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Edexcel IAL Chemistry 11.3 Selecting rate-monitoring techniques

Identify what changes continuously and measurably, then choose colorimetry, gas collection, mass loss or timed sampling only when that signal tracks reaction progress.

Syllabus
First assessment 2019
Course
Chemistry YCH11
Level
A2

Exam points

  • Choose colorimetry when a coloured species changes concentration during reaction.
  • Use gas volume or mass loss only when the stoichiometric change produces a measurable signal.
  • Distinguish continuous monitoring from quench-and-titrate sampling methods.

11.3—Select and justify a suitable experimental technique to obtain rate data for a given reaction question 1

[Maximum number: 1]

A reaction vessel contained nitrogen monoxide and oxygen in a 2: 1 molar ratio. The mixture was allowed to come to equilibrium forming nitrogen dioxide. The equation for the reaction is shown.

2NO+O2⇌2NO22 \mathrm{NO}+\mathrm{O}_{2} \rightleftharpoons 2 \mathrm{NO}_{2}

The volume of the vessel was 15dm315 \mathrm{dm}^{3} and the reaction was carried out at a constant temperature and at a pressure of 200000 Pa .

At equilibrium there was a total of 0.69625 mol of gas in the reaction vessel and the mass of oxygen was 7.000 g .

Under a different set of conditions, the reaction was carried out to find the initial rate of reaction.

Table for Question 11.3—Select and justify a suitable experimental technique to obtain rate data for a given reaction question 1 — Edexcel A-Level Chemistry A2

The rate equation for this reaction is

 rate =k[NO]2[O2]\text { rate }=k[\mathrm{NO}]^{2}\left[\mathrm{O}_{2}\right]

Give the reason why colorimetry can be used to monitor the rate of the reaction.

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