Enthalpy of Combustion of Alcohols

A simple water calorimeter is used to measure the heat released when different alcohols burn. By tracking the water’s temperature rise and the mass of fuel consumed, students estimate the molar enthalpy of combustion and compare fuels.

1. Gather an alcohol burner with a chosen alcohol, a conical flask, 100 mL of water, a thermometer, a balance, a lighter, and a draft shield if available.
2. Weigh the capped burner and record the initial mass.
3. Add 100 mL of water to the flask and record its initial temperature.
4. Place the burner under the flask, remove the cap, and ignite the wick.
5. Gently stir the water with the thermometer while heating so the temperature is uniform.
6. Extinguish the flame with the cap when the water temperature has increased by a preselected amount, for example 20 to 40 °C.
7. Record the final water temperature.
8. Allow the burner to cool briefly, recap, then reweigh to obtain the final mass.
9. Calculate the mass of alcohol burned from the mass difference.
10. Repeat for at least a second trial with the same temperature rise to improve reliability.
11. Perform the same procedure for other alcohols, keeping water volume and temperature rise constant.
12. For each trial, compute heat absorbed by water q = m × c × ΔT, then estimate ΔHcomb = −q ÷ n, where n is moles of alcohol burned.

PAG 3.3 Enthalpy of combustion - Dr David Boyce:

Combustion of Alcohols: Theory and Practical - Science Ready:

📄 Calorimetry Experiment - Learnable: https://www.learnable.education/year-11-chemistry-practical-investigation-calorimetry/

• Use different primary alcohols across a homologous series to observe the trend in ΔHcomb.
• Compare propan-1-ol and propan-2-ol to discuss isomer effects.
• Use an insulating jacket or wind shield to study heat loss reduction.

• Wear safety glasses and keep hair and loose clothing secured.
• Alcohols are highly flammable; keep fuels capped when not burning and away from ignition sources.
• Use tongs or heatproof gloves to handle hot glassware; glass may remain hot after heating.
• Work in a well ventilated area to avoid buildup of combustion products; avoid inhaling vapors.
• Do not move a lit burner; extinguish with the cap before relocating.
• Keep a fire extinguisher or fire blanket nearby and know how to use it.
• Avoid overheating water; do not allow the flask to boil dry or exceed safe temperatures.
• Wipe up any fuel spills immediately and allow surfaces to evaporate dry before ignition.

• Why is ΔH for combustion negative? (Because combustion is exothermic and releases heat to the surroundings, making the system’s enthalpy decrease.)
• What is the largest source of error in this setup? (Heat loss to the surroundings rather than all heat going into the water.)
• How would using a metal calorimeter cup with a lid change results? (It would reduce heat loss and increase the measured efficiency, giving ΔH values closer to reference data.)
• Why should the same temperature rise be used for all fuels? (To keep conditions comparable so differences arise from the fuels, not from varying experimental parameters.)
• How does molar mass or carbon chain length relate to ΔHcomb across alcohols? (Each additional CH2 unit adds similar bond energy changes, so |ΔHcomb| increases roughly by a constant increment along the series.)
• If 1.00 g of ethanol heats 100 g of water by 25 °C, what is q and what additional information is needed for ΔHcomb? (q = 100 × 4.18 × 25 = 10,450 J; you also need moles of ethanol burned to convert to kJ mol−1.)