Date | May 2014 | Marks available | 8 | Reference code | 14M.2.hl.TZ1.4 |
Level | HL | Paper | 2 | Time zone | TZ1 |
Command term | Determine, Explain, and Predict | Question number | 4 | Adapted from | N/A |
Question
Buta-1,3-diene can be hydrogenated to produce butane, according to the reaction below.
C4H6(g)+2H2(g)→C4H10(g)
State the conditions necessary for this reaction.
Determine the standard enthalpy change of reaction, ΔHΘ, in kJmol−1, at 298 K for the hydrogenation reaction, using Table 11 of the Data Booklet.
Calculate the standard free energy change, ΔGΘ, in kJmol−1, at 298 K for the hydrogenation reaction, using Table 11 of the Data Booklet.
(i) Determine the standard entropy change of the reaction, ΔSΘ, at 298 K, in kJK−1mol−1, using your answers from (b) and (c).
(ii) Explain why the standard entropy change for the hydrogenation of buta-1,3-diene has a negative sign.
(iii) Predict whether the hydrogenation reaction becomes more or less spontaneous as the temperature increases.
(iv) Determine the temperature, in K, at which the spontaneity changes.
(v) Determine the standard entropy, SΘ, for hydrogen in JK−1mol−1, using Table 11 of the Data Booklet and your answer for (d)(i).
Markscheme
heat /warm / 140–225 °C;
Do not accept high temperature.
(finely divided) catalyst / Zn/Cu/Ni/Pd/Pt;
ΔHΘ=(ΣΔHΘf(products)−ΣΔHΘf(reactants)=−127−(110+0)=)−327 (kJmol−1);
ΔGΘ=(ΣΔGΘf(products)−ΣΔGΘf(reactants)=−16−(152+0)=)−168 (kJmol−1);
(i) ΔSΘ=(ΔHΘ−ΔGΘT=)−237−(−168)209;
=−0.232 (kJK−1mol−1);
Award [2] for correct final answer.
Award [2] for –232 J K–1 mol–1 (units must be given).
(ii) 3 mol of gaseous reactants and 1 mol of gaseous products / fewer moles of gas in products;
(iii) spontaneity decreases (as temperature increases because TΔSΘ becomes a larger negative value/ΔGΘ becomes positive at higher temperatures);
(iv) ΔGΘ=ΔHΘ−TΔSΘ=0/−237−T(−0.232)=0;
T=1020 (K);
Remember to allow ECF from 4(d)(i).
(v) ΔSΘ=ΣSΘ(products)−ΣSΘ(reactants)/−232=310−(279+2SΘ(H2));
SΘ(H2)=12(310−279+232)=132 JK−1mol−1);
Award [2] for correct final answer.
Remember to allow ECF from 4(d)(i).
Examiners report
Conditions for the reaction, even though the mark scheme accepted fairly vague answers, were not well known. The calculations of enthalpy, entropy and Gibbs free energy scored well; it was pleasing to note that many realised the importance of conversion of units in part d(v). The link between the changes in temperature and the effect on spontaneity was understood, but many lost credit on part d(ii) for failing to mention the change in the number of gaseous moles. In d(v) most candidates missed the fact that 2 moles of hydrogen were present in the equation.
Conditions for the reaction, even though the mark scheme accepted fairly vague answers, were not well known. The calculations of enthalpy, entropy and Gibbs free energy scored well; it was pleasing to note that many realised the importance of conversion of units in part d(v). The link between the changes in temperature and the effect on spontaneity was understood, but many lost credit on part d(ii) for failing to mention the change in the number of gaseous moles. In d(v) most candidates missed the fact that 2 moles of hydrogen were present in the equation.
Conditions for the reaction, even though the mark scheme accepted fairly vague answers, were not well known. The calculations of enthalpy, entropy and Gibbs free energy scored well; it was pleasing to note that many realised the importance of conversion of units in part d(v). The link between the changes in temperature and the effect on spontaneity was understood, but many lost credit on part d(ii) for failing to mention the change in the number of gaseous moles. In d(v) most candidates missed the fact that 2 moles of hydrogen were present in the equation.
Conditions for the reaction, even though the mark scheme accepted fairly vague answers, were not well known. The calculations of enthalpy, entropy and Gibbs free energy scored well; it was pleasing to note that many realised the importance of conversion of units in part d(v). The link between the changes in temperature and the effect on spontaneity was understood, but many lost credit on part d(ii) for failing to mention the change in the number of gaseous moles. In d(v) most candidates missed the fact that 2 moles of hydrogen were present in the equation.