AQA A-Level Chemistry Paper 2, 2021: Question 3

7 marks · Medium difficulty · Practical Techniques & Data Analysis

This question assesses students' understanding of experimental design and data accuracy when determining the enthalpy of combustion. It begins by asking for a suitable table to record necessary measurements (temperature and mass of fuel burnt). Students must then critique the use of less effective equipment (glass beaker vs. copper calorimeter), and identify why experimental results tend to be less exothermic than theoretical values. Finally, it tests their ability to suggest improvements to the apparatus setup to enhance accuracy.

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Question

Question text

03 A student does an experiment to determine a value for the enthalpy of combustion of

heptane.

Figure 2 shows some of the apparatus used.

Figure 2

03.1 Design a table to record all the readings necessary to determine an experimental

value for the enthalpy of combustion for heptane in this experiment.

[2 marks]

03.2 The student considered using a glass beaker on a tripod and gauze instead of the

clamped copper calorimeter.

Suggest two disadvantages of using a glass beaker on a tripod and gauze.

[2 marks]

Disadvantage 1

Disadvantage 2

03.3 Suggest two reasons why the value of enthalpy of combustion from this experiment is

less exothermic than a data book value.

[2 marks]

Reason 1

Reason 2

03.4 Suggest one addition to this apparatus that would improve the accuracy of the

enthalpy value obtained.

[1 mark]

Mark scheme

Show the mark scheme

Question Answers Additional Comments/Guidelines Mark

Temp/ oC Mass /g M1 for Temperature data including units M1

Initial Burner before M2 for Burner mass data including units M2

03.1 Final Burner after If either unit missing MAX 1

(∆T) (Mass heptane

burned)

Any two from: M1

Glass is a poorer conductor than copper Heat capacity of metal is less than glass or vice M2

03.2 versa

Tripod and gauze would reduce heat transfer

Tripod and gauze would have a fixed height above the flame

Heat loss to surroundings or to copper/calorimeter M1

03.3

Incomplete combustion M2

Use a wind shield( to reduce heat loss) Allow use a lid 1

03.4

Insulate the sides of the calorimeter

How to answer it

Study Guide: Measuring Enthalpy of Combustion (Heptane)

Q1. Designing a Results Table

Step-by-step: You need to measure the change in temperature and the mass of heptane burned.

	Temp / °C	Mass / g
Initial	e.g. 20.0	Burner before
Final	e.g. 36.0	Burner after
ΔT or Δmass	Final - Initial	(Heptane burned)

Common Error: Missing units or vague column headings like "before" and "after". Be specific!

Q2. Disadvantages of Using a Glass Beaker

Why not use a beaker on a tripod?

1: Glass is a poor conductor of heat → less efficient energy transfer to water.
2: Fixed height from the flame reduces control of heating → less efficient combustion.

Don't mention "glass can break" — that’s not a chemical disadvantage!

Q3. Why Is Experimental Enthalpy Less Exothermic?

Reason 1: Heat loss to surroundings — not all energy heats the water.
Reason 2: Incomplete combustion — not all heptane burns to CO₂.

Tip: Be specific about energy loss — "heat escapes" is too vague.

Q4. Improving Accuracy

Answer: Use a wind shield (or a lid / insulate sides of the calorimeter)

Anything that reduces heat loss = improved accuracy. Shields, lids or insulation all help.

Topics

Required Practicals · Physical Chemistry · Required Practical 2: Measurement of an enthalpy change · 3.1.4 Energetics

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.