodium nitrate (\(\mathrm{NaNO}_{3}\)) decomposes to form sodium nitrite (\(\mathrm{NaNO}_{2}\)) and oxygen gas (\(\mathrm{O}_{2}\)).

Step 2: Analyze the Effect of Adding Reactants or Products

Adding \(\mathrm{NaNO}_{2}\), a product, shifts the equilibrium towards the reactants to counteract the change (Le Chatelier's principle), so (a) is correct. Adding \(\mathrm{NaNO}_{3}\), a reactant, shifts the equilibrium towards the products to consume the added reactant, making (b) correct.

Step 3: Consider Temperature Changes

This reaction is endothermic as it consumes heat to decompose \(\mathrm{NaNO}_{3}\). According to Le Chatelier's principle, increasing the temperature favors the endothermic direction, which is the forward reaction, making (c) correct.

Step 4: Analyze the Impact of Pressure Changes

The forward reaction produces gas, increasing the number of moles of gas. Hence, increasing pressure will favor the side with fewer gas moles, which is the reverse reaction, making (d) correct.

Key Concepts

Le Chatelier's Principle

Le Chatelier's Principle is a fundamental concept in chemical equilibrium. It describes how a system at equilibrium responds to disturbances or changes in conditions.

• If a reactant or product is added to a system at equilibrium, the system shifts to counteract the change by favoring the consumption of the added substance.
• Conversely, if a substance is removed, the equilibrium will shift to produce more of that substance.

In the context of the chemical reaction involving \( ext{NaNO}_{3} \), adding \( ext{NaNO}_{2} \) enhances the reverse reaction to form more \( ext{NaNO}_{3} \).
This is because \( ext{NaNO}_{2} \) is a product, so adding it encourages the system to "use up" this product by shifting back to the reactants. Similarly, adding \( ext{NaNO}_{3} \) shifts the equilibrium towards the formation of more \( ext{NaNO}_{2} \) and \( ext{O}_{2} \) to consume the added reactant.

Endothermic Reaction

Endothermic reactions are processes that absorb heat from their surroundings. This means they require energy to proceed, and an increase in temperature can affect equilibrium.In the reaction \( 2 ext{NaNO}_{3(s)} \rightarrow 2 ext{NaNO}_{2(s)} + ext{O}_{2(g)} \),
the process involves absorbing heat to decompose.
According to Le Chatelier's Principle, increasing the temperature will favor the endothermic process. Thus, a rise in temperature encourages more of the \( ext{NaNO}_{3} \) decomposition to occur.

• More heat means more energy is available for breaking bonds in \( ext{NaNO}_{3} \).
• This increase in energy supply promotes the formation of \( ext{NaNO}_{2} \) and \( ext{O}_{2} \) as products.

This principle can be applied to predict how equilibrium will shift with temperature changes in other endothermic reactions as well.

Decomposition Reaction

Decomposition reactions involve breaking down a complex molecule into simpler substances. In the case of \( ext{NaNO}_{3} \),
it decomposes into sodium nitrite (\( ext{NaNO}_{2} \)) and oxygen gas (\( ext{O}_{2} \)).

• Such reactions often require energy, sometimes evident as an increase in temperature, to proceed.
• The decomposition of \( ext{NaNO}_{3} \) involves converting the solid substance into another solid (\( ext{NaNO}_{2} \)) and a gaseous product (\( ext{O}_{2} \)).

As a result, the reaction involves a change in phase for some products, impacting pressure and volume considerations.Decomposition is a crucial process not only in chemical reactions but also in nature, where it breaks down nutrients and organic matter. This concept highlights the transformation and conservation of matter in chemical processes.