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Topic: Thermochemistry / Fuels
Level: Ordinary & Higher Level
Exam Focus: Section A Practical + Calculations
1. Experiment Snapshot
Feature
Details
Experiment
To measure the heat of combustion of a liquid fuel such as ethanol.
Main skill
Temperature measurement, mass measurement, heat calculation and evaluation of heat loss.
Difficulty
⭐⭐⭐ Medium
Exam importance
⭐⭐⭐⭐⭐ Very high because it combines practical method with calculations.
Typical marks
8–18 marks depending on calculation and evaluation questions.
2. Aim
To determine the heat of combustion of a liquid fuel by burning a known mass of the fuel and using the heat released to warm a known mass of water.
3. Quick Theory
The heat of combustion of a substance is the heat released when one mole of the substance is completely burned in oxygen.
Heat released by water: q = mcΔT
Symbol
Meaning
Common Unit
q
Heat energy absorbed by water
J
m
Mass of water heated
g
c
Specific heat capacity of water
4.18 J g⁻¹ °C⁻¹
ΔT
Temperature rise of water
°C
The heat of combustion is usually expressed in kJ mol⁻¹. Combustion is exothermic, so the value is often written as negative.
Examiner Tip: Students often calculate q correctly but forget to convert from joules to kilojoules, or from grams of fuel to moles.
4. Apparatus and Chemicals
Apparatus
Chemicals
Spirit burner, copper can or calorimeter, thermometer, measuring cylinder, balance, clamp stand, draught shield, heatproof mat, lighter or splint.
Liquid fuel such as ethanol, propanol or methanol; water.
Safety: Wear goggles. Alcohol fuels are flammable. Keep fuel bottles away from flames. Use a heatproof mat. Allow hot apparatus to cool before touching.
5. Labelled Experiment Diagram
Heat of Combustion Setup
6. Procedure
Measure a known volume of water using a measuring cylinder and pour it into a copper can.
Record the mass of water. Since 1 cm³ of water has mass about 1 g, 100 cm³ of water has mass about 100 g.
Measure and record the initial temperature of the water.
Weigh the spirit burner with fuel and cap.
Place the burner under the copper can and light it.
Stir the water gently and record the highest temperature reached.
Extinguish the flame by replacing the cap on the burner.
Reweigh the spirit burner with fuel and cap.
Calculate the mass of fuel burned.
Use q = mcΔT to calculate the heat absorbed by the water.
Use the moles of fuel burned to calculate the heat of combustion in kJ mol⁻¹.
Examiner Tip: Always record the mass of the burner before and after burning. The difference gives the mass of fuel burned.
7. Results Table Template
Measurement
Example Value
Volume of water used
100 cm³
Mass of water
100 g
Initial temperature of water
20.0 °C
Final temperature of water
45.0 °C
Temperature rise, ΔT
25.0 °C
Mass of burner before
185.60 g
Mass of burner after
184.45 g
Mass of fuel burned
1.15 g
8. Calculation Method
Step 1: Calculate heat absorbed by water
q = mcΔT
Using m = 100 g, c = 4.18 J g⁻¹ °C⁻¹ and ΔT = 25.0 °C:
q = 100 × 4.18 × 25.0 = 10,450 J = 10.45 kJ
Step 2: Calculate moles of fuel burned
moles = mass ÷ relative molecular mass
For ethanol, C₂H₅OH, relative molecular mass = 46.
If 1.15 g ethanol burns:
moles = 1.15 ÷ 46 = 0.0250 mol
Step 3: Calculate heat of combustion
heat of combustion = heat released ÷ moles burned
Heat of combustion = 10.45 ÷ 0.0250 = 418 kJ mol⁻¹
Since combustion is exothermic: ΔH = −418 kJ mol⁻¹
Examiner Trap: This experimental value is usually much less exothermic than the data-book value because of heat loss to surroundings.
9. Sources of Error and Improvements
Source of Error
Effect
Improvement
Heat lost to surroundings
Water absorbs less heat, so calculated heat of combustion is too low.
Use draught shield, insulation, lid or better calorimeter.
Heat absorbed by copper can
Not all heat goes into water.
Include calorimeter heat capacity or use better apparatus.
Incomplete combustion
Less heat released; soot may form.
Ensure good oxygen supply and correct burner position.
Evaporation of fuel before/after burning
Mass loss may not all be due to combustion.
Keep burner capped when not burning and weigh quickly.
Temperature reading inaccurate
Wrong ΔT.
Stir water and read thermometer at eye level.
Flame too far from can
More heat lost to air.
Place burner close enough while avoiding soot and unsafe contact.
10. Student Common Mistakes
Forgetting to calculate the mass of fuel burned.
Using volume of water but forgetting to convert it to mass.
Forgetting that 1 cm³ water is approximately 1 g.
Using final temperature instead of temperature rise.
Forgetting to convert joules to kilojoules.
Forgetting to divide by moles of fuel burned.
Writing a positive enthalpy value for combustion without noting combustion is exothermic.
Not mentioning heat loss as the main reason for a low experimental value.
11. Examiner Tips
Tip 1: The most common formula is q = mcΔT.
Tip 2: Always calculate ΔT as final temperature − initial temperature.
Tip 3: Use mass of fuel burned, not mass of burner.
Tip 4: If asked why the result differs from the accepted value, mention heat loss, incomplete combustion and evaporation of fuel.
12. Examiner Traps
Trap: The water absorbs only some of the heat released by the fuel.
Trap: Temperature rise is not the final temperature.
Trap: A high temperature rise does not automatically mean a correct heat of combustion unless mass of fuel burned is considered.
Trap: Heat of combustion is per mole, so a mole calculation is needed.