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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, H, for this reaction, using section 11 of the data booklet.

 

[3]
a.

Calculate the standard enthalpy change, H, for this reaction using section 12 of the data booklet.

[2]
b.

Predict, giving a reason, whether the entropy change, S, for this reaction is negative or positive.

 

[1]
c.

Calculate S for the reaction in JK-1, using section 12 of the data booklet.

The standard molar entropy for oxygen gas is 205JK-1mol-1.

[2]
d.

Calculate the standard Gibbs free energy change, G, in kJ, 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 -1952kJ and +113JK-1 respectively, although these are not the correct answers.)

[2]
e.

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 change=bonds broken-bonds formed =6494kJ8528kJ=»2034«kJ» 

Award [3] for correct final answer.

 

a.

4(241.8«kJ») AND 3(393.5«kJ») AND «1»(105«kJ») ✔
«ΔH=4(241.8«kJ»)+3(393.5«kJ»)«1»(105«kJ»)=»2043«kJ» ✔

Award [2] for correct final answer.

Award [1 max] for 2219«kJ».

b.

positive AND more moles «of gas» in products ✔

c.

4×188.8«JK1» AND 3×213.8«JK1» AND «1×»270«JK1» AND 5×205«JK1» ✔

«S=4(188.8JK1)+3(213.8JK1)[1(270JK1)+5(205JK1)]=»102«JK1»  ✔

Award [2] for correct final answer.

d.

«T=5+273=»278K ✔

«ΔG=2043kJ(278K×0.102kJK1)=»2071«kJ» ✔

Award [2] for correct final answer.

e.

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.

a.

Enthalpy calculations using enthalpy of formation data were generally well done.

b.

Most knew that entropy increased however some lost the mark by not including an explanation based on increase number of mol of gaseous products.

c.

Calculating ΔSө, like most other calculations, was well done.

d.

ΔGө calculations were also well done, with some not seeing that specific units were to be used.

e.

Syllabus sections

Additional higher level (AHL) » Topic 15: Energetics/thermochemistry » 15.2 Entropy and spontaneity
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