OCR A-Level Chemistry Unified chemistry (03), November 2020: Question 3
10 marks · Hard difficulty · Calculations
Determine the enthalpy change of combustion of propane using experimental calorimetry data and bond enthalpies accounting for vaporisation.
Practise this questionQuestion
Question text
3 Propane, C3H8, (boiling point −42 °C) is used as ‘camping gas’. A student plans to determine the
enthalpy change of combustion of propane, ∆cH (C3H8), by two methods.
(a) The student first carries out an experiment using the apparatus below.
thermometer
beaker
100 cm3 water
clamp
propane burner
Results
Mass of propane burner before burning / g 99.218
Mass of propane burner after burning / g 98.976
Initial temperature / °C 21.60
Maximum temperature reached / °C 46.10
(i) Determine the enthalpy change of combustion of propane, in kJ mol−1.
Give your answer to 3 significant figures.
(ii) The student finds that the experimental enthalpy change ∆cH (C3H8) is much less
exothermic than the accurate standard enthalpy change ∆cH (C3H8) in databases.
One reason could be that the student’s experiment had not been carried out under
standard conditions.
Suggest two other reasons for this difference in enthalpy change.
1 …
2 …
[1]
∆ H (C H ) = … kJ mol−1 [3]
(b)* The student determines the standard enthalpy change of combustion of propane using thec38
bond enthalpies in the table. An experiment is not needed.
Bond enthalpy
Bond −1
/ kJ mol
C−H +413
C−C +347
C=O +805
O=O +498
O−H +464
The bond enthalpies can be used to determine the standard enthalpy change of reaction,
∆rH, for equation 3.1:
C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g) ∆rH Equation 3.1
Enthalpy change of vaporisation, ∆vapH
The standard enthalpy change of vaporisation of water, ∆vapH, is the enthalpy change for the
conversion of 1 mol of H2O(l) into 1 mol of H2O(g) under standard conditions:
H O(l) → H O(g) ∆ H = +40.65 kJ mol−1
22 vap
Determine the standard enthalpy change of combustion of propane (boiling point −42 °C)
using the ∆rH value for equation 3.1 and ∆vapH for water. [6]
Additional answer space if required
Mark scheme
Show the mark scheme
AO
Question Answer Marks Guidance
element
3 (a) (i) FIRST, CHECK THE ANSWER ON ANSWER LINE 3 FULL ANNOTATIONS MUST BE USED
IF ∆ H = –1860 OR –1850 (kJ mol–1) with evidence of ----------------------------------------------------------------
c
working, award 3 marks ALLOW ECF throughout
IF ∆cH = –1862, award 2 marks (not 3 SF)
---------------------------------------------------------------- DO NOT ALLOW c = 4.2 → 10290
Energy released in J OR kJ Next 2 marks available by ECF → –1870
= 100 × 4.18 × 24.5 = ±10241 (J) OR ±10.241 (kJ) 2.4
3 SF minimum required ALLOW 10240/10200 J OR 10.24/10.2 kJ
IGNORE units
Calculates n(C3H8)
0.242 ALLOW ECF from initial 3 SF rounding to 10.2 kJ:
= = 0.0055(0) (mol)
44(.0) 2.4 10200
± → ±1854.545455 → 1850
0.0055 × 1000
Calculates ∆cH with – sign AND 3 SF (appropriate) ------------------------------------------------------------------
10241 Common errors
∆cH = = –1862 No mark
0.0055 × 1000 ∆H = –54.6 OR –54.7 2 marks by ECF from mc∆T
m wrong as 0.242 and ∆T wrong as 297.5 K)
= –1860 OR –1.86 × 103 (kJ mol–1) 2.8 → mc∆T wrong as 300.9391 (J)
– sign AND 3 SF required
∆H = –4.51 2 marks by ECF from mc∆T
m wrong as 0.242 and ∆T correct as 24.5)
→ mc∆T wrong as 24.78322 (J)
∆H = –22600 2 marks by ECF from mc∆T
m correct as 100 and ∆T wrong as 297.5)
→ mc∆T wrong as 124355 (J)
(a) (ii) Any two from: 1 MARK ONLY 1 1.2 IGNORE incomplete ‘reaction’
• Heat loss/released to surroundings Needs link to combustion/burning/reaction with
• Incomplete combustion/reaction with oxygen or air air/O2
OR not everything burns
IGNORE evaporation of C3H8
• Evaporation of water
AO
element
Refer to marking instructions on page 5 of mark scheme for 14
(b)* 6 2.4×2 Indicative scientific points may include:
guidance on marking this question. 3.1×2
Bond enthalpy calculation of ∆ rH
Level 3 (5-6 marks) 3.2×2 Bonds broken
Calculates ∆rH f or reaction 3.1 correctly with correct sign = (2 × 347) + (8 × 413) + (5 × 498)
AND = (694) + (3304) + (2490)
Calculates a value f or ∆ H o of propane –1
c = ± 6488 kJ mol
using ∆rH AND ± 4 × ∆vapH Bonds made
= (6 × 805) + (8 × 464)
There is a well-developed line of reasoning which is clear and –1
logically structured. The information presented is relevant and = (4830) + (3712) = ± 8542 kJ mol
∆ H = 6488 – 8542 = –2054 kJ mol–1
substantiated. r
Level 2 (3-4 marks) NOTE: 3 C–C → 6835 f or bond broken: ∆H = –1707
Calculates ∆rH f or reaction 3.1 correctly with correct sign 2 C–C omitted f rom bonds broken gives: ∆H = –2748
OR ----------------------------------------------
Calculates bonds broken OR bonds made correctly to Determination of ∆cH(C3H8)
obtain a value of ∆ H f or reaction 3.1 ∆ H o of propane using ∆ H AND ± 4 × ∆ H
r c r vap
AND attempts to link ∆rH with ∆vapH OR calculates 4 × ∆vapH Correct
∆cH(C3H8) = ∆rH – 4 × ∆vapH
There is a line of reasoning presented with some structure. = –2054 – (4 × 40.65)
The information presented is relevant and supported by some
evidence. = –2054 – 162.6
= –2216.6 / –2217 kJ mol–1
Level 1 (1-2 marks) Incorrect
Uses bond enthalpies for bonds broken and bonds made but ∆cH(C3H8) = –2054 + (4 × 40.65)
may contain errors or omissions AND obtains a value for ∆rH. = –2054 + 162.6
OR = –1891.4 / –1891 kJ mol–1
Calculates bonds broken OR bonds made correctly.
There is an attempt at a logical structure with a line of NOTE: A clear and logically structured response would
reasoning. The information is in the most part relevant. include a correct energy cycle for ∆cH(C3H8)
using ∆rH AND 4 × ∆vapH in energy cycle or expression:
0 marks No response or no response worthy of credit.
ALLOW trailing zeroes OR minor slips
Total 10
How to answer it
Determining Enthalpy Change of Combustion of Propane
This question assesses practical calorimetry, calculation of enthalpy changes using experimental data (q = mcΔT), evaluation of experimental limitations, application of mean bond enthalpies, and thermodynamic cycles relating vaporization enthalpy to standard enthalpy of combustion.
Experimental Enthalpy of Combustion
Calculate ΔcH of propane from calorimeter data (3 significant figures).
📐 Step-by-Step Calculation
- Calculate heat energy released (q):
q = m × c × ΔT
m = 100 g, c = 4.18 J g⁻¹ K⁻¹, ΔT = 46.10 - 21.60 = 24.5 °C
q = 100 × 4.18 × 24.5 = 10241 J (or 10.241 kJ ) - Calculate moles of propane burned (n):
Mass lost = 99.218 - 98.976 = 0.242 g
M(C₃H₈) = (3 × 12.0) + (8 × 1.0) = 44.0 g mol⁻¹
n = 0.242 / 44.0 = 0.00550 mol - Calculate enthalpy change per mole (ΔcH):
ΔcH = - q / (n × 1000) = - 10241 / (0.00550 × 1000) = -1862 kJ mol⁻¹
Rounded to 3 sig fig: -1860 kJ mol⁻¹ (or -1.86 × 10³ kJ mol⁻¹ )
✅ Correct Answer
Final Answer: -1860 kJ mol⁻¹
❌ Common Errors & Traps
- Forgetting to apply a negative sign to an exothermic combustion reaction.
- Using an incorrect specific heat capacity (e.g., c = 4.2 instead of 4.18 J g⁻¹ K⁻¹).
- Failing to convert Joules to kilojoules (dividing by 1000).
🧠 Exam Technique
Always write down individual intermediate values (q in J, moles of fuel) clearly. Examiners can award ECF (Error Carried Forward) if your intermediate steps are logged correctly even if a final rounding error occurs.
Evaluating Experimental Limitations
Suggest two reasons why experimental ΔcH is less exothermic than database values.
💡 Key Knowledge
Calorimetry experiments in school laboratories suffer from significant systematic losses. Database values represent pure thermodynamic standards.
- Heat loss to the surrounding environment (beaker, air, clamp).
- Incomplete combustion of propane (producing carbon or carbon monoxide instead of only carbon dioxide).
- Non-standard conditions (temperatures and pressures deviating from 298 K and 100 kPa).
- Evaporation of water from the beaker during heating.
✅ Acceptable Points (Any two)
- Heat loss / heat released to surroundings.
- Incomplete combustion (must reference air/oxygen/combustion).
- Evaporation of water.
Hess's Law & Bond Enthalpies
Determine standard enthalpy of combustion of propane using bond enthalpies and vaporisation enthalpy (6-mark extended response).
📐 Step-by-Step Calculation
- Calculate bonds broken in Equation 3.1:
C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g)
Bonds broken: 2(C–C) + 8(C–H) + 5(O=O)
= 2(347) + 8(413) + 5(498) = 694 + 3304 + 2490 = +6488 kJ mol⁻¹ - Calculate bonds made in Equation 3.1:
Bonds made: 3 × 2(C=O) + 4 × 2(O–H) = 6(C=O) + 8(O–H)
= 6(805) + 8(464) = 4830 + 3712 = -8542 kJ mol⁻¹ - Calculate ΔrH for Equation 3.1:
ΔrH = Σ(Bonds broken) - Σ(Bonds made) = 6488 - 8542 = -2054 kJ mol⁻¹ - Account for state of water (Enthalpy of Vaporisation):
Equation 3.1 produces gaseous water (H₂O(g)), but standard combustion produces liquid water (H₂O(l)).
Water must condense: H₂O(g) → H₂O(l) is the reverse of vaporisation (-ΔvapH).
Since 4 moles of water are produced, we must subtract 4 × ΔvapH:
ΔcH(C₃H₈) = ΔrH - 4(ΔvapH)
= -2054 - (4 × 40.65) = -2054 - 162.6 = -2216.6 kJ mol⁻¹ (or -2217 kJ mol⁻¹ )
✅ Level 3 Response Criteria (5-6 Marks)
To secure top marks, responses must:
- Accurately calculate ΔrH for Equation 3.1 using correct bond quantities ( -2054 kJ mol⁻¹ ).
- Correctly link ΔrH with 4 × ΔvapH to find the final enthalpy of combustion.
- Present a clear, logically structured line of reasoning with proper scientific terminology.
❌ Common Level 2 / Level 1 Traps
- Sign error on vaporisation: Adding instead of subtracting 4 × ΔvapH (yielding -1891 kJ mol⁻¹).
- Stoichiometry errors: Forgetting that propane contains 2 C–C bonds and 8 C–H bonds, or missing the coefficient 5 for O=O bonds.
- Omitting state changes entirely and stopping at ΔrH.
🧠 Top-Level Exam Strategy
Constructing a mini thermochemical cycle or energy level diagram linking Equation 3.1, vaporisation, and combustion ensures your logical reasoning is transparent to the examiner, securing Level 3 access effortlessly.
Topics
Module 3: Periodic table and energy · Practical Activity Groups · PAG 3: Enthalpy determination · 3.2 Physical chemistry
Question and mark scheme from the OCR A-Level Chemistry examination, Unified chemistry (03), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.