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Understanding Chemical Equilibrium

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Understanding Chemical Equilibrium

Analyse equilibrium models and data, then explain how a system responds to change. Show working where calculations are required.

A dynamic balance

Consider the reversible reaction: N₂O₄(g) ⇌ 2NO₂(g). The forward reaction is colourless N₂O₄ forming brown NO₂; the reverse reaction forms N₂O₄.

1.Explain what is happening to the forward and reverse reactions at dynamic equilibrium. State what is equal and what remains constant.

2.Select every statement that correctly describes a chemical equilibrium.

It is established in a closed system.
The forward and reverse reaction rates are equal.
Reactant and product concentrations must be equal.
The macroscopic concentrations remain constant.
Both reactions stop when equilibrium is reached.

3.At dynamic equilibrium, the numbers of N₂O₄ and NO₂ particles remain constant over time. Explain why this does not mean the reactions have stopped.

Following concentration data

A sealed vessel is used to study A(g) ⇌ B(g). The concentrations are measured at regular intervals.

Time (s)

[A]

[B]

0.80

0.10

0.50

0.40

0.30

0.60

0.30

0.60

4.Using the concentration measurements, identify the earliest recorded time at which the system appears to have reached equilibrium. Justify your choice using both concentrations.

5.For A(g) ⇌ B(g), Kc = [B]/[A]. Use the data at 40 s to calculate Kc, including the appropriate units or a note about whether it has units.

6.Two sealed vessels contain the same reaction, A(g) ⇌ B(g), at the same temperature. At equilibrium, vessel X has a greater concentration of B than A ([B] > [A]), while vessel Y has a greater concentration of A than B ([A] > [B]). Must the equilibrium constant be different in the two vessels? Explain.

Predicting a shift

Apply Le Châtelier’s principle: when an equilibrium system is disturbed, it shifts in the direction that tends to oppose the disturbance.

7.For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), extra H₂ is injected into a system already at equilibrium. Predict the direction of the shift and describe the immediate change in the forward and reverse reaction rates.

8.For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the volume of the vessel is suddenly decreased at constant temperature. Predict which way the equilibrium shifts and explain using the number of gas particles.

9.The forward reaction N₂O₄(g) ⇌ 2NO₂(g) is endothermic. Predict the shift when the temperature is increased, and state what happens to Kc.

10.A student says, “Adding a catalyst increases the amount of product at equilibrium.” Evaluate the claim and explain what a catalyst actually changes.