Edexcel A-Level Chemistry Paper 3, June 2017: Question 4

21 marks · Hard difficulty · Practical Techniques and Data Analysis

Calculate the enthalpy change of combustion of methanol using experimental data from a calorimetry practical and analyze measurement uncertainties and errors.

Practise this question

Question

Question text

4 This question is about the enthalpy change of combustion of methanol.

A teacher asked two students to carry out a practical task to determine the enthalpy

change of combustion of methanol.

Both students were provided with the same apparatus and chemicals.

The following procedure was provided for the students.

Procedure

• Measure out 150 cm3 of distilled water, using a 250 cm3 measuring cylinder.

• Transfer the water to a copper calorimeter and note the initial temperature of the

water (to the nearest 0.5°C) in Table 1.

• Weigh the spirit burner containing methanol and record its mass in Table 1.

• Place the spirit burner under the copper calorimeter, as shown in the diagram.

• Ignite the spirit burner and burn the methanol, whilst stirring the water with the

thermometer.

• After heating the water for three minutes, extinguish the flame and immediately

record the highest temperature reached by the water.

• As soon as possible, reweigh the spirit burner containing the methanol and

record its mass in Table 1.

thermometer

copper calorimeter

water

spirit burner

methanol

The results of Student 1 are recorded in Table 1.

Mass of spirit burner plus methanol before burning / g 213.47

Mass of spirit burner plus methanol after burning / g 211.87

Mass of methanol burned / g

Highest temperature of the water / °C 64.5

Initial temperature of the water / °C 22.0

Temperature change of the water / °C

Table 1

12(a) Complete Table 1, giving the values to an appropriate number of decimal places.

*P48954A01236* (2)

(b) Write the equation that represents the reaction that occurs when the standard

enthalpy change of combustion of methanol, CH3OH(l), is measured. Include

state symbols.

(2)

methanol in kJ mol–1.

Give your answer to an appropriate number of significant figures.

Specific heat capacity of water = 4.18 J g–1 °C–1

Density of water = 1.00 g cm–3

(4)

(d) Student 1 compared the experimental value for the enthalpy change of combustion of

methanol obtained in part (c) with the standard value given on the internet.

The student’s value was less exothermic than the standard value.

Student 1 decided to evaluate the uncertainty in the measurements made in this

experiment.

(i) Student 1 used a 250 cm3 measuring cylinder to measure the volume of 150 cm3

distilled water. The uncertainty in this volume measurement is ±1 cm3.

Calculate the percentage uncertainty in the volume of distilled water that

Student 1 measured in the experiment.

(1)

*P48954A01336*

(ii) Compare and contrast the use of a 250 cm3 measuring cylinder to measure out the

150 cm3 distilled water with the use of a 25 cm3 measuring cylinder (uncertainty

±0.2 cm3 for each volume measurement) six times to measure the same volume.

(3)

(iii) Student 1 calculated the uncertainties in the remaining measurements. However,

14 Student 1 realised that the measurement uncertainties did not explain the difference

between the experimental value for the enthalpy change of combustion of methanol*P48954A01436*

calculated in part (c) and the value obtained from the internet.

Other than human error, give three reasons for the difference in the values.

(3)

… *P48954A01536*

(e) Student 1 decided to repeat the experiment.

Student 1 used the copper calorimeter and water from the first experiment and

recorded the initial temperature as 60.0°C.

Student 1 burned exactly the same mass of methanol as in the first experiment.

Explain, with a reason, how the value for the enthalpy change of combustion of

methanol from this experiment would differ, if at all, from the value obtained in the first

experiment.

(2)

(f ) Student 2 followed the original instructions provided, but extinguished the flame

after four minutes rather than after three minutes.

Explain how the value calculated by Student 2 for the enthalpy change of combustion of

methanol compared with that obtained in Student 1’s first experiment.

(2)

… 16

… *P48954A01636*

(g) Another student, Student 3, used the results from Student 1’s first experiment to

find the enthalpy change of combustion of methanol. Student 3 incorrectly used

a value of 46.0 g mol–1 for the molar mass of methanol.

State and justify how this mistake would affect the calculated value for the

enthalpy change of combustion of methanol.

(2)

(Total for Question 4 = 21 marks)

Mark scheme

Show the mark scheme

Question

Acceptable Answers Additional Guidance Mark

Number

4(a) 1.60 Do not award MP1 for “1.6” (must be to 2 D.P.) (2)

(1)

(+) 42.5 Do not award MP2 for “42.50” (must be to 1

(1) D.P.)

Penalise D.P. error once only

Question

Acceptable Answers Additional Guidance Mark

Number

4(b) CH3OH(l) + 1.5O2(g) → CO2(g) + 2H2O(l) (2)

Balanced equation

(1) Do not award multiples (enthalpy change is for

the complete combustion of one mole) for MP1

State symbols all correct

(1) MP2 depends on the award of MP1 or correct

species

Question

Acceptable Answers Additional Guidance Mark

Number

4(c) Example of calculation (4)

Calculation of energy change (= mcΔT = 150 x 4.18 x 42.5 =)

(1) 26647.5 (J)

Calculation of moles of CH3OH Moles CH3OH = 1.60/32 (= 0.05(00))

(1)

Calculation of energy ÷ moles CH OH 26647.5 = 532950 (J mol−1)

(1) 0.05(00)

Ignore any signs at this stage

-1 −533 (kJ mol−1)

ΔH final answer in kJ mol and negative sign

included Or

−530 (kJ mol−1)

and

ΔH final answer to 2 or 3 S.F.

(1)

Correct answer with no working gains full marks

Penalise incorrect units for MP4 only

Allow TE at each stage

Allow correct rounding to 2SF or more at each stage

Question

Acceptable Answers Additional Guidance Mark

Number

4(d)(i) (±)0.7 (%) Allow from 1 SF up to calculator value correctly (1)

rounded where

(% uncertainty =) (±) 1 x 100 = 0.66666…7 (%)

Allow 0.6 or 3

Do not award 0.66/0.6

Question

Acceptable Answers Additional Guidance Mark

Number

4(d)(ii) An answer that makes reference to the following points: Needs to show combined error in using the (3)

25 cm3 six times is greater than using 250 cm3

measuring cylinder once only

Calculation of the % uncertainty using the 25 cm3 Award MP1

measuring cylinder (1) EITHER

if multiplies errors:

100 x (0.2 /25) x 6 = 4.8%

If adds errors

100 x (1.2 /150) = 0.8%

Then any two from: Do not award (0.2 / 25) x 100 = 0.8 %

% uncertainty with use of 25 cm3 measuring cylinder

is greater (1)

Repeated use of the small measuring cylinder will

lead to greater transfer losses (1)

Repeated use will take more time (1)

Do not award ‘easier’ to use larger measuring

cylinder

Question

Acceptable Answers Additional Guidance Mark

Number

4(d)(iii) An answer that makes reference to any three of the Ignore (3)

following points: experiment carried out under non-standard

conditions

heat/energy loss (to the surroundings) (1) Ignore just ‘no lid’

evaporation of methanol / water from the calorimeter

(1)

incomplete combustion (of methanol) (1)

(specific) heat capacity of the calorimeter/apparatus Allow calorimeter has not been calibrated

has been ignored (1)

Question

Acceptable Answers Additional Guidance Mark

Number

4(e) An explanation that makes reference to the following (2)

points:

The second value will be less exothermic / less negative Allow ‘more positive’ or ‘smaller in magnitude’

(1) Do not accept ‘greater’ or ‘smaller’ for ‘less

negative’

Some energy will be used to boil the water/boiling Do not award just “the water boils”

water is endothermic

Water can only be heated to 100oC/

Temperature rise (measured) can only be (a maximum)

of 40oC

Greater heat losses in the 60oC to 100oC range

(1) Mark points M1 and M2 independently

Question

Acceptable Answers Additional Guidance Mark

Number

4(f) An explanation that makes reference to the following (2)

points:

Either

student 2’s value will be similar / the same (1)

(As) both the energy change and moles/mass (of Allow ‘temperature change’ for ‘energy change’

methanol) burned will be higher/

Ratio of energy change to moles/mass (of methanol)

burned will be the same/

The energy change is proportional to the moles/mass

(of methanol) burned (1)

student 2’s value will be less negative/ less Allow ‘more positive’ or ‘smaller in magnitude’

exothermic (1) or ‘smaller’ for ‘less negative’

greater heat loss because higher temperature/heated

for longer (1) Mark points MP1 and MP2 independently within

each route

Question

Acceptable Answers Additional Guidance Mark

Number

4(g) An answer that makes reference to the following points: (2)

(Calculated) value of moles (of methanol) burned will Allow both marks for a calculation using Mr of

be less / too small 46.0 (instead of 32.0), giving a final ΔH value

(1) (approx.) of −766 (kJ mol−1)

The calculated value will be more exothermic / more Allow ‘increase’ or ‘greater’ for ‘more negative’

negative (1)

MP2 depends on MP1

(Total for Question 4 = 21 marks)

Question

Acceptable Answers Additional Guidance Mark

Number

How to answer it

Enthalpy of Combustion of Methanol

What this question tests

This multi-part practical chemistry question assesses your ability to process calorimetric experimental data, construct balanced thermochemical equations with state symbols, perform enthalpy calculations involving heat capacity and moles, analyse measurement uncertainties, evaluate practical limitations of calorimetry, and predict the impact of experimental errors.

Question Part (a)

Table Completion & Data Processing

✅ Correct Answers

Mass of methanol burned: 1.60 g
Temperature change of water: (+)42.5 °C

❌ Common Errors

Failing to match the precision of the raw data (e.g. writing 1.6 instead of 1.60 ).
Omitting the positive sign for temperature change.

Marks available: 2

Question Part (b)

Equation for Standard Enthalpy of Combustion

✅ Correct Answers

CH₃OH(l) + 1.5O₂(g) → CO₂(g) + 2H₂O(l)

💡 Key Knowledge

Enthalpy of combustion is defined for the complete combustion of one mole of a substance. Therefore, use a balancing fraction ( 1.5 or 3/2 ) for oxygen.
State symbols must match standard conditions: liquid alcohol, gaseous oxygen, gaseous carbon dioxide, and liquid water.

Marks available: 2

Question Part (c)

Enthalpy of Combustion Calculation

📐 Step-by-Step Calculation

Calculate heat energy released (q = mcΔT):
q = 150 × 4.18 × 42.5 = 26647.5 J = 26.6475 kJ
Calculate moles of methanol:
Moles = mass / Mr = 1.60 / 32.0 = 0.0500 mol
Calculate enthalpy change per mole (ΔH = -q / n):
26647.5 / 0.0500 = 532950 J mol⁻¹ = 533 kJ mol⁻¹
Apply correct sign and significant figures:
Final Answer: -533 kJ mol⁻¹ (or -530 kJ mol⁻¹ to 2 SF).

❌ Common Calculation Traps

Forgetting to include the negative sign for an exothermic reaction ( ΔH must be negative).
Using the mass of methanol instead of the mass of water ( 150 g ) in the mcΔT equation.

Marks available: 4

Question Part (d)(i)

Percentage Uncertainty in Volume

✅ Correct Answers

(±)0.7 (%) (derived from (1 / 150) × 100 )

🧠 Exam Technique

Always state percentage uncertainty using the formula: (total error / measured value) × 100 . Account for both top and bottom meniscus readings on measuring equipment where applicable.

Marks available: 1

Question Part (d)(ii)

Comparing Measuring Equipment Uncertainties

✅ Correct Answers

Calculate % uncertainty using the 25 cm³ cylinder used 6 times: 100 × (0.2 / 25) × 6 = 4.8% (or cumulative error approach).
Conclude that the percentage uncertainty using the 25 cm³ measuring cylinder is significantly greater.
Identify practical drawbacks: repeated use leads to greater transfer losses and takes considerably more time.

Marks available: 3

Question Part (d)(iii)

Evaluating Experimental Discrepancies

💡 Key Knowledge (Any three)

Heat/energy loss to the surroundings (incomplete thermal transfer to the copper calorimeter).
Evaporation of methanol from the spirit burner or water from the calorimeter.
Incomplete combustion of methanol (producing carbon or carbon monoxide instead of carbon dioxide).
Ignoring the specific heat capacity of the copper calorimeter itself.

Marks available: 3

Question Part (e)

Higher Initial Temperature Evaluation

✅ Correct Answers

The second value will be less exothermic / have a smaller magnitude (less negative).
Reason: Starting at 60.0°C means water reaches boiling point (100°C) sooner; energy is lost to boiling/evaporation, and greater heat losses occur across the higher 60°C to 100°C temperature gradient.

Marks available: 2

Question Part (f)

Extended Burning Time Analysis

✅ Correct Answers

Route 1: Value will be similar/the same because the ratio of energy change to moles burned remains constant (energy change is proportional to mass burned).

Route 2: Value will be less exothermic/less negative due to greater heat loss over a longer heating duration.

Marks available: 2

Question Part (g)

Incorrect Molar Mass Impact

✅ Correct Answers

Calculated moles of methanol will be too small (using 46.0 g mol⁻¹ instead of 32.0 g mol⁻¹ ).
Dividing energy by a smaller number of moles results in a final calculated ΔH value that is more exothermic / more negative (e.g. approx -766 kJ mol⁻¹ ).

Marks available: 2

Topics

Physical Chemistry · Core Practicals · Core Practical 8: Determine the enthalpy change of a reaction using Hess’s law · Topic 8: Energetics I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.