Question

Consider the equilibrium \(\mathrm{N}_{2} \mathrm{O}(g)+\mathrm{NO}_{2}(g) \rightleftharpoons 3 \mathrm{NO}(g) \quad \Delta H^{\circ}=+155.7 \mathrm{~kJ}\) In which direction will this equilibrium be shifted by the following changes? (a) Adding \(\mathrm{N}_{2} \mathrm{O}\) (b) Removing \(\mathrm{NO}_{2}\) (c) Adding NO (d) Increasing the temperature of the reaction mixture (e) Adding helium gas to the reaction mixture at constant volume (f) Decreasing the volume of the container at constant temperature

Short answer

The equilibrium shifts (a) to the right with the addition of \(\mathrm{N}_{2}\mathrm{O}\), (b) to the left with the removal of \(\mathrm{NO}_{2}\), (c) to the left with the addition of NO, (d) to the right with an increase in temperature, (e) remains unchanged with the addition of helium at constant volume, and (f) to the left with a decrease in container volume.

Step-by-step solution

Understand Le Chatelier's Principle

Le Chatelier's Principle states that when a stress is applied to a system at equilibrium, the system adjusts in such a way as to minimize the stress. This can be caused by changes in concentration of reactants or products, temperature, or pressure.

Analyze Change (a): Adding \(\mathrm{N}_{2}\mathrm{O}\)

Adding more \(\mathrm{N}_{2}\mathrm{O}\) will increase its concentration, causing the equilibrium to shift to the right, toward the products side to reduce the increased concentration of \(\mathrm{N}_{2}\mathrm{O}\).

Analyze Change (b): Removing \(\mathrm{NO}_{2}\)

Removing \(\mathrm{NO}_{2}\) will decrease its concentration, causing the equilibrium to shift to the left, toward the reactants side to replace the lost \(\mathrm{NO}_{2}\).

Analyze Change (c): Adding NO

Adding more NO will increase its concentration, causing the equilibrium to shift to the left, toward the reactants side to reduce the increased concentration of NO.

Analyze Change (d): Increasing the Temperature of the Reaction Mixture

Increasing the temperature adds heat to the system. Since the reaction is endothermic (\( \Delta H^{\circ}=+155.7 \mathrm{~kJ} \)), the equilibrium will shift to the right, toward the products side, to absorb the extra heat added.

Analyze Change (e): Adding Helium Gas to the Reaction Mixture at Constant Volume

Adding an inert gas like helium at constant volume does not change the partial pressures of the reacting gases, so there is no effect on the position of the equilibrium.

Analyze Change (f): Decreasing the Volume of the Container at Constant Temperature

Decreasing the volume increases the pressure of the system. According to Le Chatelier's Principle, the equilibrium will shift toward the side with fewer moles of gas. In this reaction, there are 2 moles of gas on the reactant side and 3 moles of gas on the product side, so the equilibrium will shift to the left, toward the reactants.

Key concepts

Chemical Equilibrium

When we talk about chemical equilibrium, we refer to a state in a chemical reaction where the rate of the forward reaction is equal to the rate of the reverse reaction. As a result, the concentrations of reactants and products remain constant over time. It's important to note that the process is dynamic—molecules are continuously reacting and re-forming, but in such a way that the overall amounts stay steady.

A common misconception is that a system at equilibrium has equal concentrations of reactants and products, but this isn't always the case. The equilibrium position, which tells us the relative concentrations of reactants and products at equilibrium, varies for different reactions and depends on several factors, including temperature, pressure, and concentrations.

Reaction Direction Shift

Understanding how equilibrium can be disrupted is crucial. Le Chatelier's Principle helps us predict how a reaction will respond to a change in conditions. If a change is made that disturbs the equilibrium, such as altering the concentrations of reactants or products, the system will adjust to counteract that change. This often involves the reaction shifting direction.

For instance,

• if a reactant is added, the system will shift towards making more products,
• if a product is removed, it shifts to make more of that product,
• adding a reactant or removing a product will shift the equilibrium to the right—to form more products,
• conversely, adding a product or removing a reactant shifts it to the left—to form more reactants.

Remember, the reaction will favor the direction that helps restore balance to the system.

Effects of Temperature on Equilibrium

Temperature changes can have a significant impact on the equilibrium position. The direction of the shift depends on whether the reaction is exothermic (releases heat) or endothermic (absorbs heat). If the temperature increases in an exothermic reaction, the system will shift to favor the reverse reaction, absorbing the added heat and producing more reactants.

H4: Understanding Endothermic Reactions
For an endothermic reaction, like the one in the example where \( \Delta H^{\circ}=+155.7 \mathrm{~kJ} \) indicates heat is absorbed, an increase in temperature will shift the equilibrium toward the products to absorb more heat. Conversely, decreasing the temperature would shift an endothermic reaction to favor the reactants. Temperature effects on equilibrium serve as a great reminder of how energy changes are as influential as changes in concentration or pressure.