Long free-response question

Making hydrogen iodide from H₂ and I₂

Units 3 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

Hydrogen gas and iodine vapor react reversibly to form hydrogen iodide: H₂(g) + I₂(g) ⇌ 2 HI(g). I₂(g) is violet; H₂(g) and HI(g) are colorless. For this question, take Kc = 49.0 at 700. K.

Experiments at 700. K

Experiment 1: 0.200 mol of H₂(g) and 0.200 mol of I₂(g) are injected into an evacuated, rigid 2.00 L flask held at 700. K. The mixture is allowed to reach equilibrium.

Experiment 2: 0.400 mol of HI(g) alone is injected into a second evacuated, rigid 2.00 L flask held at 700. K and allowed to reach equilibrium.

Source: Hypothetical data

Suggested time: 23 minutes

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Part (a)

1 point

Calculate the total pressure, in atm, in the flask in Experiment 1 before any reaction occurs.

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Part (b)

1 point

As the reaction in Experiment 1 proceeds toward equilibrium at constant temperature, will the total pressure in the flask increase, decrease, or remain the same? Justify your answer.

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Part (c)

2 points

Calculate the equilibrium concentrations of H₂(g), I₂(g), and HI(g) in Experiment 1.

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Part (d)

1 point

In Experiment 1, the violet color in the flask fades at first, but after some time it stops changing and stays visible. Explain these observations at the particle level.

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Part (e)

1 point

Calculate the value of Kc for the reaction HI(g) ⇌ 1/2 H₂(g) + 1/2 I₂(g) at 700. K.

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Part (f)

1 point

After equilibrium is reached in Experiment 1, a small amount of I₂(g) is added to the flask at constant temperature. Will [HI] increase, decrease, or remain the same as the system returns to equilibrium? Justify your answer in terms of Q and K.

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Part (g)

1 point

Predict whether the equilibrium concentration of HI in Experiment 2 will be greater than, less than, or equal to the equilibrium concentration of HI in Experiment 1. Justify your answer.

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Part (h)

1 point

For the forward reaction, ΔH° = −9.4 kJ/mol_rxn. If 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 (i)

1 point

Calculate the partial pressure of HI(g), in atm, in the flask in Experiment 1 at equilibrium.

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