Long free-response question

The PCl₅ decomposition equilibrium

Units 2 and 7
10 points
About 23 minutes

You can use a calculator on this question, just like on exam day.

A multipart question, often built around a lab setup or a data table, that mixes several topics. You calculate, explain and justify your answers across many lettered parts. On the exam: 3 questions (Questions 1–3), part of the 1-hour-45-minute free-response section; calculator allowed.

The question and its sources

Phosphorus pentachloride decomposes when heated: PCl₅(g) ⇌ PCl₃(g) + Cl₂(g). The forward reaction is endothermic.

Experiment 1

A 0.200 mol sample of pure PCl₅(g) is placed in an evacuated, rigid 2.00 L container and heated to a constant temperature T. When equilibrium is reached, the concentration of Cl₂(g) is 0.0450 M.

Source: Hypothetical data

Suggested time: 23 minutes

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Part (a)

1 point

Write the expression for the equilibrium constant, Kc, for the reaction.

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Part (b)

2 points

Calculate the value of Kc at temperature T.

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Part (c)

2 points

In Experiment 2, a different mixture at temperature T has [PCl₅] = 0.100 M, [PCl₃] = 0.0500 M, and [Cl₂] = 0.0500 M. Calculate the reaction quotient, Q, and predict the direction in which the reaction will proceed to reach equilibrium. Justify your prediction.

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Part (d)

1 point

The equilibrium mixture from Experiment 1 is quickly compressed from 2.00 L to 1.00 L at constant temperature. Predict the direction in which the net reaction will proceed. Justify your answer by comparing Q with Kc immediately after the compression.

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Part (e)

1 point

The temperature of the equilibrium mixture is increased. Will the value of Kc increase, decrease, or remain the same? Justify your answer.

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Part (f)

1 point

Describe a complete Lewis electron-dot diagram for the PCl₃ molecule: state each bond, every lone pair, and the formal charge on each atom.

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Part (g)

1 point

Identify the molecular geometry of PCl₃ and state whether the molecule is polar or nonpolar. Justify your answer.

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Part (h)

1 point

A student draws a particle-level diagram of a mixture in a 1.00 L container at temperature T, letting one drawn molecule stand for 0.0100 mol. The drawing contains 6 PCl₅ molecules, 4 PCl₃ molecules, and 4 Cl₂ molecules. Is this mixture at equilibrium? If not, describe how the numbers of each type of molecule in the diagram would change as the system reaches equilibrium. Justify your answer.

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