Question

Predict whether each of the following pressure changes would favor the forward or reverse reaction. $$2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \rightleftarrows 2 \mathrm{NO}_{2}(g)$$ a. increased pressure b. decreased pressure

Short answer

Increased pressure favors the forward reaction, whereas decreased pressure favors the reverse reaction.

Step-by-step solution

- Understand the Reaction

The given reaction is: \[2 \, \mathrm{NO}(g) + \mathrm{O}_{2}(g) \rightleftarrows 2 \mathrm{NO}_{2}(g)\] This reaction involves both reactants and products in the gas phase.

- Apply Le Chatelier's Principle

Le Chatelier's Principle states that a system at equilibrium will adjust to counteract any changes imposed on it. For changes in pressure, the system will shift to favor the side of the reaction with fewer gas molecules.

- Count Gas Molecules

In the reaction, there are 3 gas molecules on the reactant side \[(2 \ \mathrm{NO} + 1 \ \mathrm{O}_{2})\] and 2 gas molecules on the product side \[(2 \ \mathrm{NO}_{2})\].

- Analyze Increase in Pressure

An increase in pressure will favor the side with fewer gas molecules because the system will shift to reduce pressure. Therefore, the forward reaction is favored because the product side has fewer gas molecules (2 molecules).

- Analyze Decrease in Pressure

A decrease in pressure will favor the side with more gas molecules because the system will shift to increase pressure. Thus, the reverse reaction is favored because the reactant side has more gas molecules (3 molecules).

Key concepts

equilibrium reactions

When we talk about equilibrium reactions, we refer to reactions that can move in both directions - forward and reverse. At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction.
This balance means the concentrations of reactants and products remain constant over time.
However, equilibrium does not mean that the reactants and products are in equal amounts. Different factors can change this balance.
One of these factors is pressure, especially in reactions involving gases.
Le Chatelier’s Principle helps us understand how the system responds to these changes.

pressure changes in gases

Pressure changes can have significant effects on equilibrium reactions involving gases.
Le Chatelier’s Principle helps us predict how a system at equilibrium will respond to a change in pressure.
When the pressure of a system increases, it will shift in the direction that has fewer gas molecules.
This shift helps decrease the pressure.
For example, in the reaction \[2 \, \mathrm{NO}(g) + \mathrm{O}_2(g) \rightleftarrows 2 \, \mathrm{NO}_2(g)\] there are fewer gas molecules on the products side (2 compared to 3 on the reactants side).
So, an increase in pressure favors the formation of \(\mathrm{NO}_2\).
On the other hand, reducing the pressure causes the equilibrium to shift towards the side with more gas molecules.
This results in increased amounts of \(\mathrm{NO}\) and \(\mathrm{O}_2\).

molecular gas count in reactions

Counting the number of gas molecules on each side of a reaction helps predict the direction of the equilibrium shift due to pressure changes.
In the given reaction - \[2 \, \mathrm{NO}(g) + \mathrm{O}_2(g) \rightleftarrows 2 \, \mathrm{NO}_2(g)\] the reactants side has 3 gas molecules \( (2 \, \mathrm{NO} + 1 \, \mathrm{O}_2)\), and the products side has 2 gas molecules \( (2 \, \mathrm{NO}_2)\).
This count plays a crucial role when we apply Le Chatelier’s Principle.
If the pressure increases, the system will favor the side with fewer gas molecules to lower the pressure.
If the pressure decreases, the system favors the side with more gas molecules to increase the pressure.
So, for increases in pressure, the products are favored, and for decreases in pressure, the reactants are favored.