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Date May 2017 Marks available 1 Reference code 17M.2.hl.TZ2.5
Level HL Paper 2 Time zone TZ2
Command term Identify Question number 5 Adapted from N/A

Question

Nitrogen dioxide and carbon monoxide react according to the following equation:

NO2(g) + CO(g) NO(g) + CO2(g)                  ΔH = –226 kJ

Experimental data shows the reaction is second order with respect to NO2 and zero order with respect to CO.

State the rate expression for the reaction.

[1]
b.i.

The following mechanism is proposed for the reaction.

Step I N O 2 (g) + N O 2 (g) NO(g) + N O 3 (g) Step II N O 3 (g) + CO(g) N O 2 (g) + C O 2 (g)

Identify the rate determining step giving your reason.

[1]
b.ii.

State one method that can be used to measure the rate for this reaction.

[1]
b.iii.

Sketch the relationship between the rate of reaction and the concentration of NO2.

[1]
b.v.

The Arrhenius equation, k = A e E a R T , gives the relationship between the rate constant and temperature.

State how temperature affects activation energy.

[1]
c.

Markscheme

«rate =» k [NO2]2

 

Accept rate = k [NO2]2[CO]0.

[1 mark]

b.i.

«step» I AND CO does not appear in the rate law expression
OR
«step» I AND only «2 molecules of» NO2 appears in rate expression

 

Do not allow ECF from (i).

[1 mark]

b.ii.

«IR or UV-vis» spectroscopy
OR
colorimetry
OR
colour change «over time»

 

Accept GC/gas chromatography.

[1 mark]

b.iii.

 

Curve must go through origin.

[1 mark]

b.v.

activation energy is independent of temperature

 

Accept “no relationship”.

[1 mark]

c.

Examiners report

[N/A]
b.i.
[N/A]
b.ii.
[N/A]
b.iii.
[N/A]
b.v.
[N/A]
c.

Syllabus sections

Additional higher level (AHL) » Topic 16: Chemical kinetics » 16.1 Rate expression and reaction mechanism
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Additional higher level (AHL) » Topic 16: Chemical kinetics
Additional higher level (AHL)

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