AP Chemistry Quiz: Introduction To Equilibrium

What this quiz covers

This quiz focuses on Introduction To Equilibrium, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Chemistry.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. No ionization occurs, so the concentrations of $H_3O^+$ and $OH^-$ are zero.
2. Water molecules continuously form ions, and ions continuously recombine to form water molecules. (correct answer)
3. The reaction proceeds only in the forward direction until the pH reaches 7.
4. The concentration of water molecules is equal to the concentration of hydronium ions.

Explanation: The correct answer is B. This statement perfectly captures the dynamic nature of the autoionization equilibrium. Even in pure water, there is constant activity at the molecular level, with forward and reverse reactions occurring at equal rates. A is incorrect, as ions are present. C is incorrect as the reverse reaction is crucial for establishing equilibrium. D is incorrect, as the concentration of water is vastly greater than the very small concentrations of the ions.

Question 2

1. The total mass of the gaseous mixture in the vessel.
2. The total pressure of the gaseous mixture in the vessel. (correct answer)
3. The individual partial pressures of each gas species in the vessel.
4. The rate of the forward reaction relative to the reverse reaction.

Explanation: The correct answer is B. At equilibrium, the concentrations (and therefore partial pressures) of all species become constant. Since the total pressure is the sum of the partial pressures, it also remains constant. A is incorrect because mass is always conserved in a closed system, whether at equilibrium or not. C is incorrect because individual partial pressures are constant at equilibrium, but this describes the equilibrium condition itself rather than a consequence. D is incorrect because at equilibrium the rates are equal, not that their relative values remain constant.

Question 3

1. The reaction has gone to completion because the product concentration is now constant.
2. The limiting reactant has been completely consumed, stopping the reaction.
3. The system has reached equilibrium, and the rates of the forward and reverse reactions are now equal. (correct answer)
4. The reverse reaction rate has become greater than the forward reaction rate, stabilizing the concentrations.

Explanation: The correct answer is C. Constant concentrations are the key macroscopic evidence that a reversible system has reached equilibrium. The reason concentrations are constant is that the forward and reverse rates have become equal, leading to no net change. A and B are possible explanations for an irreversible reaction, but for a general chemical process, equilibrium is the most appropriate conclusion for constant concentrations. D is incorrect because if the reverse rate were greater, the product concentration would decrease, not remain constant.

Question 4

1. The concentration of $Pb^{2+}$ ions in the solution is zero because the solid is present.
2. The processes of dissolving and precipitation have both completely stopped.
3. The rate of dissolution of $PbCl_2(s)$ is equal to the rate of precipitation of $Pb^{2+}(aq)$ and $Cl^-(aq)$. (correct answer)
4. The concentration of $Pb^{2+}(aq)$ is exactly half the concentration of $Cl^-(aq)$ due to the stoichiometry of the solid.

Explanation: The correct answer is C. A saturated solution in contact with excess solid represents a dynamic equilibrium. The solid dissolves at a certain rate, and the ions in solution precipitate at a certain rate. At equilibrium, these two rates are equal. A is incorrect because the ions exist in the saturated solution. B is incorrect because equilibrium is dynamic, not static. D is incorrect because while the stoichiometry of dissolution produces two chloride ions for every one lead ion, the statement is about the equilibrium condition itself, which is defined by rates, not simply concentration ratios.

Question 5

1. The concentrations of $\text{PCl}_5$, $\text{PCl}_3$, and $\text{Cl}_2$ are all equal.
2. The forward and reverse reactions continue, and their rates are equal. (correct answer)
3. The forward reaction has stopped because all reactant has been used up.
4. The reverse reaction stops, so only decomposition continues slowly.
5. The reaction stops because particles no longer collide effectively.

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.

Question 6

1. The forward and reverse reaction rates are equal, so there is no net change. (correct answer)
2. The reaction has stopped completely, so molecules no longer interconvert.
3. The concentrations of $\text{PCl}_5$, $\text{PCl}_3$, and $\text{Cl}_2$ are equal.
4. The forward reaction rate is greater than the reverse rate, but amounts stay constant.
5. Only $\text{PCl}_5$ decomposes at equilibrium; recombination does not occur.

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.

Question 7

1. The forward reaction rate equals the reverse reaction rate. (correct answer)
2. The forward reaction rate is zero, but the reverse reaction rate is nonzero.
3. The reverse reaction rate is zero, but the forward reaction rate is nonzero.
4. The rates alternate between forward and reverse, causing no net change overall.
5. The reaction is at equilibrium only if all species have the same concentration.

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.

Question 8

1. The reaction stops, so no $\mathrm{NH_4^+}$ forms anymore.
2. The forward and reverse reactions proceed at the same rate. (correct answer)
3. The concentrations of $\mathrm{NH_3}$ and $\mathrm{NH_4^+}$ must be equal.
4. All $\mathrm{NH_3}$ has reacted, leaving only ions in solution.
5. The reverse reaction rate is zero because products cannot react.

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.

Question 9

1. The forward rate is greater than the reverse rate, but both are nonzero.
2. The forward rate equals the reverse rate, so concentrations stay constant. (correct answer)
3. Both reaction rates are zero because no further reaction can occur.
4. The concentrations of all aqueous species are equal at equilibrium.
5. The reverse rate is greater than the forward rate, but both are nonzero.

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.

Question 10

1. The concentrations are constant because the reaction has completely stopped.
2. The concentrations are constant because the reactant and product concentrations are equal.
3. The forward and reverse reaction rates are equal, so composition stays constant. (correct answer)
4. The forward reaction rate is greater than the reverse rate, but the net change is zero.
5. Only the reverse reaction occurs, preventing any further concentration change.

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.

Question 11

1. The system has reached equilibrium, so the reaction has stopped permanently.
2. The equilibrium mixture must contain equal concentrations of all three gases.
3. The forward and reverse reactions still occur, and their rates are equal. (correct answer)
4. Only the decomposition of $\text{PCl}_5$ occurs once concentrations become constant.
5. 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.

Question 12

1. Evaporation and condensation both occur, and their rates are equal. (correct answer)
2. All bromine becomes gas, so there is no longer any liquid present.
3. The amounts of liquid and gas must be equal at equilibrium.
4. Molecules stop moving between phases once equilibrium is reached.
5. Only condensation continues because the system has reached equilibrium.

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.

Question 13

1. The forward reaction continues, but the reverse reaction has stopped.
2. The reaction has stopped because the system has reached maximum product.
3. The concentrations of all species are equal because the rates are equal.
4. Both reactions occur, and the forward and reverse rates are equal. (correct answer)
5. No molecular-level changes occur because particles stop moving at equilibrium.

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.

Question 14

1. The forward reaction rate equals the reverse reaction rate. (correct answer)
2. The reaction stops, so no ions form or break apart anymore.
3. The concentrations of $\text{Fe}^{3+}$ and $\text{FeSCN}^{2+}$ must be equal.
4. The reverse reaction rate is zero because complex formation is complete.
5. The system contains only $\text{FeSCN}^{2+}$ once equilibrium is reached.

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.

Question 15

1. The reaction has stopped, so the forward and reverse rates are both zero.
2. The forward rate equals the reverse rate, so concentrations remain constant. (correct answer)
3. The concentrations of $\text{H}^+$ and $\text{CH}_3\text{COO}^-$ must equal the concentration of $\text{CH}_3\text{COOH}$.
4. The forward rate is greater than the reverse rate, but changes are too small to detect.
5. Only the reverse reaction occurs once equilibrium is reached.

Explanation: This question probes the understanding of forward and reverse processes in acid dissociation equilibrium. In the reaction CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq), constant concentrations indicate equilibrium, where the forward dissociation rate equals the reverse recombination rate, resulting in no net change. This means acetic acid molecules continue to ionize while ions reform the acid at the same rate. The principle of dynamic equilibrium applies, emphasizing ongoing reactions in both directions. Choice A is a tempting distractor, stating the reaction has stopped with zero rates, which embodies the misconception that equilibrium is static rather than a dynamic balance of rates. To tackle equilibrium problems, identify constant concentrations as a sign of rate equality, not reaction termination, and apply this to predict system behavior.

Question 16

1. The reaction has stopped, so no $\text{COCl}_2$ molecules form or break apart.
2. The forward and reverse reactions continue at equal rates, maintaining constant concentrations. (correct answer)
3. The amounts of reactants and products are equal, so the rates must be equal.
4. The reverse reaction rate is greater than the forward reaction rate, but concentrations stay constant.
5. Only molecules with the highest kinetic energy continue reacting at equilibrium.

Explanation: This question evaluates knowledge of the molecular-level behavior at chemical equilibrium for reversible reactions. In the system CO(g) + Cl₂(g) ⇌ COCl₂(g), the unchanged amounts of reactants and product signify that equilibrium has been established. At this point, the forward and reverse reactions continue but at identical rates, ensuring constant concentrations without net change. The underlying principle is that equilibrium is a dynamic state where molecular collisions and reactions persist in both directions equally. Choice C tempts by suggesting equal amounts imply equal rates, but this misinterprets equilibrium as requiring stoichiometric equality instead of rate balance. A useful strategy is to differentiate between static cessation and dynamic balance when describing equilibrium conditions.

Question 17

1. Complex formation continues while dissociation stops, keeping the color constant.
2. Formation and dissociation occur at equal rates, so concentrations remain constant. (correct answer)
3. All ions have been converted to complex, so no further reaction can occur.
4. The concentrations of $\text{Fe}^{3+}$ and $\text{FeSCN}^{2+}$ must be equal.
5. Particles stop moving between states because equilibrium eliminates collisions.

Explanation: This question examines the equilibrium dynamics in complex ion formation. For Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq), the constant color after mixing shows equilibrium, as color relates to the concentration of the red complex. At equilibrium, the rates of complex formation and dissociation are equal, maintaining steady concentrations and thus constant color. The dynamic equilibrium principle means ions continue to form and break apart the complex, but with no net change observable. Choice D is a common error, suggesting equal concentrations of Fe³⁺ and FeSCN²⁺, which confuses the equilibrium condition with stoichiometric balance instead of rate equality. For equilibrium problems involving observables like color, link stability to equal opposing rates as a key strategy.

Question 18

1. The forward and reverse reactions occur at equal rates, so there is no net change. (correct answer)
2. The concentrations of $\text{H}_2$, $\text{I}_2$, and $\text{HI}$ must be equal.
3. The collision frequency becomes zero, preventing further reaction.
4. The reaction has stopped because the reactants have been completely used up.
5. Only product molecules react at equilibrium, converting back to reactants.

Explanation: This question assesses the concept of dynamic equilibrium where reaction rates equalize in a closed system. For the reaction H₂(g) + I₂(g) ⇌ 2HI(g), the constant concentrations after time indicate that the system has reached equilibrium. This occurs because the rate of the forward reaction equals the rate of the reverse reaction, leading to no net change in the amounts of reactants and products. The principle of chemical equilibrium emphasizes that this balance is dynamic, with continuous formation and decomposition of HI molecules. Choice C is a common distractor, wrongly assuming concentrations must be equal at equilibrium, which confuses equilibrium with equal amounts rather than equal rates. When analyzing equilibrium questions, focus on rate equality rather than concentration equality to identify the correct description.

Question 19

1. The forward and reverse processes occur at equal rates, so amounts stay constant. (correct answer)
2. Equilibrium occurs only if the amounts of $\text{NH}_3$ and $\text{HCl}$ are equal.
3. The gases have been completely consumed, so no further change is possible.
4. All particle motion ceases, preventing additional reaction.
5. The solid stops forming, but the solid continues decomposing into gases.

Explanation: This question tests understanding of equilibrium in heterogeneous systems involving gases and solids. At equilibrium, NH₃ and HCl gases continue to combine to form solid NH₄Cl, while the solid simultaneously sublimes back into gaseous NH₃ and HCl, with both processes occurring at equal rates. This dynamic balance maintains constant amounts of both gaseous and solid phases. Choice B incorrectly suggests solid formation stops while decomposition continues, which would lead to decreasing solid and increasing gas amounts. Remember that equilibrium in heterogeneous systems still involves equal rates of opposing processes, regardless of the phases involved.

Question 20

1. Molecules continue reacting in both directions, but at the same rate. (correct answer)
2. All molecules stop reacting because there is no driving force remaining.
3. Only reactant molecules collide; product molecules are unreactive.
4. Concentrations are constant because the forward reaction is much faster.
5. Concentrations are constant because reactants and products are equal in amount.

Explanation: This question tests understanding of the dynamic nature of chemical equilibrium at the particle level. At equilibrium, SO₂ and NO₂ molecules continue to collide and react to form SO₃ and NO, while simultaneously SO₃ and NO collide to reform the original reactants, with both processes occurring at equal rates. This continuous, bidirectional molecular activity maintains constant concentrations macroscopically. Choice B incorrectly assumes equilibrium means all molecular reactions cease, which contradicts the dynamic nature of equilibrium. To understand equilibrium, visualize it as a busy intersection where traffic flows equally in both directions, not as a parking lot where all motion stops.