Measuring heat energy of fuels
The combustion of alcohols is exothermic and in this experiment the energy released from burning a known mass of alcohol is used to heat a known amount of water. A comparison of various alcohols as fuels can be made by calculating the quantity of energy transferred to the water.
Students could undertake this experiment in pairs or small groups, perhaps using a different alcohol each and then pooling their results. Alternatively they could obtain their own results for each alcohol if time allows.
Burners containing the alcohols (or dropper bottles) need to be prepared in advance (see Technical notes).
All students will need access to the following alcohols in labelled spirit burners (or in dropper bottles if using crucibles):
Methanol (HIGHLY FLAMMABLE, TOXIC)
Ethanol (HIGHLY FLAMMABLE) or Industrial Denatured Alcohol (IDA) (HIGHLY FLAMMABLE, HARMFUL)
Propan-1-ol (HIGHLY FLAMMABLE, IRRITANT)
Butan-1-ol (FLAMMABLE, HARMFUL)
Refer to Health & Safety and Technical notes section below for additional information.
Eye protection for each student
All students will need access to:
Balances (2 or 3 d.p.)
Each student or pair of students will need:
Clamp, stand and boss
Metal can (such as a copper calorimeter) or conical flask (250 cm3)
Measuring cylinder (100 cm3)
Spirit burner with cap or small crucible with wide lid (Note 1 to 4 and alternative method in teaching notes)
Stirring thermometer (0-110 °C)
Heat resistant mat
Matches or Bunsen burner (Note 5)
Balance (Note 6)
Health & Safety and Technical notes
Wear eye protection. Keep bottles of alcohol well away from flames. Keep all stock bottles of alcohol in the prep room during the lesson.
Methanol, CH3OH(l), (HIGHLY FLAMMABLE, TOXIC) - see CLEAPSS Hazcard.
Ethanol, C2H5OH(l), (HIGHLY FLAMMABLE) and Industrial Denatured Alcohol (HIGHLY FLAMMABLE, HARMFUL) - see CLEAPSS Hazcard.
Propan-1-ol, C3H7OH(l), (HIGHLY FLAMMABLE, IRRITANT) - see CLEAPSS Hazcard.
Butan-1-ol, C4H9OH(l), (FLAMMABLE, HARMFUL) - see CLEAPSS Hazcard.
1 If possible, provide spirit burners which have removable glass caps or, if using crucibles, provide lids. This makes it easier and safer to extinguish the flames.
2 Make sure the wick fits tightly in the wick holder and that the wick holder fits tightly in the burner.
3 If burner is large capacity, reduce it by partly filling with epoxy resin or packing with cotton wool.
4 Fill and label spirit burners or dropper bottles with alcohols in advance of the lesson. Ensure any excess alcohol is wiped off the side of the burners or dropper bottles.
5 Careful consideration must be given to how the students will ignite the alcohols. They could be given matches. Alternatively, a single Bunsen burner could be provided at the front of the lab (well away from the spirit burners or dropper bottles) and the students given access to wooden spills. The spirit burners must be kept upright when lighting. Do not tip onto the side.
6 The experiments should be carried out well away from the balances. Ensure, if possible, that students have access to balances at a number of different stations around the lab.
a Clamp the metal can (or flask) at a suitable height to allow room for the spirit burner to be placed below. Allow a gap of around 2-5 cm between the base of the flask/can and the top of the spirit burner. This gap may need to be adjusted depending on the height of the flame.
b Using the measuring cylinder, fill the metal can with 100 cm3 of water.
c Using the thermometer, measure and record the initial temperature of the water.
d Weigh the spirit burner (and cap) containing the alcohol and record the initial mass and name of the alcohol in a suitable table.
e Place the spirit burner on the heat-resistant mat under the metal can, remove the cap, and light the wick.
f Allow the alcohol to heat the water so the temperature rises by about 40°C. Use a glass rod or the thermometer to stir the water gently whilst the alcohol burns.
g Replace the cap on the spirit burner to extinguish the flame.
h Record the final temperature of the water using the thermometer. Work out the temperature change.
i Reweigh the spirit burner and cap. Work out the mass of alcohol used.
j Repeat the experiment for different alcohols using 100 cm3 of fresh cold water each time.
k If time allows repeat the experiment for each alcohol at least twice.
Students should use their measurements to work out the temperature change and the mass of alcohol burned for each alcohol.
The energy transferred to the water from the burning alcohol can be calculated using the equation
q = mcΔT
where q = energy transferred (in J), m = mass of water (in g) c = specific heat capacity of water (in J/[g°C]) and ΔT = temp change (in °C or K).
Assume that 1 cm3 of water has a mass of 1 g.
Assume that the specific heat capacity of water, c = 4.18 J/(g°C) or c = 4.18 J g-1 K-1 (i.e. 4.18 J are required for every 1 °C rise in temperature per g of water).
Students could be asked to work out the answer which fuel (alcohol) provides the most energy per gram of fuel burned?
The experimental results are often much less than values given in data books, or predicted using models and the energies needed to break the bonds. This is largely due to the fact that not all the energy produced is transferred to the water. Some is used to heat the air and surroundings, plus the flask. Also, combustion of the alcohols is likely to be incomplete. You may like to ask the students how they could tell this from their observations of the fuels burning.
Additional work: Students could investigate methods of reducing heat loss to the environment and thus discuss how the experiment could be improved.
For advanced courses, the experiment can be adapted to determine values for the enthalpy of combustion. The experimental set-up can be calibrated by using, say, propan-1-ol, and then the method used for other alcohols. This gives more accurate results.
If using small crucibles to burn the alcohol then students should:
a Measure the mass of the crucible and, well away from naked flames, add about 0.5 g of the alcohol from the dropper bottle. Measure the mass again.
b Place the crucible on the heat resistant mat under the conical flask and ignite the alcohol.
c Stir the water in the flask with the glass rod or thermometer and record the highest temperature reached by the water.
d Allow all the alcohol to burn away.
Health and Safety checked November 2006
Page last updated on 02 December 2011