OCR A-Level Chemistry AS Depth in chemistry (02), November 2020: Question 2

12 marks · Medium difficulty · Structured Questions

Define enthalpy change of combustion, calculate $\Delta_c H$ for cyclohexane from experimental calorimetry data, and evaluate the procedure including percentage uncertainties and sources of error.

Practise this question

Question

Question text

2 Enthalpy changes of combustion can be determined directly by experiment.

(a) Explain the term enthalpy change of combustion, ΔcH.

… [2]

(b) A student carries out an experiment to determine the enthalpy change of combustion of

cyclohexane, C6H12, using the apparatus shown in the diagram.

thermometer

beaker

clamp 200 cm3 water

wick spirit burner

cyclohexane

In the experiment, 0.525 g of cyclohexane are burnt, and the temperature of the 200 cm3 of

water changes from 21.0 °C to 41.0 °C.

Calculate the enthalpy change of combustion, Δ H, of cyclohexane in kJ mol−1.

Give your answer to 3 significant figures.

Δ H = … kJ mol–1 [4]

data book.

The student evaluates the experimental results.

(i) The uncertainty in each thermometer reading is ±0.5 °C and the uncertainty in the

measured volume of water is ±2 cm3.

Determine whether the temperature change or the measured volume of water has the

greater percentage uncertainty.

[2]

(ii) Suggest two reasons, apart from measurement uncertainties, why the experimental

value for ΔcH is less exothermic than the data book value.

Reason 1 …

Reason 2 …

[2]

(iii) In the experiment the water in the beaker was heated for 5 minutes. The student thought

that the experiment could be improved by heating the water for 10 minutes.

Explain whether the accuracy in the student’s calculated value for ΔcH may or may not

be improved by heating for longer.

… [2]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

element

2 (a) (The enthalpy change) for complete combustion 2 1.1 ×2 ALLOW energy change for combustion in excess

oxygen

OR reacts in excess oxygen

OR reacts completely in oxygen

OR energy released during complete combustion

OR energy change for combustion in excess air

IGNORE energy required

of 1 mol (of substance) ALLOW element OR compound OR reactant

DO NOT ALLOW atoms

(b) FIRST CHECK ANSWER ON THE ANSWER LINE 4 ALLOW 16700 J or 16.7 kJ up to calculator value

If answer = – 2680 (kJ mol–1) award 4 marks of 16720 J (Must be at least 3 SF)

If answer = (+) 2680 (kJ mol-1) award 3 marks

Energy released in J OR kJ = 200 × 4.18 × 20.0 3.1 ×2

= 16720 (J) OR 16.72 (kJ) ALLOW ECF from incorrect M(C6H12) or energy

0.525 change

n(C6H12) = = 0.00625 (mol)

16.72 –1 IF energy released above rounded to 16700,

Energy per mole = OR (–)2675.2 (kJ mol ) 3.2 ×1 Energy per mole = (–)2672 by ECF 3 marks

0.00625

∆cH = – 2670 to 3SF 4 marks

∆cH = – 2680 (kJ mol–1)

Value to 3SF COMMON ERROR

1.2 ×1 -7.02 (kJ mol-1) award 3 marks

AND ‘–‘ sign

% uncertainty in temp. rise = × 100 = 5%

2 Award 1 mark if uncertainties are given as 0.05

% uncertainty in volume = × 100 = 1% AND 0.01 with correct statement

AND temp rise has greater % uncertainty

Question Answer 8 Marks Guidance

element

(ii) Any two from: 2 3.2 ×2

Heat released to the surroundings ALLOW heat loss

Incomplete combustion OR incomplete reaction IGNORE reference to evaporation

OR not everything burns

Non-standard conditions

(iii) Less accurate due to greater heat losses 2 3.4 ×2 ALLOW less accurate due to evaporation of water

More accurate due to smaller % uncertainty in ALLOW error for uncertainty

temperature change OR mass of fuel burnt

ALLOW for both marks

May not change as

increase in temperature change

OR increase in mass of fuel burned would

decrease % uncertainty

BUT

may be outweighed by increased heat loss to

surroundings

OWTTE

How to answer it

Enthalpy Change of Combustion of Cyclohexane

📋 Revision Overview

What this question tests

This question evaluates your core knowledge of experimental calorimetry and error analysis:

Recalling the standard definition of enthalpy change of combustion (Δ_cH).
Executing multi-step thermochemical calculations using q = mcΔT , converting units (J to kJ), and determining molar enthalpy changes to the correct sign and significant figures.
Calculating percentage uncertainties (accounting for two temperature readings from a single thermometer).
Identifying procedural heat loss mechanisms and systematically evaluating trade-offs between measurement uncertainty and heat transfer to surroundings.

Part (a) — Definition of Enthalpy Change of Combustion

Explaining Δ_cH [2 Marks]

✅ Model Answer

The enthalpy change that takes place when 1 mole of a substance undergoes complete combustion (or reacts completely in excess oxygen).

💡 Key Knowledge

Mark 1: Complete combustion / complete reaction in excess oxygen.
Mark 2: Specifically 1 mole of substance / compound / reactant.

❌ Common Errors

Writing "1 mole of atoms" (Strictly penalized by OCR; say "substance" or "compound").
Omitting the word "complete" when referring to combustion.
Writing "energy required" instead of "enthalpy change" (combustion is exothermic; it releases energy).

Examiner Insight: Precision is essential for definition marks. Ensure both conditions are clearly articulated: "1 mole" and "complete combustion".

Part (b) — Calorimetry Calculation

Calculating Δ_cH from Experimental Data [4 Marks]

📐 Step-by-Step Calculation

Calculate heat transferred to water (q):
m = 200 g (since 1 cm³ water = 1 g)
ΔT = 41.0 − 21.0 = 20.0 °C
q = mcΔT = 200 × 4.18 × 20.0 = 16,720 J = 16.72 kJ
Calculate moles of cyclohexane burnt:
M_r(C₆H₁₂) = (6 × 12.0) + (12 × 1.0) = 84.0 g mol⁻¹
n = 0.525 / 84.0 = 0.00625 mol
Calculate enthalpy change per mole:
Energy per mole = 16.72 / 0.00625 = 2675.2 kJ mol⁻¹
Apply sign and 3 significant figures:
Because the reaction is exothermic (temperature increased):
Δ_cH = −2680 kJ mol⁻¹

🧠 Exam Technique & Mark Breakdown

Mark 1: Correct calculation of q ( 16720 J or 16.72 kJ ).
Mark 2: Correct moles of C₆H₁₂ ( 0.00625 mol ).
Mark 3: Evaluating q / n ( 2675.2 kJ mol⁻¹ ).
Mark 4: Final answer given to 3 sig figs WITH the negative sign ( −2680 ).

❌ Common Calculation Traps

Using the mass of fuel in q = mcΔT: Substituting m = 0.525 g gives −7.02 kJ mol⁻¹ . Remember: m is always the mass of the material whose temperature changes (the water)!
Forgetting the negative sign: Combustion is exothermic. Quoting +2680 caps the score at 3 out of 4 marks.
Incorrect rounding: Forgetting the "3 significant figures" instruction and leaving the answer as −2675 or −2675.2 loses the final mark.

Part (c)(i) — Percentage Uncertainty Comparison

Temperature Change vs. Volume [2 Marks]

📐 Uncertainty Calculations

1. Temperature change (ΔT):
A temperature change requires two independent readings (initial and final).
Total uncertainty = 2 × (±0.5 °C) = ±1.0 °C
% uncertainty = (1.0 / 20.0) × 100 = 5.0%

2. Water volume:
Measured once with a measuring cylinder.
% uncertainty = (2 / 200) × 100 = 1.0%

Conclusion:
The temperature change has the greater percentage uncertainty.

🧠 Exam Technique: Two Readings Trap

Whenever you calculate an uncertainty for a difference (such as a temperature rise or a titration titre), you must multiply the instrument uncertainty by 2 unless told otherwise.

Mark Scheme Note: 1 mark for calculating both percentages correctly ( 5% and 1% ); 1 mark for explicitly concluding that temperature change has the greater percentage uncertainty.

Part (c)(ii) — Discrepancies in Experimental Enthalpy

Why the Experimental Value is Less Exothermic [2 Marks]

✅ Any Two Valid Reasons

Heat loss to the surroundings: Heat escapes to the air or warms the beaker/clamp rather than the water.
Incomplete combustion of cyclohexane: Some fuel burns to form carbon monoxide (CO) or soot (C) rather than CO₂, releasing less energy.
Non-standard conditions: The experiment was not performed at standard temperature and pressure (298 K, 100 kPa).

❌ Disallowed Responses

Do NOT list measurement uncertainty (the question explicitly states "apart from measurement uncertainties").
Do NOT state "evaporation of water" or "evaporation of fuel" for this part (the mark scheme states: IGNORE reference to evaporation).

Part (c)(iii) — Evaluating Experimental Modifications

Heating for 10 Minutes vs. 5 Minutes [2 Marks]

💡 The Competing Factors (2 Marks)

Heating for longer (10 minutes) creates two opposing effects on experimental accuracy:

Factor 1 (Reduces accuracy): Water reaches a higher temperature, causing greater heat losses to the surroundings (or increased evaporation of water).
Factor 2 (Increases accuracy): Larger temperature rise (and greater mass of fuel burned) results in a smaller percentage uncertainty in measurements.

✅ How to Structure Full-Mark Explanations

You can argue either perspective, or synthesize both:

Argument A: Less accurate because of significantly greater heat losses to the surroundings.
Argument B: More accurate because a larger ΔT / larger fuel mass decreases percentage uncertainty.
Balanced Synthesis: Accuracy may not change because the reduction in percentage uncertainty is counterbalanced/outweighed by greater heat loss to surroundings.

Examiner Insight: Candidates who scored both marks demonstrated deep evaluative thinking by identifying both the beneficial reduction in percentage uncertainty and the detrimental increase in heat dissipation.

Topics

Module 1: Development of practical skills in chemistry · Module 3: Periodic table and energy · Practical Activity Groups · 1.1 Practical skills assessed in a written examination · 3.2 Physical chemistry · PAG 3: Enthalpy determination

Question and mark scheme from the OCR A-Level Chemistry examination, AS Depth in chemistry (02), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.