Introduction to Equilibrium
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AP Chemistry › Introduction to Equilibrium
In a sealed container, the reversible reaction $\text{PCl}_5(g)\rightleftharpoons \text{PCl}_3(g)+\text{Cl}_2(g)$ reaches a point where the measured amounts of each gas do not change with time. Which statement best describes the system at equilibrium?
The concentrations of $\text{PCl}_5$, $\text{PCl}_3$, and $\text{Cl}_2$ are all equal.
The reverse reaction stops, so only decomposition continues slowly.
The reaction stops because particles no longer collide effectively.
The forward reaction has stopped because all reactant has been used up.
The forward and reverse reactions continue, and their rates are equal.
Explanation
This question tests understanding of equilibrium in decomposition reactions. At equilibrium, PCl₅ molecules continue to decompose into PCl₃ and Cl₂, while PCl₃ and Cl₂ simultaneously recombine to form PCl₅, with both the forward and reverse reactions proceeding at equal rates. This dynamic balance results in constant concentrations of all species over time. Choice A incorrectly assumes equilibrium requires equal concentrations of all species, which confuses the equilibrium condition with a specific equilibrium position. To recognize equilibrium, focus on the equality of reaction rates in both directions, not on concentration relationships or the misconception that reactions stop.
A sealed vessel contains the reversible reaction $\text{PCl}_5(g) \rightleftharpoons \text{PCl}_3(g) + \text{Cl}_2(g)$ at constant temperature. After some time, the amounts of each gas remain constant. Which statement correctly describes the system at equilibrium?
The concentrations of $\text{PCl}_5$, $\text{PCl}_3$, and $\text{Cl}_2$ are equal.
The forward and reverse reaction rates are equal, so there is no net change.
Only $\text{PCl}_5$ decomposes at equilibrium; recombination does not occur.
The reaction has stopped completely, so molecules no longer interconvert.
The forward reaction rate is greater than the reverse rate, but amounts stay constant.
Explanation
This question assesses comprehension of equilibrium in decomposition reactions. In PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), the constant amounts of gases indicate equilibrium has been reached. This happens because the forward decomposition and reverse recombination rates are equal, resulting in no net change. Dynamic equilibrium implies ongoing molecular interconversions, balanced to keep compositions stable. Choice C misleads by stating concentrations must be equal, reflecting the misconception that equilibrium requires equal quantities rather than equal rates. To tackle these questions, emphasize the equality of rates over equality of concentrations.
The reversible reaction $2\text{NO}(g)+\text{O}_2(g)\rightleftharpoons 2\text{NO}_2(g)$ occurs in a sealed container. After some time, the concentrations of $\text{NO}$, $\text{O}_2$, and $\text{NO}_2$ remain constant. Which statement correctly describes the rates at equilibrium?
The rates alternate between forward and reverse, causing no net change overall.
The forward reaction rate equals the reverse reaction rate.
The reaction is at equilibrium only if all species have the same concentration.
The forward reaction rate is zero, but the reverse reaction rate is nonzero.
The reverse reaction rate is zero, but the forward reaction rate is nonzero.
Explanation
This question tests understanding of reaction rates at equilibrium. At equilibrium, NO and O₂ molecules continue to react to form NO₂, while NO₂ molecules simultaneously decompose back to NO and O₂, with the forward reaction rate exactly equal to the reverse reaction rate. This equality of rates maintains constant concentrations of all species. Choice B incorrectly suggests only the reverse reaction continues, which would cause concentrations to change over time. When analyzing equilibrium, remember that constant concentrations result from equal rates of opposing processes, not from one reaction stopping while the other continues.
In a closed container at constant temperature, the reaction $\mathrm{NH_3(g) \rightleftharpoons NH_4^+(aq) + OH^-(aq)}$ is established in water. After some time, the measured concentrations remain constant. Which statement is true at equilibrium?
The forward and reverse reactions proceed at the same rate.
The concentrations of $\mathrm{NH_3}$ and $\mathrm{NH_4^+}$ must be equal.
The reverse reaction rate is zero because products cannot react.
The reaction stops, so no $\mathrm{NH_4^+}$ forms anymore.
All $\mathrm{NH_3}$ has reacted, leaving only ions in solution.
Explanation
This question tests understanding of equilibrium in aqueous base reactions. At equilibrium, NH₃ continues to react with water to form NH₄⁺ and OH⁻ at the same rate that these ions recombine to form NH₃ and water. This dynamic process maintains constant concentrations of all species in solution. The misconception that the reaction stops (choice A) incorrectly interprets constant concentrations as meaning no molecular activity occurs. Remember that equilibrium is characterized by equal forward and reverse reaction rates, resulting in no net change in concentrations over time.
A student sets up the reversible reaction $\mathrm{Fe^{3+}(aq) + SCN^-(aq) \rightleftharpoons FeSCN^{2+}(aq)}$ in a closed beaker at constant temperature. After a period of time, the color intensity stays constant, indicating constant concentrations. What does equilibrium imply about the reaction rates?
The forward rate is greater than the reverse rate, but both are nonzero.
The reverse rate is greater than the forward rate, but both are nonzero.
The concentrations of all aqueous species are equal at equilibrium.
Both reaction rates are zero because no further reaction can occur.
The forward rate equals the reverse rate, so concentrations stay constant.
Explanation
This question tests understanding of equilibrium through observable properties. The constant color intensity indicates constant concentration of FeSCN²⁺ (the colored complex), which occurs when the forward rate of complex formation equals the reverse rate of complex dissociation. Both reactions continue simultaneously, maintaining the steady-state concentrations. The misconception that both rates are zero (choice C) incorrectly assumes that constant macroscopic properties mean no reactions occur at the molecular level. When analyzing equilibrium, remember that unchanging observable properties result from balanced reaction rates, not from reaction cessation.
The reversible decomposition reaction $2\text{NOCl}(g) \rightleftharpoons 2\text{NO}(g)+\text{Cl}_2(g)$ is carried out in a sealed container at constant temperature. After some time, the measured concentrations remain constant.
Which statement correctly describes the system at equilibrium?
The forward and reverse reaction rates are equal, so composition stays constant.
The concentrations are constant because the reactant and product concentrations are equal.
Only the reverse reaction occurs, preventing any further concentration change.
The forward reaction rate is greater than the reverse rate, but the net change is zero.
The concentrations are constant because the reaction has completely stopped.
Explanation
This question assesses the description of equilibrium in terms of rates and concentrations for decomposition reactions. For 2NOCl(g) ⇌ 2NO(g) + Cl₂(g), constant concentrations at equilibrium mean the forward and reverse reaction rates are equal, preventing net changes in composition. This indicates ongoing decomposition and reformation at matching rates. The core principle is that equilibrium is dynamic, with continuous molecular activity. Choice A is a distractor suggesting the reaction has completely stopped, which stems from the misconception that constant concentrations imply no reaction, whereas reactions proceed but balance out. A strategy for these problems is to link macroscopic observations like constant concentrations to microscopic rate equality, aiding in distinguishing true equilibrium characteristics.
A student studies the reversible reaction $\text{PCl}_5(g) \rightleftharpoons \text{PCl}_3(g)+\text{Cl}_2(g)$ in a closed container at constant temperature. After sufficient time, the measured concentrations of all gases remain constant.
Which statement best describes the meaning of equilibrium for this system?
Only the decomposition of $\text{PCl}_5$ occurs once concentrations become constant.
The forward and reverse reactions still occur, and their rates are equal.
The equilibrium mixture must contain equal concentrations of all three gases.
The system has reached equilibrium, so the reaction has stopped permanently.
The reverse reaction rate is zero because no more $\text{PCl}_5$ can form.
Explanation
This question evaluates the meaning of equilibrium in a dissociation reaction context. For PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), constant gas concentrations mean the forward and reverse reactions occur at equal rates, with decomposition and recombination balancing out. This dynamic state keeps the system stable over time. The principle emphasizes that equilibrium involves persistent reactions in both directions, not a halt. Choice B distracts by implying equal concentrations of all gases, based on the misconception that equilibrium equates to equal amounts rather than just rate equality, as actual ratios depend on the equilibrium constant. A transferable approach is to interpret equilibrium as rate balance, using this to evaluate statements about reaction progress and system states.
A reversible reaction $\text{Br}_2(l) \rightleftharpoons \text{Br}_2(g)$ occurs in a closed container at constant temperature. Over time, the amount of liquid bromine and bromine vapor each becomes constant.
Which statement best describes equilibrium for this system?
Molecules stop moving between phases once equilibrium is reached.
Evaporation and condensation both occur, and their rates are equal.
The amounts of liquid and gas must be equal at equilibrium.
Only condensation continues because the system has reached equilibrium.
All bromine becomes gas, so there is no longer any liquid present.
Explanation
This question tests the concept of phase equilibrium in a closed system. For Br₂(l) ⇌ Br₂(g), when the amounts of liquid and vapor become constant, evaporation and condensation are both occurring, with their rates equal, maintaining a balance. This means liquid molecules continue to enter the gas phase while gas molecules return to liquid at the same rate. The principle is dynamic equilibrium, where phase changes persist but net transfer is zero. Choice C is a common distractor, suggesting equal amounts of liquid and gas, which comes from the misconception that equilibrium requires equal quantities in each phase, whereas it actually depends on vapor pressure and temperature, not equal masses. A transferable strategy is to recognize that equilibrium in any system, chemical or physical, involves equal rates of opposing processes, ensuring constant macroscopic properties.
A reversible reaction $\text{CO}(g)+\text{Cl}_2(g) \rightleftharpoons \text{COCl}_2(g)$ is allowed to proceed in a sealed container at constant temperature. After a period of time, the concentrations no longer change.
Which description best matches the system at equilibrium?
The reaction has stopped because the system has reached maximum product.
The concentrations of all species are equal because the rates are equal.
The forward reaction continues, but the reverse reaction has stopped.
No molecular-level changes occur because particles stop moving at equilibrium.
Both reactions occur, and the forward and reverse rates are equal.
Explanation
This question assesses the understanding of what occurs at equilibrium in a reversible reaction system. In the reaction CO(g) + Cl₂(g) ⇌ COCl₂(g), once concentrations no longer change in a sealed container, both the forward and reverse reactions are proceeding, but their rates are equal, leading to no net concentration changes. This dynamic equilibrium means molecules are constantly reacting in both directions at the same rate. The principle highlights that equilibrium is not a cessation of reaction but a balance of opposing processes. Choice C is a common distractor, suggesting all species have equal concentrations because rates are equal, which arises from the misconception that rate equality implies concentration equality, whereas actual concentrations are determined by the equilibrium constant. When analyzing equilibrium questions, always recall that constant concentrations result from equal forward and reverse rates, not from reactions stopping or concentrations being identical.
A student investigates the reversible reaction $\text{Fe}^{3+}(aq)+\text{SCN}^-(aq) \rightleftharpoons \text{FeSCN}^{2+}(aq)$ in a closed beaker at constant temperature. After mixing, the solution’s composition becomes constant with time.
Which statement best describes the equilibrium state?
The reaction stops, so no ions form or break apart anymore.
The system contains only $\text{FeSCN}^{2+}$ once equilibrium is reached.
The reverse reaction rate is zero because complex formation is complete.
The forward reaction rate equals the reverse reaction rate.
The concentrations of $\text{Fe}^{3+}$ and $\text{FeSCN}^{2+}$ must be equal.
Explanation
This question examines the rate perspective of equilibrium in complex ion formation. In Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq), constant composition means the forward rate of complex formation equals the reverse rate of dissociation, so both processes continue equally. This balance keeps the concentrations stable despite ongoing reactions. The dynamic equilibrium principle underscores that ions are constantly associating and dissociating at the same rate. Choice C tempts by claiming Fe³⁺ and FeSCN²⁺ concentrations must be equal, reflecting the misconception that equilibrium demands equal reactant and product amounts, but concentrations are governed by the equilibrium constant, not equality. When solving equilibrium questions, emphasize that rate equality, not concentration equality or reaction stoppage, defines the state.