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Date May 2017 Marks available 2 Reference code 17M.2.sl.TZ2.8
Level SL Paper 2 Time zone TZ2
Command term Determine Question number 8 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


 

Calculate the enthalpy change, in kJ, for the spray reaction, using the data below.

C 6 H 4 (OH) 2 (aq) C 6 H 4 O 2 (aq) + H 2 (g) Δ H θ = + 177.0 kJ 2 H 2 O(l) + O 2 (g) 2 H 2 O 2 (aq) Δ H θ = + 189.2 kJ H 2 O(l) H 2 (g) + 1 2 O 2 (g) Δ H θ = + 285.5 kJ

[2]
a.i.

The energy released by the reaction of one mole of hydrogen peroxide with hydroquinone is used to heat 850 cm3 of water initially at 21.8°C. Determine the highest temperature reached by the water.

Specific heat capacity of water = 4.18 kJ kg−1 K−1.

(If you did not obtain an answer to part (i), use a value of 200.0 kJ for the energy released, although this is not the correct answer.)

[2]
a.ii.

Identify the species responsible for the peak at m/z = 110 in the mass spectrum of hydroquinone.

[1]
b.

Identify the highest m/z value in the mass spectrum of quinone.

[1]
c.

Markscheme

ΔH = 177.0 – 189.2 2 –285.5 «kJ»

«ΔH =» –203.1 «kJ»

 

Accept other methods for correct manipulation of the three equations.

Award [2] for correct final answer.

[2 marks]

a.i.

203.1 «kJ» = 0.850 «kg» x 4.18 «kJ kg–1 K–1» x ΔT «K»
OR
«ΔT =» 57.2 «K»
«Tfinal = (57.2 + 21.8) °C =» 79.0 «°C» / 352.0 «K»


If 200.0 kJ was used:
200.0 «kJ» = 0.850 «kg» x 4.18 «kJ kg–1 K–1» x ΔT «K»
OR
«ΔT =» 56.3 «K»
«Tfinal = (56.3 + 21.8) °C =» 78.1 «°C» / 351.1 «K»

 

Award [2] for correct final answer.

Units, if specified, must be consistent with the value stated.

[2 marks]

a.ii.

C6H4(OH)2+

 

Accept “molecular ion”.

Do not accept “C6H4(OH)2” (positive charge missing).

[1 mark]

b.

«highest m/z» 108

 

Only accept exactly 108, not values close to this.

[1 mark]

c.

Examiners report

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

Syllabus sections

Core » Topic 5: Energetics/thermochemistry » 5.1 Measuring energy changes
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