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Question 18N.2.hl.TZ0.5

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Date November 2018 Marks available [Maximum mark: 9] Reference code 18N.2.hl.TZ0.5
Level hl Paper 2 Time zone TZ0
Command term Calculate, Predict, State Question number 5 Adapted from N/A
5.
[Maximum mark: 9]
18N.2.hl.TZ0.5

This reaction is used in the manufacture of sulfuric acid.

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)        Kc = 280 at 1000 K

(a)

State why this equilibrium reaction is considered homogeneous.

[1]

Markscheme

all «species» are in same phase ✔

 

Accept “all species are in same state”.

Accept “all species are gases”.

(b)

Predict, giving your reason, the sign of the standard entropy change of the forward reaction.

[1]

Markscheme

negative AND fewer moles/molecules «of gas» in the products ✔

(c)

Calculate the standard Gibbs free energy change, ΔGΘ, in kJ, for this reaction at 1000 K. Use sections 1 and 2 of the data booklet.

[2]

Markscheme

ΔGΘ =«–RT ln Kc =» –8.31 J K–1 mol–1 × 1000 K × ln 280

OR

ΔGΘ = – 4.7 × 104 «J» ✔

 

«ΔGΘ =» – 47 «kJ» ✔

 

Award [2] for correct final answer.

(d)

Predict, giving your reasons, whether the forward reaction is endothermic or exothermic. Use your answers to (b) and (c).

[2]

Markscheme

ΔGΘ < 0/spontaneous AND ΔSΘ < 0/unfavourable ✔

exothermic AND ΔHΘ  «must be» negative/favourable ✔

(e)

0.200 mol sulfur dioxide, 0.300 mol oxygen and 0.500 mol sulfur trioxide were mixed in a 1.00 dm3 flask at 1000 K.

Predict the direction of the reaction showing your working.

[3]

Markscheme

«reaction quotient/Q =»  [ S O 3 ] 2 [ S O 2 ] 2 [ O 2 ] / 0.500 2 0.200 2 × 0.300 / 20.8 ✔

 

reaction quotient/Q/20.8/answer < Kc/280

OR

mixture needs more product for the number to equal Kc ✔

 

reaction proceeds to the right/products ✔

 

Do not award M3 without valid reasoning.

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