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Dynamic equilibrium: NEET MCQs with answers

89 questions on this topic of Equilibrium (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Which of the following conditions is an essential criterion for the establishment and persistence of a state of dynamic chemical equilibrium?

  1. The system must be open to allow continuous exchange of matter with the surroundings
  2. The Gibbs energy of the system must reach a minimum value at constant temperature and pressure
  3. The forward and reverse reaction rate constants must become identical
  4. The total moles of gaseous reactants must equal the total moles of gaseous products
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Answer: (B) The Gibbs energy of the system must reach a minimum value at constant temperature and pressure

At dynamic chemical equilibrium under constant temperature and pressure, the Gibbs free energy (G) of the closed system reaches a minimum, where ΔG = 0. A closed system is required, and rate constants need not be equal, only the reaction rates must be equal.

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2. Given below are two statements: Statement I: At dynamic equilibrium, the free energy change (ΔG) of the reacting system is zero at constant temperature and pressure. Statement II: Dynamic equilibrium is characterized by a state of minimum entropy of the universe. In the light of the above statements, choose the most appropriate answer from the options given below:

  1. Both Statement I and Statement II are correct.
  2. Statement I is correct but Statement II is incorrect.
  3. Statement I is incorrect but Statement II is correct.
  4. Both Statement I and Statement II are incorrect.
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Answer: (B) Statement I is correct but Statement II is incorrect.

At dynamic chemical equilibrium under constant temperature and pressure, the Gibbs free energy change ΔG = 0, which corresponds to the minimum free energy of the system. However, the total entropy of the universe is at a maximum (ΔS_total = 0 for changes at equilibrium, not minimum), making Statement II incorrect.

hard

3. For a general reversible reaction 2X(g) ⇌ Y(g), the system reaches dynamic equilibrium at a constant temperature T. If the rate of forward reaction is given by r_f = k_f[X]² and backward reaction by r_b = k_b[Y], which statement is TRUE regarding this state?

  1. Doubling the volume of the container will momentarily make r_b > r_f before a new equilibrium is established
  2. k_f must be equal to k_b at equilibrium
  3. The ratio r_f / r_b remains greater than 1 as long as products are present
  4. A catalyst alters the value of the ratio k_f / k_b
Show answer and explanation

Answer: (A) Doubling the volume of the container will momentarily make r_b > r_f before a new equilibrium is established

For 2X(g) ⇌ Y(g), when volume is doubled, all concentrations halve. New r_f = k_f([X]/2)² = r_f/4, while new r_b = k_b([Y]/2) = r_b/2. Since r_b decreases by a factor of 2 while r_f decreases by a factor of 4, r_b > r_f momentarily, shifting equilibrium to the left (consistent with Le Chatelier's principle).

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4. For an elementary reversible gas-phase reaction A(g) + B(g) ⇌ C(g) + D(g) in a 1.0 L closed vessel at 300 K, the forward rate constant is k_f = 4.0 × 10⁻² L mol⁻¹ s⁻¹ and the reverse rate constant is k_b = 1.0 × 10⁻² L mol⁻¹ s⁻¹. If the equilibrium concentration of C(g) and D(g) are both 0.40 mol L⁻¹, what must be the equilibrium concentration of A(g), given that [A] = [B] at equilibrium?

  1. 0.80 mol L⁻¹
  2. 0.20 mol L⁻¹
  3. 0.40 mol L⁻¹
  4. 0.10 mol L⁻¹
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Answer: (B) 0.20 mol L⁻¹

At dynamic equilibrium, forward rate = reverse rate: k_f[A][B] = k_b[C][D]. Since [A] = [B] and [C] = [D] = 0.40 mol L⁻¹: (4.0 × 10⁻²)[A]² = (1.0 × 10⁻²)(0.40)². This gives [A]² = (1.0 × 10⁻² / 4.0 × 10⁻²) × 0.16 = 0.25 × 0.16 = 0.04. Taking the square root gives [A] = 0.20 mol L⁻¹.

easy

5. Which of the following statements regarding the approach to dynamic equilibrium for the reaction 2A(g) ⇌ B(g) starting from pure A is correct?

  1. The rate of the reverse reaction starts from zero and gradually increases until it equals the forward rate
  2. The rate of the forward reaction increases until it reaches a maximum at equilibrium
  3. The concentrations of A and B must become equal at the equilibrium point
  4. The reaction stops once the concentration of B becomes constant
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Answer: (A) The rate of the reverse reaction starts from zero and gradually increases until it equals the forward rate

Starting from pure reactant A, the initial concentration of B is zero, so the reverse reaction rate starts at zero. As B forms, the reverse rate increases while the forward rate decreases, until both rates become equal at dynamic equilibrium.

hard

6. An elementary gas-phase reaction 2A(g) ⇌ B(g) reaches dynamic equilibrium in a rigid 1.0 L container at 400 K. The equilibrium concentrations are [A] = 0.50 mol L⁻¹ and [B] = 0.25 mol L⁻¹. If the rate of formation of B at equilibrium is 1.25 × 10⁻³ mol L⁻¹ s⁻¹, what is the rate of disappearance of A due to the forward reaction at that instant?

  1. 1.25 × 10⁻³ mol L⁻¹ s⁻¹
  2. 5.00 × 10⁻³ mol L⁻¹ s⁻¹
  3. 6.25 × 10⁻⁴ mol L⁻¹ s⁻¹
  4. 2.50 × 10⁻³ mol L⁻¹ s⁻¹
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Answer: (D) 2.50 × 10⁻³ mol L⁻¹ s⁻¹

For the reaction 2A ⇌ B, stoichiometry dictates that rate of reaction = -(1/2)(d[A]/dt)_f = (d[B]/dt)_f. Thus, the forward rate of disappearance of A is twice the rate of formation of B: 2 × (1.25 × 10⁻³ mol L⁻¹ s⁻¹) = 2.50 × 10⁻³ mol L⁻¹ s⁻¹.

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7. For the gaseous reaction PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), which of the following graphs correctly represents the attainment of dynamic equilibrium when starting with pure PCl₅?

  1. Both forward and backward reaction rates become zero at equilibrium
  2. The forward rate increases and backward rate decreases until both become zero
  3. Concentration of PCl₅ decreases to zero while concentrations of PCl₃ and Cl₂ increase
  4. The forward rate decreases and backward rate increases until both become equal and constant
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Answer: (D) The forward rate decreases and backward rate increases until both become equal and constant

As the reaction proceeds starting from pure reactant, [PCl₅] decreases, so the forward rate decreases. As products form, the reverse rate increases from zero until the two rates become identical and constant at equilibrium.

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8. For the elementary gas-phase reaction 2NO₂(g) ⇌ N₂O₄(g), the rate of the forward reaction is given by r_f = k_f [NO₂]² and the rate of the reverse reaction by r_r = k_r [N₂O₄]. At dynamic equilibrium:

  1. k_f [NO₂]² = k_r [N₂O₄] and both rates are non-zero
  2. k_f [NO₂]² = k_r [N₂O₄] and both rates are zero
  3. k_f / k_r = [NO₂]² / [N₂O₄]
  4. [NO₂] must equal [N₂O₄] at all temperatures
Show answer and explanation

Answer: (A) k_f [NO₂]² = k_r [N₂O₄] and both rates are non-zero

At dynamic equilibrium, the forward rate equals the reverse rate (k_f [NO₂]² = k_r [N₂O₄]), but reactions continue indefinitely at the microscopic level, meaning both rates are non-zero.

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9. For the reversible reaction A(g) ⇌ 2B(g), initially pure A is introduced into a rigid vessel at T K. Let r_f and r_b represent the rates of forward and backward reactions respectively. As the system approaches dynamic equilibrium:

  1. r_f increases from zero while r_b decreases continuously
  2. r_f remains constant throughout while r_b increases linearly
  3. Both r_f and r_b decrease exponentially until they become zero
  4. r_f decreases from an initial maximum while r_b increases from zero until r_f = r_b
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Answer: (D) r_f decreases from an initial maximum while r_b increases from zero until r_f = r_b

Initially, only reactant A is present, so the forward rate r_f is at its maximum and the backward rate r_b is zero. As A is consumed, r_f decreases; as B accumulates, r_b increases until both rates become equal at dynamic equilibrium.

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10. When a small amount of radioactive deuterium gas (D₂) is added to a sealed system at dynamic equilibrium containing H₂(g), I₂(g), and HI(g) at 700 K, which of the following species will eventually be detected in the mixture?

  1. Both HD(g) and DI(g) along with D₂(g)
  2. Only DI(g) and D₂(g)
  3. Only HD(g) and D₂(g)
  4. Only unchanged D₂(g) because equilibrium prevents further reaction
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Answer: (A) Both HD(g) and DI(g) along with D₂(g)

Because chemical equilibrium is dynamic, forward and reverse reactions continuously take place. Added D₂ participates in dynamic exchange reactions: D₂ + I₂ ⇌ 2DI, and HI + DI ⇌ HD + I₂. Hence, both DI and HD will be formed in the mixture alongside the original species.

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11. For an elementary reversible isomerisation reaction M(g) ⇌ N(g), the rate constant for the forward step is 4.8 × 10⁻³ s⁻¹ and that for the backward step is 1.2 × 10⁻³ s⁻¹. At dynamic equilibrium, what is the equilibrium concentration of N if the equilibrium concentration of M is 0.50 mol L⁻¹?

  1. 0.125 mol L⁻¹
  2. 1.50 mol L⁻¹
  3. 0.50 mol L⁻¹
  4. 2.00 mol L⁻¹
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Answer: (D) 2.00 mol L⁻¹

At dynamic equilibrium, rate of forward reaction equals rate of reverse reaction: k_f [M] = k_b [N]. Therefore, [N] = (k_f / k_b) × [M] = (4.8 × 10⁻³ / 1.2 × 10⁻³) × 0.50 = 4 × 0.50 = 2.00 mol L⁻¹.

hard

12. Consider the dynamic equilibrium: N₂O₄(g) ⇌ 2NO₂(g). When the system is perturbed by a small temperature change, a new dynamic equilibrium is established. Which parameter is identical at both equilibrium states?

  1. The equilibrium constant K_c
  2. The equilibrium mole fraction of NO₂
  3. The net reaction rate (r_forward - r_backward)
  4. The total pressure inside the container at fixed volume
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Answer: (C) The net reaction rate (r_forward - r_backward)

At any true dynamic equilibrium, the forward and reverse rates are exactly equal, meaning the net reaction rate (r_forward - r_backward) is always zero. Since K_c, composition, and pressure depend on temperature, they change when temperature changes, but net rate remains zero.

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