IB Chemistry HL Reactivity 2: How much, how fast and how far? questions

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Question 1

Paper 2Hard11 marks

During the gas-phase oxidation of nitric oxide to nitrogen dioxide, the overall reaction is:

2NO(g) + O₂(g) → 2NO₂(g)

Initial-rate data were collected at constant temperature.

Experiment[NO] / mol dm⁻³[O₂] / mol dm⁻³Initial rate / mol dm⁻³ s⁻¹
11.00 × 10⁻²2.00 × 10⁻²1.20 × 10⁻⁵
22.00 × 10⁻²2.00 × 10⁻²4.80 × 10⁻⁵
32.00 × 10⁻²4.00 × 10⁻²9.60 × 10⁻⁵

Two mechanisms have been proposed. Each step is elementary.

Mechanism I: NO(g) + O₂(g) → NO₃(g) (slow)
NO₃(g) + NO(g) → 2NO₂(g) (fast)

Mechanism II: 2NO(g) ⇌ N₂O₂(g) (fast equilibrium)
N₂O₂(g) + O₂(g) → 2NO₂(g) (slow)

For a proposed energy profile for mechanism II, the relative energies of successive stages are:

Stage along the reaction pathwayRelative energy / kJ mol⁻¹
2NO(g) + O₂(g)0
During elementary step 1+35
Between the two steps+12
During elementary step 2+60
2NO₂(g)−114
(a)

Determine the order of reaction with respect to each reactant and write the rate equation.

[3]
(b)

Assess both proposed mechanisms using the overall equation and the initial-rate data. State which mechanism is consistent with both.

[4]
(c)

Sketch an energy profile for mechanism II using the values given. Label the reactants, products, intermediate and both transition states. Mark the activation energy of the rate-determining step, and state which step the profile indicates is rate-determining.

[4]

Question 2

Paper 2Hard9 marks

In a sealed, fixed-volume vessel, nitrogen and oxygen form nitrogen monoxide at 1500 K:

N₂(g) + O₂(g) ⇌ 2NO(g)

At this temperature, Kc = 4.00 × 10⁻⁶. At a particular instant, the concentrations are [N₂] = 0.0500 mol dm⁻³, [O₂] = 0.0400 mol dm⁻³ and [NO] = 1.00 × 10⁻⁵ mol dm⁻³.

(a)

Calculate Qc for this mixture and predict the direction in which the reaction will proceed to reach equilibrium.

[2]
(b)

Determine the equilibrium concentrations of the three gases. Show how you check that any approximation used is justified.

[4]
(c)

After the mixture reaches equilibrium, an injection increases [NO] by 2.00 × 10⁻⁴ mol dm⁻³. Immediately after the injection, before further reaction occurs, calculate Qc and predict the direction in which the reaction will proceed.

[3]

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