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Date	May 2019	Marks available	1	Reference code	19M.2.hl.TZ2.1
Level	HL	Paper	2	Time zone	TZ2
Command term	Deduce	Question number	1	Adapted from	N/A

Question

Ethyne, C₂H₂, reacts with oxygen in welding torches.

Ethyne reacts with steam.

C₂H₂ (g) + H₂O (g) → C₂H₄O (g)

Two possible products are:

Product B, CH₃CHO, can also be synthesized from ethanol.

Write an equation for the complete combustion of ethyne.

[1]

Deduce the Lewis (electron dot) structure of ethyne.

[1]

b(i).

Compare, giving a reason, the length of the bond between the carbon atoms in ethyne with that in ethane, C₂H₆.

[1]

b(ii).

Identify the type of interaction that must be overcome when liquid ethyne vaporizes.

[1]

b(iii).

State the name of product B, applying IUPAC rules.

[1]

c(i).

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

[3]

c(ii).

The enthalpy change for the reaction to produce B is −213 kJ.

Predict, giving a reason, which product is the most stable.

[1]

c(iii).

The IR spectrum and low resolution ¹H NMR spectrum of the actual product formed are shown.

Deduce whether the product is A or B, using evidence from these spectra together with sections 26 and 27 of the data booklet.

Identity of product:

One piece of evidence from IR:

One piece of evidence from ¹H NMR:

[2]

c(iv).

Deduce the splitting pattern you would expect for the signals in a high resolution ¹H NMR spectrum.

2.3 ppm:

9.8 ppm:

[2]

c(v).

Suggest the reagents and conditions required to ensure a good yield of product B.

Reagents:

Conditions:

[2]

d(i).

Deduce the average oxidation state of carbon in product B.

[1]

d(ii).

Explain why product B is water soluble.

[3]

d(iii).

Markscheme

C₂H₂ (g) + 2.5O₂ (g) → 2CO₂ (g) + H₂O (l)

2C₂H₂ (g) + 5O₂ (g) → 4CO₂ (g) + 2H₂O (l) [✔]

[✔]

Note: Accept any valid combination of lines, dots and crosses.

b(i).

«ethyne» shorter AND a greater number of shared/bonding electrons

«ethyne» shorter AND stronger bond [✔]

b(ii).

London/dispersion/instantaneous dipole-induced dipole forces [✔]

b(iii).

ethanal [✔]

c(i).

«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.

c(ii).

B AND it has a more negative/lower enthalpy/«potential» energy

B AND more exothermic «enthalpy of reaction from same starting point» [✔]

c(iii).

Identity of product: «B»

IR spectrum:
1700–1750 «cm^–1 band» AND carbonyl/CO group present
OR
no «band at» 1620–1680 «cm^–1» AND absence of double bond/C=C
OR
no «broad band at» 3200–3600 «cm^–1 » AND absence of hydroxyl/OH group [✔]

¹H NMR spectrum:
«only» two signals AND A would have three
OR
«signal at» 9.4–10.0 «ppm» AND «H atom/proton of» aldehyde/–CHO present
OR
«signal at» 2.2–2.7 «ppm» AND «H atom/proton of alkyl/CH next to» aldehyde/CHO present
OR
«signal at» 2.2–2.7 «ppm» AND «H atom/proton of» RCOCH_2- present
OR
no «signal at» 4.5–6.0 «ppm» AND absence of «H atom/proton next to» double bond/C=C ✔

Note: Accept a specific value or range of wavenumbers and chemical shifts.

Accept “two signals with areas 1:3”.

c(iv).

2.3 ppm: doublet [✔]

9.8 ppm: quartet [✔]

c(v).

Reagents:
acidified/H⁺ AND «potassium» dichromate«(VI)»/K₂Cr₂O₇/Cr₂O₇^2- [✔]

Conditions:
distil «the product before further oxidation» [✔]

Note: Accept “«acidified potassium» manganate(VII)/KMnO₄/MnO₄^-/permanganate”.

Accept “H₂SO₄” or “H₃PO₄” for “H⁺”.

Accept “more dilute dichromate(VI)/manganate(VII)” or “excess ethanol”.

Award M1 if correct reagents given under “Conditions”.

d(i).

–1 [✔]

d(ii).

Any three of:

has an oxygen/O atom with a lone pair [✔]

that can form hydrogen bonds/H-bonds «with water molecules» [✔]

hydrocarbon chain is short «so does not disrupt many H-bonds with water molecules» [✔]

«large permanent» dipole-dipole interactions with water [✔]

d(iii).

Examiners report

All candidates were able to write the correct reactants/products for combustion of ethyne, but a few failed to balance correctly.

Most drew correct Lewis structures for ethyne, though some drew ethene.

b(i).

Surprisingly very few explained the difference in bond length/strength looking at electrons shared and just gave the shorter/triple or longer/single bond answer.

b(ii).

Good to see that most candidates identified the specific IMF correctly.

b(iii).

Most candidates gave the correct IUPAC name.

c(i).

Candidates were able to calculate the ΔH of the given reaction correctly; a few inverted the calculations or made mathematical errors.

c(ii).

Generally well done, most common error was stating that the enthalpy change was “larger” without the indication that it was an exothermic change or the sign.

c(iii).

Interpretation of spectra was very good and the few candidates that lost marks with ¹H NMR data rather than IR, for example simply mentioning two signals for B. However, most candidates that attempted this question got full marks.

c(iv).

The stronger candidates were able to predict the splitting pattern correctly, others inverted the answer, but many others repeated the information for protons with the given chemical shift, which is unexpected since wording was straightforward.

c(v).

Candidates seemed to be confused by the prompts, reagent and conditions, so often included the acid among conditions. Careless errors were common such as the wrong charge on the dichromate ion. Few candidates suggest permanganate as an option.

d(i).

Most candidates were able to calculate oxidation state of carbon in B.

d(ii).

Candidates did not understand that they must mention the IMF responsible for the solubility. Most candidates explained the polarity of the aldehyde and water but did not mention that this results in permanent dipole-dipole interactions; many did mention H-bonding. The mention of the lone pair on O atom and short hydrocarbon chain were very rare.

d(iii).

Syllabus sections

Core » Topic 9: Redox processes » 9.1 Oxidation and reduction

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