Long free-response question

The decomposition of nitrosyl bromide

Units 3 and 5
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

Nitrosyl bromide gas decomposes according to the equation 2 NOBr(g) → 2 NO(g) + Br₂(g). A student follows the reaction in a rigid container at 373 K and records the concentration of NOBr over time.

Table 1. Concentration of NOBr over time at 373 K

Time (s)	[NOBr] (M)	ln[NOBr]	1/[NOBr] (M⁻¹)
0	0.0400	−3.219	25.0
10.0	0.0302	−3.500	33.1
20.0	0.0243	−3.717	41.2
40.0	0.0174	−4.051	57.5
60.0	0.0136	−4.298	73.5
100.0	0.00943	−4.664	106.0

Source: Hypothetical data

Energy information

For the reaction as written, ΔH° = +47 kJ/mol_rxn.

For this question, take the activation energy of the forward reaction to be 80. kJ/mol_rxn.

Source: Hypothetical values

Suggested time: 23 minutes

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Part (a)

1 point

At a certain moment, NOBr is being consumed at a rate of 1.2 × 10⁻³ M/s. What is the rate of formation of Br₂ at that moment? Justify your answer.

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Part (b)

1 point

Determine the order of the reaction with respect to NOBr. Justify your answer using the data in Table 1.

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Part (c)

2 points

(i) Write the rate law for the reaction. (ii) Calculate the value of the rate constant, k. Include units with your answer.

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Part (d)

1 point

Calculate the time, in seconds, required for [NOBr] to decrease from 0.0400 M to 0.0100 M.

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Part (e)

1 point

A student proposes that the reaction occurs in a single elementary step: NOBr + NOBr → 2 NO + Br₂. Is this proposal consistent with the rate law? Justify your answer.

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Part (f)

1 point

Using the collision model, explain why only a small fraction of the collisions between NOBr molecules result in a reaction.

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Part (g)

1 point

Describe the reaction energy profile you would draw for this reaction. Include what is plotted on each axis, the relative energies of the reactants and products, and the values of the activation energy for the forward reaction and for the reverse reaction.

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Part (h)

1 point

Calculate the initial pressure of NOBr(g), in atm, in the container at 373 K.

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Part (i)

1 point

As the reaction proceeds at constant temperature, will the total pressure in the container increase, decrease, or remain the same? Justify your answer.

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