Problem 62 - Chemistry The Central Science In Si Units

Question

Consider the reaction $$ \begin{array}{l} 4 \mathrm{NH}_{3}(g)+5 \mathrm{O}_{2}(g) \rightleftharpoons \\ 4 \mathrm{NO}(g)+6 \mathrm{H}_{2} \mathrm{O}(g), \Delta H=-904.4 \mathrm{~kJ} \end{array} $$ Does each of the following increase, decrease, or leave unchanged the yield of \(\mathrm{NO}\) at equilibrium? (a) increase \(\left[\mathrm{NH}_{3}\right] ;(\mathbf{b})\) increase \(\left[\mathrm{H}_{2} \mathrm{O}\right] ;(\mathbf{c})\) decrease \(\left[\mathrm{O}_{2}\right] ;(\mathbf{d})\) decrease the volume of the container in which the reaction occurs; (e) add a catalyst; (f) increase temperature.

Step-by-Step Solution

Answer

(a) Increase, (b) Decrease, (c) Decrease, (d) Decrease, (e) Unchanged, (f) Decrease.

Step 1: Le Chatelier's Principle Overview

Le Chatelier's Principle states that if an external change is applied to a system at equilibrium, the system will adjust in such a way as to counteract that change. This principle will guide us in determining how various changes affect the yield of NO.

Step 2: Effect of Increasing [NH3]

Increasing the concentration of a reactant shifts the equilibrium to favor the formation of products. Here, increasing \([\mathrm{NH}_3]\) will shift the equilibrium to the right, increasing the yield of \(\mathrm{NO}\).

Step 3: Effect of Increasing [H2O]

Increasing the concentration of a product shifts the equilibrium toward the reactants to decrease the product concentration. Therefore, increasing \([\mathrm{H}_2\mathrm{O}]\) results in shifting the equilibrium to the left, decreasing the yield of \(\mathrm{NO}\).

Step 4: Effect of Decreasing [O2]

Decreasing the concentration of a reactant shifts the equilibrium toward the reactants. Thus, decreasing \([\mathrm{O}_2]\) will shift the equilibrium to the left, decreasing the yield of \(\mathrm{NO}\).

Step 5: Effect of Decreasing Volume

Decreasing the volume of a gaseous system shifts the equilibrium toward the side with fewer moles of gas. Comparing both sides, the reactant side has 9 moles (4 NH3 + 5 O2), and the product side 10 moles (4 NO + 6 H2O). Hence, decreasing volume shifts the equilibrium to the left, decreasing \(\mathrm{NO}\) yield.

Step 6: Effect of Adding a Catalyst

A catalyst speeds up both the forward and reverse reactions equally without altering the equilibrium position. Therefore, adding a catalyst leaves the yield of \(\mathrm{NO}\) unchanged.

Step 7: Effect of Increasing Temperature

An increase in temperature favors the endothermic reaction direction. Given \(\Delta H = -904.4 \text{ kJ}\), the reaction is exothermic, so increasing temperature shifts the equilibrium left, reducing \(\mathrm{NO}\) yield.

Key Concepts

Chemical Equilibrium

Chemical equilibrium occurs when the rates of the forward and reverse reactions in a chemical system become equal, leading to constant amounts of reactants and products. This state is not static but dynamic, as both reactions continue to occur, albeit at equal rates.
In the context of Le Chatelier's Principle, which helps predict how changes in conditions can affect equilibrium, the system will shift in a way to counteract any applied change:

• Adding more reactants shifts the equilibrium toward products to consume the excess.
• Adding products shifts the equilibrium toward reactants to reduce the excess products.
• Reducing the concentration of reactants or products will shift equilibrium to replace the limited component.

The principle can be especially useful for predicting how factors like concentration, pressure, and temperature alterations can affect a system's equilibrium position. Understanding equilibrium is essential for chemical processes as it influences how much product can be obtained from reactions.

Reaction Yield

Reaction yield refers to the amount of product obtained from a chemical reaction. Depending on the conditions applied, the yield of a reaction can increase or decrease. In our example, we are interested in boosting the yield of nitric oxide (NO) in the reaction.
Several factors can influence reaction yield according to Le Chatelier's Principle:

• Increasing the concentration of reactants (such as \([NH_3]\) in the reaction) will typically increase the yield of the product (NO).
• Conversely, increasing the concentration of products (like \([H_2O]\)) will generally decrease the product yield as the system attempts to reach equilibrium by reverting some products back to reactants.
• Changes in volume and pressure affect gases differently: decreasing the volume of the container for our reaction shifts the equilibrium towards fewer gas moles, impacting yield.

By manipulating these factors, scientists and engineers can maximize or reduce the yield depending on the desired outcome needed for their applications.

Endothermic and Exothermic Reactions

Reactions can either absorb energy (endothermic) or release energy (exothermic). Understanding these concepts is crucial when considering how temperature changes affect chemical reactions according to Le Chatelier's Principle.
For our specific reaction, \(\Delta H = -904.4 \text{ kJ}\), signifying it's exothermic. In exothermic reactions, heat is released:

• Increasing the temperature of an exothermic reaction results in a shift toward the reactants, because the system tries to absorb the added heat by favoring the endothermic (reverse) process.
• By contrast, decreasing the temperature would favor product formation, attempting to compensate for the lost heat.

Knowing whether a reaction is endothermic or exothermic helps in predicting and controlling the yield of desired products in practical applications.