Question
Ethane-1,2-diol, HOCH2CH2OH, has a wide variety of uses including the removal of ice from aircraft and heat transfer in a solar cell.
(i) Calculate ΔHθ, in kJ, for this similar reaction below using \(\Delta H_{\rm{f}}^\theta \) data from section 12 of the data booklet. \(\Delta H_{\rm{f}}^\theta \) of HOCH2CH2OH(l) is –454.8kJmol-1.
2CO (g) + 3H2 (g) \( \rightleftharpoons \) HOCH2CH2OH (l)
(ii) Deduce why the answers to (a)(iii) and (b)(i) differ.
(iii) ΔSθ for the reaction in (b)(i) is –620.1JK-1. Comment on the decrease in entropy.
(iv) Calculate the value of ΔGθ, in kJ, for this reaction at 298 K using your answer to (b)(i). (If you did not obtain an answer to (b)(i), use –244.0 kJ, but this is not the correct value.)
(v) Comment on the statement that the reaction becomes less spontaneous as temperature is increased.[6]
Predict the 1HNMR data for ethanedioic acid and ethane-1,2-diol by completing the table.
[2]
Answer/Explanation
Markscheme
i
«ΔH = Σ ΔHf products – ΣΔHf reactants = –454.8 kJ mol-1 – 2(–110.5 kJ mol-1) =» –233.8 «kJ»
ii
in (a)(iii) gas is formed and in (b)(i) liquid is formed
OR
products are in different states
OR
conversion of gas to liquid is exothermic
OR
conversion of liquid to gas is endothermic
OR
enthalpy of vapourisation needs to be taken into account
Accept product is «now» a liquid.
Accept answers referring to bond enthalpies being means/averages.
iii
«ΔS is negative because five mols of» gases becomes «one mol of» liquid
OR
increase in complexity of product «compared to reactants»
OR
product more ordered «than reactants»
Accept “fewer moles of gas” but not “fewer molecules”.
iv
ΔS = \(\left( {\frac{{ – 620.1}}{{1000}}} \right)\)«kJ K-1»
ΔG = –233.8 kJ – (298 K \(\left( {\frac{{ – 620.1}}{{1000}}} \right)\) kJ K-1) = –49.0 «kJ»
Award [2] for correct final answer.
Award [1 max] for «+»185 × 103.
If –244.0 kJ used, answer is:
ΔG = –244.0 kJ – (298 K \(\left( {\frac{{ – 620.1}}{{1000}}} \right)\)kJ K-1) = –59.2 «kJ»
Award [2] for correct final answer.
v
increasing T makes ΔG larger/more positive/less negative
OR
–TΔS will increase
Accept “none/no splitting” for singlet.
Question
This question is about carbon and chlorine compounds.
Ethane, \({{\text{C}}_{\text{2}}}{{\text{H}}_{\text{6}}}\), reacts with chlorine in sunlight. State the type of this reaction and the name of the mechanism by which it occurs.
[1]
Formulate equations for the two propagation steps and one termination step in the formation of chloroethane from ethane.
3]
Deduce the splitting patterns in the 1H NMR spectrum of C2H5Cl.[1]
Explain why tetramethylsilane (TMS) is often used as a reference standard in 1H NMR.[2]
One possible product, X, of the reaction of ethane with chlorine has the following composition by mass:
carbon: 24.27%, hydrogen: 4.08%, chlorine: 71.65%
Determine the empirical formula of the product.[2]
The mass and 1H NMR spectra of product X are shown below. Deduce, giving your reasons, its structural formula and hence the name of the compound.
[3]
When the product X is reacted with NaOH in a hot alcoholic solution, C2H3Cl is formed. State the role of the reactant NaOH other than as a nucleophile.[1]
Chloroethene, \({{\text{C}}_{\text{2}}}{{\text{H}}_{\text{3}}}{\text{Cl}}\), can undergo polymerization. Draw a section of the polymer with three repeating units.[1]
Answer/Explanation
Markscheme
substitution AND «free-»radical
OR
substitution AND chain
Award [1] for “«free-»radical substitution” or “SR” written anywhere in the answer.
[1 mark]
Two propagation steps:
\({{\text{C}}_{\text{2}}}{{\text{H}}_{\text{6}}} + \bullet {\text{Cl}} \to {{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}} \bullet + {\text{HCl}}\)
\({{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}} \bullet + {\text{C}}{{\text{l}}_{\text{2}}} \to {{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}{\text{Cl}} + \bullet {\text{Cl}}\)
One termination step:
\({{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}} \bullet + {{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}} \bullet \to {{\text{C}}_{\text{4}}}{{\text{H}}_{{\text{10}}}}\)
OR
\({{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}} \bullet + \bullet {\text{Cl}} \to {{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}{\text{Cl}}\)
OR
\( \bullet {\text{Cl}} + \bullet {\text{Cl}} \to {\text{C}}{{\text{l}}_{\text{2}}}\)
Accept radical without \( \bullet \) if consistent throughout.
Allow ECF for incorrect radicals produced in propagation step for M3.
[3 marks]
triplet AND quartet
[1 mark]
chemical shift/signal outside range of common chemical shift/signal
strong signal/12/all H atoms in same environment
OR
singlet/no splitting of the signal
volatile/easily separated/easily removed
OR
inert/stabl
contains three common NMR nuclei/1H and 13C and 29Si
Do not accept chemical shift = 0.
[2 marks]
\({\text{C}} = \frac{{24.27}}{{12.01}} = 2.021\) AND \({\text{H}} = \frac{{4.08}}{{1.01}} = 4.04\) AND \({\text{Cl}} = \frac{{71.65}}{{35.45}} = 2.021\)
«hence» CH2Cl
Accept \(\frac{{24.27}}{{12.01}}\) : \(\frac{{4.08}}{{1.01}}\) : \(\frac{{71.65}}{{35.45}}.\)
Do not accept C2H4Cl2.
Award [2] for correct final answer.
[2 marks]
molecular ion peak(s) «about» m/z 100 AND «so» C2H4Cl2 «isotopes of Cl»
two signals «in 1H NMR spectrum» AND «so» CH3CHCl2
OR
«signals in» 3:1 ratio «in 1H NMR spectrum» AND «so» CH3CHCl2
OR
one doublet and one quartet «in 1H NMR spectrum» AND «so» CH3CHCl2
1,1-dichloroethane
Accept “peaks” for “signals”.
Allow ECF for a correct name for M3 if an incorrect chlorohydrocarbon is identified.
[3 marks]
base
OR
proton acceptor
[1 mark]
Continuation bonds must be shown.
Ignore square brackets and “n”.
Accept 
.
Accept other versions of the polymer, such as head to head and head to tail.
Accept condensed structure provided all C to C bonds are shown (as single).
[1 mark]