Core practical
Investigate the combustion of alcohols
There are a number of ways that you could investigate the combustion of alcohols in Chemistry. This is an outline of the required steps to undertake one of these methods. It is important in this core practical to make and record a range of measurements accurately, including mass, temperature and volume.
Aims
To investigate the temperature rise produced in a known mass of water by the combustion of ethanol, propanol, butanol and pentanol. These alcohols can be supplied in spirit burners that have a wick and a lid.
Method
- Use a stand, boss and clamp to secure a steel or copper can over a spirit burner. Adjust the height of the can so that the lid of the burner can be removed and replaced safely.
- Measure and record the mass of a spirit burner with its lid.
- Use a measuring cylinder to add 100 cm3 of cold water to the can. Measure and record its temperature.
- Place the spirit burner underneath the can. Remove the lid and light the wick.
- Stir the water carefully with the thermometer. When the temperature has increased by about 20掳C, replace the lid to put the flame out.
- Measure and record the mass of the spirit burner with its lid, and the maximum temperature of the water.
- Repeat steps 2 to 6 with different alcohols, starting with fresh water each time.
Results
Record the results in a table. This table gives some example results for methanol.
| Temperature at start | Temperature at end | Change in temperature | Mass at start | Mass at end | Change in mass | |
| Methanol | 18掳颁 | 39掳颁 | 248.60 g | 248.01 g | ||
| Ethanol | ||||||
| Propanol | ||||||
| Butanol |
| Methanol | |
|---|---|
| Temperature at start | 18掳颁 |
| Temperature at end | 39掳颁 |
| Change in temperature | |
| Mass at start | 248.60 g |
| Mass at end | 248.01 g |
| Change in mass | |
| Ethanol | |
|---|---|
| Temperature at start | |
| Temperature at end | |
| Change in temperature | |
| Mass at start | |
| Mass at end | |
| Change in mass | |
| Propanol | |
|---|---|
| Temperature at start | |
| Temperature at end | |
| Change in temperature | |
| Mass at start | |
| Mass at end | |
| Change in mass | |
| Butanol | |
|---|---|
| Temperature at start | |
| Temperature at end | |
| Change in temperature | |
| Mass at start | |
| Mass at end | |
| Change in mass | |
Analysis
The change in mass is equal to the mass of fuel burned.
For each experiment, calculate the change in temperature and the change in mass.
Question
Calculate the change in mass and the change in temperature for methanol, using the results in the table.
Change in temperature = 39 - 18 = 21掳C
Change in mass = 248.60 - 248.01 = 0.59 g
For each experiment, calculate the change in temperature per gram of fuel burned.
Question
Calculate the change in temperature per gram of methanol burned, using the results in the table.
Change in temperature = 21掳C
Mass of fuel burned = 0.59 g
Change in temperature per gram of fuel burned = 21/0.59 = 35.6掳C/g
Compare the change in temperature per gram of fuel burned for all four alcohols. Describe the pattern seen in your results.
Evaluation
Heat loss from the water and the can is a major source of error in this practical. Describe one way in which this could be reduced.
Question
Explain why the height of the can above the wick should be kept the same for each experiment.
The distance between the bottom of the can and the wick affects the amount of energy transferred to the water by heating. It would affect the temperature rise if it was not kept the same.
Hazards, risks and precautions
It is important in this practical activity to use appropriate apparatus and methods. This includes the safe use and careful handling of substances.
Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:
| Hazard | Harm | Precautions |
| Alcohols are highly flammable liquids, and produce highly flammable vapours | Burns to the skin, fire and damage by fire | Light the wick with a lighted splint; do not move a spirit burner while it is alight; keep the lab well ventilated |
| Methanol is toxic in contact with the skin | Damage to health | Avoid skin contact |
| Hazard | Alcohols are highly flammable liquids, and produce highly flammable vapours |
|---|---|
| Harm | Burns to the skin, fire and damage by fire |
| Precautions | Light the wick with a lighted splint; do not move a spirit burner while it is alight; keep the lab well ventilated |
| Hazard | Methanol is toxic in contact with the skin |
|---|---|
| Harm | Damage to health |
| Precautions | Avoid skin contact |