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Difficulty: HardReversible Reactions and Dynamic Equilibrium

Consider the gaseous reversible reaction 2NO2(g)⇌N2O4(g)2\text{NO}_2(g) \rightleftharpoons \text{N}_2\text{O}_4(g) taking place in a closed container at a fixed temperature. Once the system reaches dynamic equilibrium, which of the following statements correctly describes the microscopic and macroscopic behavior of the reaction?

  1. The rate of formation of N2O4\text{N}_2\text{O}_4 equals its rate of decomposition, keeping all macroscopic properties such as pressure and intensity of color constant.Answer
  2. B
    Introducing a positive catalyst increases the net yield of N2O4\text{N}_2\text{O}_4 at equilibrium by accelerating only the forward reaction rate.
  3. C
    The concentrations of NO2\text{NO}_2 and N2O4\text{N}_2\text{O}_4 must become equal to one another for the system to achieve dynamic equilibrium.
  4. D
    All chemical reactions cease completely at equilibrium, causing the total mass of the system to stabilize permanently.

Answer

The rate of formation of N2O4\text{N}_2\text{O}_4 equals its rate of decomposition, keeping all macroscopic properties such as pressure and intensity of color constant.
In a closed system at dynamic equilibrium, the forward reaction rate equals the reverse reaction rate. As a result of this balance at the microscopic level, no net change occurs in macroscopic properties such as concentration, total gas pressure, or intensity of color.

Step-by-Step Solution

1
Analyze the microscopic nature of dynamic equilibrium in reversible chemical reactions.
Dynamic equilibrium implies that both forward (2NO2→N2O42\text{NO}_2 \rightarrow \text{N}_2\text{O}_4) and reverse (N2O4→2NO2\text{N}_2\text{O}_4 \rightarrow 2\text{NO}_2) reactions occur concurrently at identical rates.
At the molecular level, reactants continue forming products while products revert to reactants at the same speed.
2
Connect microscopic rate equality to observable macroscopic properties.
Because the forward and reverse rates are equal, the net concentrations of species, total system pressure, and color intensity stay strictly constant.
When rate of formation equals rate of consumption, observable macroscopic properties do not change with time.
3
Evaluate common misconceptions regarding equilibrium state features.
Equilibrium does not require equal concentrations, nor does chemical activity stop, nor does a catalyst alter the equilibrium position.
Concentration equality is not required by KcK_c, chemical activity continues dynamically, and catalysts alter activation energy for both directions equally.

Key Concept

Microscopic vs. Macroscopic Characteristics of Dynamic Equilibrium
Estimated Time:1m 30s
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Topics

Reversible Reactions and Dynamic Equilibrium