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Question 19M.2.sl.TZ2.c(iii)

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Date May 2019 Marks available [Maximum mark: 3] Reference code 19M.2.sl.TZ2.c(iii)
Level sl Paper 2 Time zone TZ2
Command term Determine Question number c(iii) Adapted from N/A
c(iii).
[Maximum mark: 3]
19M.2.sl.TZ2.c(iii)

Ethyne, C2H2, reacts with oxygen in welding torches.

Ethyne reacts with steam.

C2H2 (g) + H2O (g) → C2H4O (g)

Two possible products are:

Determine the enthalpy change for the reaction, in kJ, to produce A using section 11 of the data booklet.

[3]

Markscheme

«sum of bond enthalpies of reactants =» 2(C–H) + C≡C + 2(O–H)
OR
2 × 414 «kJ mol–1» + 839 «kJ mol–1» + 2 × 463 «kJ mol–1»
OR
2593 «kJ»  [✔]

«sum of bond enthalpies of A =» 3(C–H) + C=C + C–O + O–H
OR
3 × 414 «kJ mol–1» + 614 «kJ mol–1» + 358 «kJ mol–1» + 463 «kJ mol–1»
OR
2677 «kJ»  [✔]

«enthalpy of reaction = 2593 kJ – 2677 kJ» = –84 «kJ»  [✔]

 

Note: Award [3] for correct final answer.

Examiners report

Candidates were confident in using average bond enthalpies for calculating the enthalpy change for the reaction. Mistakes included forgetting to include the breaking of the O-H bonds in water and reversing the signs.

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