Equilibrium Representation & Calculations (College Board AP® Chemistry): Exam Questions

The following reaction takes place in a closed container at a constant temperature:

A (g) + B (g) ⇌ C (g)

The equilibrium constant is K_c = 2.50 at this temperature. An equilibrium mixture in a 1.00 L container is found to contain 0.40 mol of A, 0.10 mol of B, and 0.20 mol of C.

Write the expression for the equilibrium constant, K_c​, for the reaction.

Calculate the value of the reaction quotient, Q_c​.

Predict whether the system is at equilibrium. If the system is not at equilibrium, describe what will happen to the concentration of C (g) as the system moves toward equilibrium. Justify your answer in terms of Q_c​ and K_c​.

A mixture of gases in a sealed container is represented by the particulate diagram below. Each particle represents 1 molecule. Assume the reaction below is at equilibrium:

2D (g) ⇌ E (g)

Based on the particulate diagram, determine the ratio of molecules of D to E at equilibrium.

Write the expression for the equilibrium constant, K_c​, for the reaction.

The volume of the container is suddenly halved. Predict how the number of molecules of D at equilibrium will change. Justify your prediction in terms of the shift in equilibrium position.

At 700 K, carbon monoxide gas reacts reversibly with steam in a closed container according to the following balanced equation:

CO (g) + H₂O (g) ⇌ CO₂ (g) + H₂ (g)

An experiment is conducted where 0.400 mol of each species is placed in a 2.00 L container. The system reaches equilibrium at constant temperature.

Write the expression for the equilibrium constant, K_c​, for this reaction.

Determine the initial concentrations (in mol L^-1) of all four species in the container.

At equilibrium, the concentration of H₂O (g) is found to be 0.160 mol L⁻¹. Calculate the equilibrium concentrations of the other three species.

Using your answers from parts (a) and (c), calculate the value of the equilibrium constant, K_c​, for the reaction.

A second reaction mixture is prepared at the same temperature with the following concentrations:

Determine whether the system will shift to the left, right, or remain unchanged. Justify your answer using a calculation.

The reaction vessel is opened briefly, allowing some H₂ gas to escape before being resealed. Explain how this change affects the concentrations of all four species after the system re-establishes equilibrium.

Chlorine gas reacts reversibly with iodine monochloride gas in a sealed container at constant pressure:

Cl₂ (g) + ICl (g) ⇌ ICl₃ (g)

This reaction is studied at two different temperatures. The equilibrium concentrations of each species are recorded for each trial in identical 2.00 L containers:

Write the expression for the equilibrium constant, K_c​, for this reaction.

Calculate the value of K_c​ at 400 K.

Calculate the value of K_c​ at 500 K.

Determine whether the forward reaction is endothermic or exothermic. Justify your answer using the values of K_c​.

Each of the following particle diagrams shows a 2.00 L container at equilibrium.

Which diagram best represents the system at 400 K? Justify your answer based on the value of K_c​.

A new mixture is prepared at 400 K with the following concentrations:

[Cl₂] = 0.10 M, [ICl] = 0.10 M, [ICl₃] = 1.20 M

Determine whether the system will shift toward the reactants, the products, or remain unchanged. Justify your answer with a calculation.