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Date May 2017 Marks available 2 Reference code 17M.2.hl.TZ2.9
Level HL Paper 2 Time zone TZ2
Command term Predict Question number 9 Adapted from N/A

Question

The Bombardier beetle sprays a mixture of hydroquinone and hydrogen peroxide to fight off predators. The reaction equation to produce the spray can be written as:

C6H4(OH)2(aq) + H2O2(aq) C6H4O2(aq) + 2H2O(l)
hydroquinone   quinone


 

Hydrogenation of propene produces propane. Calculate the standard entropy change, ΔSθ, for the hydrogenation of propene.

[2]
b.i.

The standard enthalpy change, ΔH θ, for the hydrogenation of propene is –124.4 kJ mol–1. Predict the temperature above which the hydrogenation reaction is not spontaneous.

[2]
b.ii.

Markscheme

«ΔSθ =» 270 «J K–1 mol–1» – 267 «J K–1 mol–1» – 131 «J K–1 mol–1»

«ΔSθ =» –128 «J K–1 mol–1»

 

Award [2] for correct final answer.

[2 marks]

b.i.

«non spontaneous if» ΔG θ = ΔH θ – TΔS θ > 0
OR
ΔH θ > TΔS θ

«T above» 124.4   kJ mo l 1 0.128   kJ K 1 mo l 1 = » 972 «K»

 

Award [2] for correct final answer.

Accept 699 °C.

Do not award M2 for any negative T value.

[2 marks]

b.ii.

Examiners report

[N/A]
b.i.
[N/A]
b.ii.

Syllabus sections

Additional higher level (AHL) » Topic 15: Energetics/thermochemistry » 15.2 Entropy and spontaneity
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