Representations of Equilibrium
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AP Chemistry › Representations of Equilibrium
A closed system contains the reversible gas-phase reaction $2\text{NO}(g)+\text{Cl}_2(g) \rightleftharpoons 2\text{NOCl}(g)$. A student measures concentrations early in the run, before any concentrations become constant. Based only on the table, what is the direction of the net reaction progress during the interval shown?
Net reaction is proceeding in the forward direction (toward $\text{NOCl}$).
Net reaction is proceeding in the reverse direction (toward $\text{NO}$ and $\text{Cl}_2$).
No net reaction is occurring because the changes in concentration are not identical at each time step.
No net reaction is occurring because $[\text{NOCl}]$ is always the largest concentration.
The net direction cannot be inferred because $[\text{NO}]$ and $[\text{Cl}_2]$ are both decreasing.
Explanation
This question tests analyzing table concentrations to determine net reaction direction in a synthesis reaction. The table indicates [NO] and [Cl₂] decreasing, showing reactant depletion, and [NOCl] increasing, denoting product buildup. The changes reflect the 2:1:2 stoichiometry, with net forward progress dominating. As concentrations are not yet constant, the net shift is toward products. Choice A fails because it misreads the table, interpreting reactant decreases as reverse when they support forward. For these representations, use the pattern of decreasing reactants and increasing products to identify forward net progress, ignoring unrelated factors like relative magnitudes.
A sealed vessel contains the reversible reaction $\text{PCl}_5(g) \rightleftharpoons \text{PCl}_3(g)+\text{Cl}_2(g)$. A student records concentrations shortly after the reaction begins (before any concentration becomes constant). Using only the table, what is the direction of the net reaction progress over the interval shown?
The net direction cannot be inferred because the reaction is reversible.
No net reaction is occurring because $[\text{Cl}_2]$ is increasing while $[\text{PCl}_3]$ is also increasing.
No net reaction is occurring because $[\text{PCl}_3]$ and $[\text{Cl}_2]$ are equal at each time.
Net reaction is proceeding in the forward direction (toward $\text{PCl}_3$ and $\text{Cl}_2$).
Net reaction is proceeding in the reverse direction (toward $\text{PCl}_5$).
Explanation
This question tests the interpretation of concentration data in a table to determine net reaction progress in a reversible decomposition reaction. The table reveals [PCl₅] increasing over time, suggesting it is being formed, while [PCl₃] and [Cl₂] are decreasing, indicating they are being consumed. These changes match the reverse reaction, where products recombine to form the reactant, consistent with the 1:1:1 stoichiometry. As no concentrations are constant yet, the net progress is toward the reactant side. Choice C fails because it misreads the table by incorrectly identifying the direction; the increases and decreases actually support reverse, not forward, progress. For such analyses, prioritize the direction of change in each species' concentration over time, not their relative magnitudes, to accurately determine net progress.
A sealed container has the reversible reaction $\text{Br}_2(g) \rightleftharpoons 2\text{Br}(g)$. Concentrations are recorded during the first few seconds after heating begins; the data shown are from a period before any concentration becomes constant. Based only on the table, what is the direction of the net reaction progress during the interval shown?
No net reaction is occurring because $[\text{Br}]$ is always less than $[\text{Br}_2]$.
The net direction cannot be inferred because the reaction produces twice as much $\text{Br}$ as $\text{Br}_2$ consumed.
No net reaction is occurring because both species are present at all times.
Net reaction is proceeding in the reverse direction (toward $\text{Br}_2$).
Net reaction is proceeding in the forward direction (toward $\text{Br}$).
Explanation
This question examines using table data to determine net progress in a dissociation reaction. The table displays [Br₂] decreasing and [Br] increasing, consistent with forward dissociation where one Br₂ yields two Br. The net increase in [Br] reflects stoichiometric production exceeding consumption. Before constants, net forward progress occurs. Choice A fails due to misreading the table, confusing product increase with reverse direction. When reviewing such tables, emphasize the net accumulation of products or reactants through trends, not comparisons of absolute concentrations, for accurate direction assessment.
A sealed tube contains the reversible reaction $\mathrm{PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)}$. Concentrations are recorded at early times before equilibrium is reached. What is the direction of net reaction progress during the interval shown?
No net reaction occurs because $[\mathrm{PCl_3}]$ and $[\mathrm{Cl_2}]$ are equal at all times.
No net reaction occurs because $[\mathrm{PCl_5}]$ is not zero.
Net reaction proceeds in the forward direction (toward $\mathrm{PCl_3}$ and $\mathrm{Cl_2}$).
Net reaction proceeds in the reverse direction (toward $\mathrm{PCl_5}$).
The net direction cannot be determined because two products are formed.
Explanation
This question tests the skill of determining reaction direction from concentration trends in decomposition equilibria. The data shows [PCl₅] decreasing while [PCl₃] and [Cl₂] are increasing, indicating that PCl₅ is decomposing into PCl₃ and Cl₂. Since the forward reaction converts PCl₅ into products, the net reaction proceeds in the forward direction. Choice C incorrectly assumes equal product concentrations mean no net reaction, but the key is whether concentrations are changing, not their relative values. The strategy is to identify the decomposition pattern: one species decreasing while multiple species increase indicates forward decomposition.