Question

For the reaction system $$4 \mathrm{NH}_{3}(g)+5 \mathrm{O}_{2}(g) \rightleftharpoons 4 \mathrm{NO}(g)+6 \mathrm{H}_{2} \mathrm{O}(g)$$ which has already reached a state of equilibrium, predict the effect that each of the following changes will have on the position of the equilibrium. Tell whether the equilibrium will shift to the right, will shift to the left, or will not be affected. a. The pressure of oxygen is increased by injecting one additional mole of oxygen into the reaction vessel. b. A desiccant (a material that absorbs water) is added to the system. c. The system is compressed and the ammonia liquefies.

Short answer

a. The equilibrium will shift to the right due to the increased pressure of oxygen. b. The equilibrium will shift to the right due to the removal of water by the desiccant. c. The equilibrium will shift to the left due to the liquefaction of ammonia, removing it from the gaseous phase.

Step-by-step solution

Scenario a: Increasing the pressure of oxygen

If we increase the pressure of oxygen by injecting one additional mole into the reaction vessel, the equilibrium will be disturbed. According to Le Chatelier's principle, the system will respond by counteracting the effect of this disturbance. In this case, the equilibrium will shift to the side that will consume the excess oxygen added. The reaction, as written, on the left side has 5 moles of \(\mathrm{O_2}\) molecules and on the right side has no \(\mathrm{O_2}\) molecules. Hence, the equilibrium will shift to the right (in the forward reaction direction) to consume the excess \(\mathrm{O_2}\) and re-establish equilibrium.

Scenario b: Addition of a desiccant

When a desiccant is added to the system, it absorbs water. So effectively, the concentration of \(\mathrm{H_2O}\) molecules in the equilibrium system decreases. According to Le Chatelier's principle, the system will respond by shifting the equilibrium in such a way that it counteracts this change. In this case, since the removal of \(\mathrm{H_2O}\) molecules disturbs the equilibrium, the system will shift the equilibrium to the side that produces more \(\mathrm{H_2O}\) to restore the original state. This means the position of the equilibrium will shift to the right (in the forward reaction direction).

Scenario c: Compression of the system and liquefaction of ammonia

In this scenario, the system is compressed, and ammonia becomes a liquid. The chemical equilibrium only takes into account the gaseous phases. When the ammonia liquefies, ammonia is removed from the equilibrium in gas phase. This will cause the equilibrium to shift to counteract this change, in accordance with Le Chatelier's principle. Since we are effectively reducing the concentration of \(\mathrm{NH_3}\) by liquefying it, the equilibrium will shift to the side that produces more \(\mathrm{NH_3}\) to restore the concentration. The position of the equilibrium will shift to the left (in the reverse reaction direction).

Key concepts

Chemical Equilibrium

Chemical equilibrium occurs in a reaction system when the rate of the forward reaction equals the rate of the reverse reaction. At this point, the concentrations of reactants and products remain constant over time, although both reactions continue to proceed.
Think of it like a balance where both sides are balanced with equal forces. This doesn't mean there are equal amounts of reactants and products, just that their formation and consumption rates are equal.

• The reactions still happen, but there's no net change in the reactants or products.
• Equilibrium can be achieved from either direction, starting with reactants or products.
• The system reaches a dynamic equilibrium that depends on the reaction constants and conditions.

Understanding this concept is crucial as it helps in predicting how a reaction will respond to changes in conditions.

Reaction Systems

A reaction system involves multiple chemical species undergoing a reaction, which can be influenced by various factors. This concept is closely linked with how we perceive changes that can occur in different parts of a system at equilibrium.
In the reaction system \[4 \mathrm{NH}_{3}(g) + 5 \mathrm{O}_{2}(g) \rightleftharpoons 4 \mathrm{NO}(g) + 6 \mathrm{H}_{2} \mathrm{O}(g)\]
This is an example of a gaseous reaction system where the reactants and products exist in the gaseous state. Each component in this system can influence the equilibrium position when altered:

• Introducing or removing reactants or products.
• Changing physical conditions like pressure or temperature.

By understanding the dynamics of this system, you can predict how changes will affect it and how to achieve the desired product formation.

Pressure Effects

Pressure effects play a significant role in the behavior of gaseous reaction systems. According to Le Chatelier's principle, if a dynamic equilibrium is disturbed by changing the conditions, the system responds to nullify the effect of the disturbance and restore a new equilibrium state.
For the reaction we are considering, increasing the pressure by adding a mole of \(\mathrm{O}_{2}\) leads to a shift in equilibrium. The equilibrium will move towards the side that consumes the added substance to minimize the change. Therefore, it shifts to the right since additional \(\mathrm{O}_{2}\) needs to be consumed.

• Adding pressure favors the side of the reaction with fewer gas molecules.
• Removing gas would shift the equilibrium to produce more gas to counterbalance.

These pressure effects are notably influential in reactions where gases are involved, significantly affecting their yields.

Concentration Changes

Concentration changes are one of the primary factors that affect a chemical equilibrium. When the concentration of a reactant or product in a reaction system is changed, the equilibrium will shift in a direction that opposes the change, as per Le Chatelier's principle.
For instance, if water \(\mathrm{H_2O}\) is removed by a desiccant in our example reaction, it will cause the equilibrium to shift right to produce more water. The system compensates for the reduced concentration, balancing the reaction again:

• Increasing reactant concentration pushes the equilibrium towards products.
• Decreasing product concentration also pushes the equilibrium towards products to replace lost quantities.

This shift helps in restoring the balance of chemical species, crucial in industrial processes where maximum yield is desired.

Phase Changes

Phase changes can significantly impact chemical equilibrium, particularly when a component of the reaction changes its state. This is because equilibrium calculations typically focus only on the gaseous components.
In our given reaction, compressing the system causes \(\mathrm{NH}_3\) to liquefy, effectively removing it from the gaseous equilibrium concerns. This change leads to a shift in equilibrium towards the side that replenishes the lost \(\mathrm{NH}_3\). Therefore, the equilibrium shifts to the left, promoting the reverse reaction:

• Phase changes, like liquefaction or vaporization, alter the pressure and concentration of reactants/products.
• Reducing gas phase concentration by liquefaction shifts the equilibrium to refill the depleted species.

Understanding these influences helps chemists in manipulating conditions to achieve specific reactions and outcomes.