Questions 35-36 refer to the experiment described below. H2 gas and N2 gas were placed in a rigid ve — Equilibrium Chemistry Question

STEPS:

1. Analyze the Visual Evidence: Examine the concentration-time graph provided in the sources for the interval between $t_1$ and $t_2$. During this specific period, the lines representing the concentrations of $H_2$, $N_2$, and $NH_3$ are all flat and horizontal, indicating that the concentrations of all species are constant.
2. Identify the State of the System: In chemistry, when the concentrations of reactants and products in a reversible reaction remain constant over time, the system has reached a state of chemical equilibrium.
3. Apply the Definition of Dynamic Equilibrium: The fundamental characteristic of chemical equilibrium is that it is dynamic, not static. This means that both the forward and reverse reactions are still occurring, but they are happening at the exact same rate.
4. Relate Rates to the Graph: Because the rate of the forward reaction (forming $NH_3$) is equal to the rate of the reverse reaction (decomposing $NH_3$), there is no net change in the amount of any substance. This perfectly explains the plateau observed on the graph between $t_1$ and $t_2$.
5. Conclusion: Therefore, statement D is correct because it accurately describes the defining condition of a system at equilibrium.

WHY_OTHERS_WRONG:

• A) The concentration of $N_2$ decreased: The graph shows that while the concentration of $N_2$ decreased initially, it became constant at time $t_1$ and remained so until $t_2$.
• B) The temperature of the system decreased: There is no data in the sources to suggest a temperature change during the equilibrium period. Typically, equilibrium is established and maintained at a constant temperature unless an external stress is applied.
• C) The number of effective collisions between $H_2$ and $N_2$ was zero: Equilibrium is dynamic, meaning molecules continue to collide and react. Effective collisions are still occurring to drive the forward reaction; however, they are precisely balanced by the effective collisions driving the reverse reaction.
• E) The rate of formation of $NH_3$ molecules was equal to the rate of disappearance of $H_2$ molecules: This is incorrect due to stoichiometry. According to the balanced equation, 3 moles of $H_2$ disappear for every 2 moles of $NH_3$ formed ($3:2$ ratio); therefore, their rates of change are proportional to their coefficients, not equal to each other.