Problem 25 - Chemistry and Chemical Reactivity

Question

Dinitrogen trioxide decomposes to \(\mathrm{NO}\) and \(\mathrm{NO}_{2}\) in an endothermic process \(\left(\Delta_{\mathrm{r}} H^{\circ}=40.5 \mathrm{kJ} / \mathrm{mol}-\mathrm{rxn}\right)\) $$ \mathrm{N}_{2} \mathrm{O}_{3}(\mathrm{g}) \rightleftarrows \mathrm{NO}(\mathrm{g})+\mathrm{NO}_{2}(\mathrm{g}) $$ Predict the effect of the following changes on the position of the equilibrium; that is, state which way the equilibrium will shift (left, right, or no change) when each of the following changes is made. (a) adding more \(\mathrm{N}_{2} \mathrm{O}_{3}(\mathrm{g})\) (b) adding more \(\mathrm{NO}_{2}(\mathrm{g})\) (c) increasing the volume of the reaction flask (d) lowering the temperature

Step-by-Step Solution

Answer

(a) Right; (b) Left; (c) Right; (d) Left.

Step 1: Understanding the Reaction

The reaction given is an endothermic equilibrium: \( \mathrm{N}_2 \mathrm{O}_3(\mathrm{g}) \rightleftarrows \mathrm{NO}(\mathrm{g})+\mathrm{NO}_2(\mathrm{g}) \). Because it is endothermic, \( \Delta_{\mathrm{r}} H^{\circ} = 40.5 \text{ kJ/mol-rxn} \), heat is absorbed when the equilibrium shifts to the right.

Step 2: Effect of Adding \(\mathrm{N}_2 \mathrm{O}_3\)

According to Le Chatelier's principle, adding more \( \mathrm{N}_2 \mathrm{O}_3(\mathrm{g}) \) increases the concentration of reactants, thus shifting the equilibrium to the right to produce more products \( \mathrm{NO}(\mathrm{g}) \) and \( \mathrm{NO}_2(\mathrm{g}) \).

Step 3: Effect of Adding More \(\mathrm{NO}_2\)

Adding more \( \mathrm{NO}_2(\mathrm{g}) \) increases the concentration of products, causing the equilibrium to shift to the left to produce more \(\mathrm{N}_2 \mathrm{O}_3 \) to offset the added \(\mathrm{NO}_2\).

Step 4: Effect of Increasing Volume

Increasing the volume of the reaction flask decreases the pressure. The equilibrium will shift towards the side with more moles of gas to increase the pressure, which is to the right (2 moles on the right vs 1 mole on the left).

Step 5: Effect of Lowering Temperature

Lowering the temperature of an endothermic reaction shifts the equilibrium to the left since the system will release heat by forming more \( \mathrm{N}_2 \mathrm{O}_3 \) (reactant side), opposing the heat removal.

Key Concepts

Le Chatelier's Principle

Le Chatelier's Principle helps predict the direction of an equilibrium shift when a change occurs in a system. Imagine a reaction at equilibrium as a teeter-totter, balanced in the middle. If something new is added or a condition is changed, the teeter-totter tips to reestablish balance. In simple terms, Le Chatelier's Principle states that if a change is imposed on a system at equilibrium, the system will adjust itself to counteract the effect of the change. It is essential for identifying how reactants and products behave when disturbed.

For example:

• Adding a reactant or product will shift the equilibrium in the opposite direction to restore balance.
• If heat is added to an exothermic reaction, the equilibrium will shift to absorb the extra heat.
• Changes in pressure and volume will cause shifts favoring the side with more or fewer gas moles.

Understanding this principle helps in controlling the yields of chemical processes, making it a crucial part of chemical equilibrium studies.

Endothermic Reactions

Endothermic reactions play a significant role in chemical equilibrium. These reactions absorb heat from their surroundings. This means when a reaction shifts to the right (towards products) in an endothermic process, it requires energy in the form of heat.

Some characteristics of endothermic reactions include:

• Absorbing heat, indicated by a positive \(\Delta_r H^{\circ}\).
• Reactions feel cold to the touch as they draw in heat.
• Increased temperature favors the formation of products in an endothermic reaction.

Understanding the energy changes in reactions like the decomposition of \(\mathrm{N}_2\mathrm{O}_3\) is crucial. When you lower the temperature, the reaction will shift to the left to generate heat. Conversely, increasing the temperature helps the reaction proceed to the right, absorbing heat.

Equilibrium Shifts

Equilibrium shifts are the system's response to restore balance when a condition changes. When a disturbance occurs, the equilibrium position will move to reestablish equilibrium.

Consider these common causes of equilibrium shifts:

• Change in concentration: Adding or removing a reactant or product directly shifts the balance.
• Change in volume and pressure: Increasing the volume reduces the pressure, favoring the side with more gas molecules. Decreasing volume does the opposite, favoring the side with fewer gas molecules.
• Temperature changes: In endothermic reactions, increased temperature shifts the equilibrium toward the products.

For example, increasing the volume in the given reaction \(\mathrm{N}_2\mathrm{O}_3(\mathrm{g}) \rightleftarrows \mathrm{NO}(\mathrm{g}) + \mathrm{NO}_2(\mathrm{g})\) results in shifting the equilibrium to the right because there are more moles of gas on that side.

Reaction Dynamics

Reaction dynamics delve into the complexities of how chemical reactions transition from reactants to products. In every chemical reaction, there is a dynamic process governing how molecules collide, react, and balance. Understanding these dynamics is crucial for predicting the behavior of reactions under various conditions.

In the context of the decomposition of dinitrogen trioxide:

• The given reaction is reversible, meaning it can go forward and backward, constantly shifting around the equilibrium position.
• The reaction's direction and speed are influenced by changes in conditions like temperature, pressure, and concentration.
• Insight into reaction dynamics aids in controlling reaction rates to favor desired products and optimize efficiency.

By understanding the flow and interaction of molecules within the reaction, you can better prepare for how they respond to environmental changes, leading to improved planning and use of reactions in practical applications.