Date | November 2020 | Marks available | 1 | Reference code | 20N.2.hl.TZ0.3 |
Level | HL | Paper | 2 | Time zone | TZ0 |
Command term | Justify and Predict | Question number | 3 | Adapted from | N/A |
Question
An equation for the combustion of propane is given below.
C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)
Determine the standard enthalpy change, , for this reaction, using section 11 of the data booklet.
Calculate the standard enthalpy change, , for this reaction using section 12 of the data booklet.
Predict, giving a reason, whether the entropy change, , for this reaction is negative or positive.
Calculate for the reaction in , using section 12 of the data booklet.
The standard molar entropy for oxygen gas is .
Calculate the standard Gibbs free energy change, , in , for the reaction at 5 °C, using your answers to (b) and (d). Use section 1 of the data booklet.
(If you did not obtain an answer to (b) or (d) use values of and respectively, although these are not the correct answers.)
Markscheme
Bonds broken: 8(C–H) + 2(C–C) + 5(O=O) / 8 × 414 «kJ mol−1» + 2 × 346 «kJ mol−1» + 5 × 498 «kJ mol−1» / 6494 «kJ» ✔
Bonds formed: 6(C=O) + 8(O–H) / 6 × 804 «kJ mol−1» + 8 × 463 «kJ mol−1» / 8528 «kJ» ✔
«Enthalpy changebonds brokenbonds formed ✔
Award [3] for correct final answer.
AND AND ✔
✔
Award [2] for correct final answer.
Award [1 max] for .
positive AND more moles «of gas» in products ✔
AND AND AND ✔
✔
Award [2] for correct final answer.
✔
✔
Award [2] for correct final answer.
Examiners report
Many candidates had difficulty determining the number and type of bonds broken or formed and consequently this was the part of question 3 that was most poorly attempted. Those that could identify these bonds performed the calculations correctly.
Enthalpy calculations using enthalpy of formation data were generally well done.
Most knew that entropy increased however some lost the mark by not including an explanation based on increase number of mol of gaseous products.
Calculating ΔSө, like most other calculations, was well done.
ΔGө calculations were also well done, with some not seeing that specific units were to be used.