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Date November 2020 Marks available 2 Reference code 20N.2.sl.TZ0.3
Level SL Paper 2 Time zone TZ0
Command term Calculate 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.

Markscheme

Bonds broken: 8(CH)+2(CC)+5(O=O) /8×414«kJmol1»+2×346«kJmol1»+5×498«kJmol1»/6494«kJ»

Bonds formed: 6(C=O)+8(OH)/6×804«kJmol1»+8×463«kJmol1»/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.

Examiners report

This question was generally well answered. The mean mark on the question was 1.8 out of 3 marks. Mistakes included three C–C (instead of two), missing C–C bonds completely, subtracting bonds formed bonds broken, 3 and 4 instead of 6 and 8 for bonds formed coefficients, and bond enthalpies of C=O and O=O double bonds treated as single bonds.

a.

About half of the candidates obtained both marks. In the incorrect answers coefficients were sometimes ignored, some candidates used the wrong state for water, and some candidates did not realize the heat of formation of O2 was zero and inserted the bond enthalpy for O2 in the calculation.

b.

Syllabus sections

Core » Topic 5: Energetics/thermochemistry » 5.3 Bond enthalpies
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