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Specifications

combined science

chemistry

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Core Practicals

Core Practical 8

Combustion of alcohols (chemistry only)

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Introducing the Practical

Apparatus set up to measure the increase in temperature when the different fuels given are burnt.

  

This must lead to a comparison of the results for the four alcohols.

Risk Assessment

As a general rule, eye protection (goggles) must be worn for all practicals.

  

This risk assessment is provided as an example only, and you must perform your own risk assessment before doing this experiment.

Hazard
Possible Harm
Precuation
alcohols (highly flammable)
light wick with a lit splint, don't move a spirit burner while alight, keep lab well ventilated
methanol (toxic)
danage to health when in contact with the skin
avoid skin contact, wear gloves

Apparatus

Each group will need:

  • alcohol burners

  • 150 ml conical flasks

  • thermometers

  • retort stands and clamps

  • access to a balance (at least 2 d.p.)

  • 100 ml measuring cylinders

  • lighters or matches

  • methanol

  • ethanol

  • propan‐1‐ol

  • butan‐1‐ol

Experiment Set-up

Method

  1. secure a conical flask over a spirit burner so that the lid of the burner can be removed and replaced safely

  2. measure and record the mass of a spirit burner with its lid

  3. use a measuring cylinder to add 100 ml of cold water to the flask, and then record its temperature

  4. place the spirit burner underneath the flask, remove the lid and light the wick

  5. stir the water carefully with the thermometer until the temperature has increased by 20°C, then replace the lid to put the flame out

  6. measure and record the new mass of the spirit burner (with lid), and the maximum temperature that the water reached

  7. repeat steps 2-6 with different alcohols, starting with new water every time

Results and 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.

  

How is the temperature rise of water related to the number of carbon atoms in the alcohol molecules?

Exam Question and Model Answer

An investigation was carried out to find out which alcohol gives out the most energy when burned. The apparatus used included (see diagram above):

  • alcohol burners

  • 150 ml conical flask

  • thermometer

  • retort stands and clamp

  • balance

  • 100 ml measuring cylinder


Describe and explain three improvements that could be made to the method to obtain experimental values closer to the theoretical values of energy released.

[6 marks}

Level 1 (1-2 marks)

Wrapping the equipment in insulation or at least putting a lid on the beaker;
would reduce the heat lost after it is transferred to the water.

Level 2 (3-4 marks)

They could use a better conductor, so use a copper can instead of glass beaker - this would allow more heat to be transferred to the water.
Wrapping the equipment in insulation or at least putting a lid on the beaker would reduce the heat lost after it is transfered to the water.

Level 3 (5-6 marks)

The student should reduce the heat loss between the flame and the beaker by enclosing the apparatus/surrounding the equipment to recuce any drafts.
They could also use a better conductor, so use a copper can instead of glass beaker - this would allow more heat to be transferred to the water.
Wrapping the equipment in insulation or at least putting a lid on the beaker would reduce the heat lost after it is transfered to the water.

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