Question

Coal-burning power plants release sulfur dioxide into the atmosphere. The \(\mathrm{SO}_{2}\) is converted to \(\mathrm{SO}_{3}\) by nitrogen dioxide as follows. $$ \mathrm{SO}_{2}(g)+\mathrm{NO}_{2}(g) \rightleftarrows \mathrm{SO}_{3}(g)+\mathrm{NO}(g)+\text { heat } $$ Predict the direction of equilibrium shift for each of the following stresses: (a) increase \(\left[\mathrm{SO}_{2}\right]\) (b) decrease \(\left[\mathrm{NO}_{2}\right]\) (c) increase \(\left[\mathrm{SO}_{3}\right]\) (d) decrease \([\mathrm{NO}\) (e) increase temperature (f) decrease temperature (g) increase volume (h) decrease volume (i) add He inert gas (j) ultraviolet light

Short answer

(a) Right, (b) Left, (c) Left, (d) Right, (e) Left, (f) Right, (g) No shift, (h) No shift, (i) No shift, (j) No effect.

Step-by-step solution

Evaluate the Effect of Increasing [SO2]

An increase in the concentration of a reactant, \ \( \mathrm{SO}_{2} \ \), will drive the equilibrium position to the right to oppose the change and consume the additional \ \( \mathrm{SO}_{2} \ \). This results in the production of more products: \ \( \mathrm{SO}_{3} \ \) and \ \( \mathrm{NO} \ \).

Evaluate the Effect of Decreasing [NO2]

Decreasing the concentration of one of the reactants, \ \( \mathrm{NO}_{2} \ \), will shift the equilibrium to the left to produce more \ \( \mathrm{NO}_{2} \ \), resulting in a decrease in products and an increase in reactants.

Evaluate the Effect of Increasing [SO3]

Increasing the concentration of a product, \ \( \mathrm{SO}_{3} \ \), shifts the equilibrium to the left to oppose the change and consume the additional product, thus forming more reactants.

Evaluate the Effect of Decreasing [NO]

A decrease in the concentration of a product, \ \( \mathrm{NO} \ \), will shift the equilibrium to the right to produce more \ \( \mathrm{NO} \ \), leading to the production of more products.

Evaluate the Effect of Increasing Temperature

Since the reaction is exothermic (releases heat), increasing the temperature will shift the equilibrium to the left. This is because the system attempts to absorb additional heat by favoring the endothermic reverse reaction.

Evaluate the Effect of Decreasing Temperature

Conversely, decreasing the temperature favors the exothermic forward reaction, shifting the equilibrium to the right to produce more heat.

Evaluate the Effect of Increasing Volume

Increasing the volume of the gaseous reaction environment decreases pressure, causing the equilibrium to shift to the side with more gas moles. Both sides have two moles of gas, so there is no shift.

Evaluate the Effect of Decreasing Volume

Decreasing the volume increases pressure, and the equilibrium will favor the side with fewer moles of gas. Since both sides have two moles of gas, there is no shift in equilibrium.

Evaluate the Effect of Adding He Inert Gas

Adding helium or any inert gas at constant volume does not affect the partial pressures of the reactants and products, thus, there is no shift in equilibrium.

Evaluate the Effect of Ultraviolet Light

As the reaction does not involve photochemical processes with ultraviolet light, there will be no effect on the equilibrium position.

Key concepts

Equilibrium Shift

When we talk about equilibrium in chemistry, we're discussing the point at which the rate of the forward reaction equals the rate of the reverse reaction. However, Le Chatelier's Principle tells us that when a system at equilibrium is subjected to a change or "stress," it will shift to counteract that stress and restore balance. For the reaction \[ \mathrm{SO}_{2}(g)+\mathrm{NO}_{2}(g) \rightleftarrows \mathrm{SO}_{3}(g)+\mathrm{NO}(g)+\text{heat} \]this means that adding more \(\mathrm{SO}_{2}\) will shift the equilibrium to the right to produce more \(\mathrm{SO}_{3}\) and \(\mathrm{NO}\), since the system will try to use up the excess \(\mathrm{SO}_{2}\). Conversely, if you decrease the concentration of \(\mathrm{NO}_{2}\), the reaction will shift to the left to produce more \(\mathrm{NO}_{2}\). By understanding how equilibrium shifts in response to changes, you can predict the direction of shift and the concentrations of products and reactants at any given time.

Exothermic Reactions

Exothermic reactions are those that release heat as they proceed. In our example, the given reaction is exothermic.This simply means that as \(\mathrm{SO}_{2}\) and \(\mathrm{NO}_{2}\) react to form \(\mathrm{SO}_{3}\) and \(\mathrm{NO}\), heat is released as a product. According to Le Chatelier's Principle, if you increase the temperature for an exothermic reaction, the equilibrium shifts to absorb this extra heat, which usually involves favoring the reverse reaction (endothermic direction).On the other hand, if you cool down an exothermic reaction, equilibrium will shift toward the production of heat, thus making more products. These thermal dynamics are pivotal in industrial processes where controlling reaction temperatures is crucial for maximizing yield.

Gaseous Reactions

Reactions that occur in the gas phase have unique characteristics, often affected by pressure and volume changes. The following equation represents a gaseous reaction:\[ \mathrm{SO}_{2}(g)+\mathrm{NO}_{2}(g) \rightleftarrows \mathrm{SO}_{3}(g)+\mathrm{NO}(g) \]In gaseous reactions, altering the volume of the system can impact equilibrium. Increasing the volume reduces pressure, possibly shifting the equilibrium towards the side with more moles of gas. Decreasing volume increases pressure and shifts equilibrium toward the side with fewer moles. However, in this particular reaction, both the reactant and product sides have two moles of gas each. Hence, changes in volume or pressure do not affect the reaction's equilibrium, showcasing that the number of moles of gases involved can neutralize certain expected shifts.

Reaction Conditions

The conditions under which a reaction takes place significantly impact its behavior and equilibrium. In our reaction, the starting concentrations of \(\mathrm{SO}_{2}\), \(\mathrm{NO}_{2}\), \(\mathrm{SO}_{3}\), and \(\mathrm{NO}\) can change the equilibrium position. Altering these concentrations can push the reaction forward or backward.Temperature adjustments, as discussed earlier, affect equilibrium shifts based on whether a reaction is exothermic or endothermic. Inert gases, like helium, when added at constant volume, do not participate in the reaction and thus have no effect. Reaction conditions can act like dials that we adjust to control the direction and extent of the reaction. This knowledge is crucial in optimizing reactions in both lab settings and industrial applications.

Stress Effect on Equilibrium

The effect of stress on equilibrium can be because of various factors, including concentration, temperature, and volume changes. According to Le Chatelier’s Principle, a system will react to stress by trying to restore a new equilibrium.

• Increasing a reactant or product causes a shift to consume the added material.
• Decreasing a reactant or product results in a shift to produce more of what's been reduced.
• Temperature changes shift the equilibrium depending on the reaction's exothermic or endothermic nature.
• Volume changes affect gaseous equilibria by altering pressure.
• Inert gases generally don't affect equilibrium under constant volume conditions, as they don't change the partial pressures of reactants and products.

Understanding these effects is essential for predicting how a reaction will behave under varying conditions, which is invaluable in fields such as chemical engineering and environmental science.