Question

Consider the following exothermic reaction: $$ 4 \mathrm{NH}_{3}(g)+5 \mathrm{O}_{2}(g) \rightleftharpoons 4 \mathrm{NO}(g)+6 \mathrm{H}_{2} \mathrm{O}(l) $$ Which of the following changes will increase the number of moles of NO at equilibrium? Explain why or why not for each change. (a) remove \(\mathrm{H}_{2} \mathrm{O}\) (b) decrease volume (c) decrease temperature (d) add \(\mathrm{O}_{2}\) (e) add a catalyst

Short answer

(a) Yes, removing H2O will increase the number of moles of NO at equilibrium. (b) Yes, decreasing volume will increase the moles of NO. (c) Yes, decreasing temperature will increase the moles of NO. (d) Yes, adding O2 will increase the number of moles of NO. (e) No, adding a catalyst will not affect the moles of NO at equilibrium.

Step-by-step solution

Considering the removal of H2O

According to Le Chatelier’s Principle, removing a component will cause the equilibrium to shift in a direction that replaces that component. In this case, removing H2O will make the reaction shift to the right to produce more H2O, thus increasing the number of moles of NO. So, this change will increase the number of moles of NO at equilibrium.

Considering the decrease in volume

Decreasing the volume in which the reaction is occurring will increase the pressure on the system. According to Le Chatelier’s Principle, the reaction will shift towards the side with fewer moles of gas to alleviate the increased pressure. There are 9 moles of gas on the left and 4 moles on the right. Therefore, if the volume is decreased, the reaction will shift to the right, increasing the quantity of NO. So, decreasing volume will increase the number of moles of NO at equilibrium.

Considering the decrease in temperature

Given that the reaction is exothermic (release heat), a decrease in temperature will cause the equilibrium to shift towards the right to produce more heat (making more NO in this case), according to Le Chatelier's principle. So, a decrease in temperature will increase the moles of NO.

Considering the addition of O2

Adding O2 will cause the reaction to shift to the right, trying to reduce the increased pressure by producing more products. This will increase the moles of NO as a result. So, adding O2 will increase the number of moles of NO at equilibrium.

Considering the addition of a catalyst

A catalyst speeds up both the forward and reverse reactions equally, but does not shift the position of equilibrium. Therefore, the addition of a catalyst does not change the number of NO moles at equilibrium. So, adding a catalyst will not affect the number of moles of NO at equilibrium.

Key concepts

Equilibrium Shift

In any chemical reaction, when external conditions such as concentration, pressure, or volume change, the equilibrium will adjust to counteract that change. This behavior is explained by Le Chatelier’s Principle. For our given reaction, if we remove a product like water (\(\mathrm{H}_{2} \mathrm{O}\)) from the system, the equilibrium will shift to the right. This shift attempts to replace the missing water, thus increasing the amount of nitrogen monoxide (\(\mathrm{NO}\)).
Another example is decreasing the volume of the reaction vessel. This action increases the pressure. Since there are fewer moles of gas on the right side (4 moles) compared to the left side (9 moles), the system will shift to the right to decrease pressure. Therefore, this reduces the number of gas moles present. In both instances, more \(\mathrm{NO}\) forms at equilibrium as the reaction adjusts to these changes.

Exothermic Reaction

An exothermic reaction is one in which heat is released as the reaction proceeds. In the context of our reaction that forms \(\mathrm{NO}\), heat is considered a product. When the temperature decreases, according to Le Chatelier's Principle, the equilibrium shifts toward producing more heat.
This results in a rightward shift, favoring the formation of excess products, such as \(\mathrm{NO}\). On the other hand, if the temperature increases, the equilibrium will move in the opposite direction, decreasing \(\mathrm{NO}\) production. So, by lowering the temperature, one can maximize the amount of \(\mathrm{NO}\) produced in this exothermic reaction.

Catalyst Effect

Catalysts play a crucial role in speeding up reactions, but they do not change the equilibrium position. When you add a catalyst, it provides an alternative pathway for the reaction with a lower activation energy. This results in a faster reaction rate.

• Both forward and reverse reactions speeds up equally.
• The equilibrium itself remains unchanged.
• The ratio of concentrations of products and reactants stays the same.

So, for our \(\mathrm{NO}\) forming reaction, adding a catalyst will merely allow the system to reach equilibrium faster. The catalyst doesn't affect the amount of \(\mathrm{NO}\) at equilibrium. It's important to remember that catalysts improve efficiency but do not alter the balance between products and reactants.