IB Chemistry HL R2.2 How fast? The rate of chemical change questions

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Question 1

Paper 2Medium6 marks

Nitric oxide is oxidized by oxygen to form nitrogen dioxide:

2NO(g) + O₂(g) → 2NO₂(g)

Initial rates were measured at 298 K.

Trial[NO] / mol dm⁻³[O₂] / mol dm⁻³Initial rate / mol dm⁻³ s⁻¹
10.02000.01002.40 × 10⁻⁴
20.04000.01009.60 × 10⁻⁴
30.02000.02004.80 × 10⁻⁴

At 308 K, an experiment with [NO] = 0.0200 mol dm⁻³ and [O₂] = 0.0100 mol dm⁻³ gave an initial rate of 3.60 × 10⁻⁴ mol dm⁻³ s⁻¹. Assume the rate equation is unchanged between these temperatures.

(a)

Determine the order of the reaction with respect to NO and O₂.

[2]
(b)

Write the rate equation for the reaction.

[1]
(c)

Calculate the rate constant, k, at 298 K, including its units.

[2]
(d)

State which temperature has the larger value of k, using the measurements to justify your answer.

[1]

Question 2

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]

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