Practical: The temperature rise in water by the combustion of alcohols

It is important in this practical to make and record a range of measurements accurately, including mass, temperature and volume. It is also important to handle harmful and flammable liquids safely, as well as taking care to avoid hazards with hot apparatus.

This method outlines one way to carry out the practical using four different alcohols - methanol, ethanol, propanol and butanol. The reaction can also be carried out using other alcohols, or other fuels. Eye protection must be worn.

Aims

To investigate the temperature rise produced in a known mass of water by the combustion of different alcohols. These alcohols can be supplied in spirit burners that have a wick and a lid.

Method

A spirit burner using alcohol is underneath a conical flask containing water and a thermometer, held up by a clamp stand.
  • use a measuring cylinder to add 100 cm3 of water to the conical flask
  • clamp the conical flask at a suitable height above the spirit burner
  • record the initial temperature of the water in the conical flask
  • place some alcohol into the spirit burner, put the lid on and record the mass
  • place the spirit burner under the conical flask, remove the lid and ignite it
  • stir the water gently with the thermometer and continue until the temperature of the water increases by about 30°C
  • extinguish the flame of the spirit burner and put the lid on
  • record the final temperature of the water in the conical flask
  • record the final mass of the spirit burner, including the unburned alcohol and lid
  • repeat the experiment with a different alcohol in the spirit burner

Record the masses of the spirit burner, alcohol and lid, and the temperatures for each experiment in a suitable table. The tables below give some example results for methanol.

Temperature

AlcoholAt startAt endChange
Methanol18°C59°C
Ethanol
Propanol
Butanol

Mass

AlcoholAt startAt endChange
Methanol248.60 g247.41 g
Ethanol
Propanol
Butanol

Analysis

  • calculate the temperature change and the mass change for the experiment using methanol
  • calculate the energy released during the combustion of methanol and the other alcohols using this equation:
    • energy released per gram of alcohol (J) = mass of water (g) × temperature change (°C) × 4.2 ÷ mass of alcohol (g)
Question

Calculate the change in temperature and the change in mass for methanol, using the results in the table.

Change in temperature = 59 - 18 = 41°C

Change in mass = 248.60 - 247.41 = 1.19 g

Question

Calculate the energy released per gram of methanol burned, using the results in the table.

Change in temperature 41°C

Mass of fuel burned = 1.19 g

Energy released per gram of alcohol = 100 g × 41°C × 4.2 ÷ 1.19 g = 14.5 kJ

Evaluation

Question

Explain why water in the conical flask is stirred while it is being heated.

The water is stirred to make sure that the temperature is consistent throughout.

Question

Explain why it is important that the lid is kept on the spirit burner when it is not in use.

The lid prevents evaporation of the alcohol, which would make the experiment less accurate.

Hazards, risks and precautions

Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:

HazardHarmPrecaution
Alcohols are flammableMay cause burns, or firesMake sure that spirit burner has the lid in place. Clean up any spillages quickly
Clamping the conical flaskGlass may shatter and lead to cutsDo not clamp the flask too tightly
Conical flask will get hotMay cause burnsDo not heat water to a high temperature. Let apparatus cool down before dismantling