Question

Suppose the reaction system $$ 2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \rightleftharpoons 2 \mathrm{NO}_{2}(g) $$ has already reached equilibrium. Predict the effect of each of the following changes 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. Additional oxygen is injected into the system. b. \(\mathrm{NO}_{2}\) is removed from the reaction vessel. c. 1.0 mole of helium is injected into the system.

Short answer

a. The equilibrium will shift to the right due to the increased concentration of \(\mathrm{O}_{2}\). b. The equilibrium will shift to the right due to the decreased concentration of \(\mathrm{NO}_{2}\). c. There will be no shift in the position of the equilibrium since the addition of helium does not affect the concentrations of the reactants or products.

Step-by-step solution

Identify the chemical reaction at equilibrium

The given chemical reaction system is: \[ 2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \rightleftharpoons 2 \mathrm{NO}_{2}(g) \]

Apply Le Chatelier's Principle for each change

a. Additional oxygen is injected into the system: Le Chatelier's Principle states that the system will shift its equilibrium position to counteract the change. Injecting additional oxygen (\(\mathrm{O}_{2}\)) increases its concentration. To counter this, the system will shift towards the side that consumes \(\mathrm{O}_{2}\). Thus, the equilibrium will shift to the right. b. \(\mathrm{NO}_{2}\) is removed from the reaction vessel: Removing \(\mathrm{NO}_{2}\) will decrease its concentration. According to Le Chatelier's Principle, the system will shift towards the side that produces \(\mathrm{NO}_{2}\) to counteract this change. Therefore, the equilibrium will shift to the right. c. 1.0 mole of helium is injected into the system: Injecting helium into the system does not affect the concentration of the reactants or the products, as it is an inert gas that does not participate in the reaction. As a result, the equilibrium will not be affected, and there will be no shift in the position of the equilibrium.

Key concepts

Chemical Equilibrium

Chemical equilibrium in a reaction occurs when the rate of the forward reaction equals the rate of the reverse reaction. At this stage, the concentrations of the reactants and products remain constant over time. This does not mean they are equal, but they have reached a balance. Imagine a see-saw perfectly balanced; any slight nudge to one side will cause it to tilt. In much the same way, changes in concentration, temperature, or pressure can disturb this balance, but the system will always aim to restore equilibrium where possible.

Reaction Shift

When a system at equilibrium is disturbed, it responds by shifting the equilibrium position to reduce the disturbance, as described by Le Chatelier's Principle. This "reaction shift" can go in two directions:

• To the right: More products are formed to restore balance.
• To the left: The system produces more reactants to counteract the change.

For instance, in the given reaction, adding more either reactant (like oxygen ) can shift the equilibrium to the right, promoting the formation of nitrogen dioxide. On the other hand, removing product ( such as nitrogen dioxide), will also shift the equilibrium right as the system works to replace the removed substance.

Effects of Concentration Changes

The concentration of reactants and products at equilibrium is critical. If you increase the concentration of a reactant or product, the equilibrium will shift toward consuming the added substance to re-establish balance. Conversely, removing a component will shift the direction to produce more of the removed substance. In the example given:

• Adding more oxygen causes the system to shift right, producing additional nitrogen dioxide until equilibrium is restored.
• Removing nitrogen dioxide results in a shift to the right to replenish it.

Inert Gases in Reaction Systems

Inert gases, such as helium, are non-reactive and do not participate in the chemical reaction. When an inert gas is added to a system, it increases the total pressure but does not affect the partial pressures of the reactants or products involved in the reaction. Therefore, the addition of an inert gas does not change the equilibrium position. In the original exercise, injecting helium into the reaction does not lead to a shift in equilibrium because it does not affect the concentrations of the substances that are actually reacting.