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

Correct answers are (c) and (d): increasing pressure favors the reverse, and increasing temperature favors the forward reaction.

Step 1: Understand the Reaction

The decomposition of sodium nitrate (\(\mathrm{NaNO}_3\)) can be represented by the equation: \[ \mathrm{2NaNO}_3 (s) \rightarrow \mathrm{2NaNO}_2 (s) + \mathrm{O}_2 (g) \]. It is a reversible reaction, and at equilibrium, the rates of the forward and reverse reactions are equal.

Step 2: Analyze the Effects of Addition of Solids

Adding \(\mathrm{NaNO}_2\) or \(\mathrm{NaNO}_3\) to the system won't shift the equilibrium because solids do not appear in the equilibrium expression and their concentration does not affect equilibrium. Therefore, options (a) and (b) are incorrect.

Step 3: Examine the Impact of Pressure

An increase in pressure shifts the equilibrium towards the side with fewer moles of gas. The forward reaction produces 1 mole of gas, while the reverse produces none, so increasing pressure favors the reverse reaction, supporting option (c).

Step 4: Consider the Effect of Temperature

The reaction of decomposition of \(\mathrm{NaNO}_3\) is endothermic. Increasing temperature favors the endothermic direction, which is the forward reaction. Thus, option (d) is correct.

Key Concepts

Le Chatelier's Principle

Le Chatelier's Principle helps us understand how a chemical equilibrium shifts in response to changes. It essentially tells us that if a dynamic equilibrium system is disturbed by changing the conditions, the system will adjust itself to counteract the effect of the disturbance and re-establish equilibrium.
For example, if we alter the concentration of a reactant or a product in the equilibrium mixture, the system responds by shifting the equilibrium position to reduce the change. This principle is particularly useful when predicting the behavior of gases and solutions during a reaction.

• Adding a reactant will generally shift the equilibrium towards the products (forward direction).
• Adding a product will usually shift the equilibrium towards the reactants (reverse direction).

In terms of applications, Le Chatelier's Principle is crucial in chemical engineering and industrial manufacturing processes where conditions like temperature, pressure, and concentration are manipulated to maximize yield.

Endothermic Reactions

Endothermic reactions are those that absorb energy from their surroundings. This often takes the form of heat, causing the surrounding temperature to decrease.
An example is the decomposition of \(\mathrm{NaNO}_3\), where solid sodium nitrate breaks down into sodium nitrite and oxygen gas. During this reaction, energy is absorbed, making it endothermic.
A key point with endothermic reactions is that increasing the temperature typically favors the forward reaction. That's because the added heat acts as a reactant. To reach a new equilibrium, the system tends to produce more products. This is particularly important in processes where heat is a controllable variable.

Gas Pressure Effects

Gas pressure plays a significant role in the equilibrium position of reactions involving gases.
According to Le Chatelier's Principle, if you increase the pressure on a system at equilibrium, the system will adjust itself to decrease the pressure. This often means moving toward the side of the reaction with fewer moles of gas.
In the decomposition of \(\mathrm{NaNO}_3\), the forward reaction produces one mole of oxygen gas, while the reverse reaction involves no gases. Therefore, increasing the pressure will favor the reverse reaction, pushing the equilibrium towards the side without gas (the reactants).

• More moles of gas on one side? Increased pressure favors the opposite side.
• Less gas volume is often preferred at higher pressures.

Solid State Reactions

Solid state reactions involve solid reactants and/or products without the presence of a solvent. Unlike gases and liquids, the concentration of solids doesn't change significantly.
This means that adding more solid reactants or products usually has minimal direct impact on the equilibrium position.
In our given reaction of sodium nitrate, the substances \(\mathrm{NaNO}_3\) and \(\mathrm{NaNO}_2\) are both solids. Despite changing their quantities, the equilibrium position remains largely unaffected because their concentrations do not appear in the equilibrium expression.

• Solid concentration is considered constant in equilibrium expressions.
• Generally, solids do not impact the equilibrium shift compared to gases or liquids in a reaction.

As a result, changes in the physical quantity of these solids won't noticeably shift the equilibrium direction.