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Question 18M.2.hl.TZ2.6c.ii

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Date May 2018 Marks available [Maximum mark: 2] Reference code 18M.2.hl.TZ2.6c.ii
Level hl Paper 2 Time zone TZ2
Command term Deduce Question number c.ii Adapted from N/A
c.ii.
[Maximum mark: 2]
18M.2.hl.TZ2.6c.ii

A mixture of 1.00 mol SO2(g), 2.00 mol O2(g) and 1.00 mol SO3(g) is placed in a 1.00 dm3 container and allowed to reach equilibrium.

2SO2(g) + O2(g) ⇌  2SO3(g)

(c.ii)

A two-step mechanism is proposed for the formation of NO2(g) from NO(g) that involves an exothermic equilibrium process.

First step: 2NO(g) ⇌  N2O2(g)     fast

Second step: N2O2(g) + O2 (g) → 2NO2(g)     slow

Deduce the rate expression for the mechanism.

[2]

Markscheme

ALTERNATIVE 1:

«from equilibrium, step 1»

K c = [ N 2 O 2 ] [NO] 2

OR

[N2O2] = Kc[NO]2

«from step 2, rate «= k1[N2O2][O2] = k2K[NO]2[O2]»

rate = k[NO]2[O2]

 

ALTERNATIVE 2:

«from step 2» rate = k2[N2O2][O2]

«from step 1, rate(1) = k1[NO]2 = k–1[N2O2], [N2O2] = k 1 k − 1  [NO]2»

«rate = k 1 k − 1  k2[NO]2[O2]»

rate = k[NO]2[O2]

 

Award [2] for correct rate expression.

[2 marks]

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