Energy from Fuels | WJEC GCSE Chemistry Revision Notes 2018

Specified Practical: Determining Energy Released from Fuels

Introduction

When a fuel, such as alcohol, burns it reacts with oxygen to release energy
Carbon dioxide and water are also produced
The general equation for the combustion/burning of an alcohol is:

alcohol + oxygen → carbon dioxide + water

Different alcohols will release different amounts of energy when they burn
This investigation will determine which fuel releases the most energy per gram when it burns in oxygen

Apparatus

Clamp stand
Clamp and boss
250 cm³conical flask
100 cm³ measuring cylinder
Thermometer
Electronic balance ± 0.01g
Spirit burners containing:
- Methanol
- Ethanol
- Propanol
- Butanol

Diagram

How to set up your apparatus

Method

Use a measuring cylinder to measure 100 cm³ of water into a conical flask
Clamp the conical flask to the clamp stand at a suitable height so the spirit burner can be placed below
Record the temperature of the water using the thermometer
Record the mass of the spirit burner (including lid) using the balance
Place the spirit burner under the conical flask and light it
Allow the burner to heat the water until the temperature increases by approximately 40 ^oC
Record the temperature of the water
Extinguish the flame and record the mass of the spirit burner
Repeat steps 1-8 with the other alcohols

Practical Tip

Make sure the thermometer does not touch the bottom of the conical flask or the temperature will be inaccurate
Alcohols are highly flammable so make sure this experiment is conducted in a well ventilated lab and away from other flames

Analysis of Results

You will be required to create a suitable results table for this experiment:

Fuel	Initial mass of burner (g)	Final mass of burner (g)	Change in mass of burner (g)	Initial temperature (^oC)	Final temperature (^oC)	Change in temperature (^oC)	Energy released per gram (J)
Methanol
Ethanol
Propanol
Butanol

Evaluation

Calculate the temperature rise for each fuel
Calculate the mass of each alcohol burnt
Calculate the energy released for each alcohol using:

$energy released per gram of fuel = \frac{mass of water (g) \times temp change (° C) \times 4.2}{mass of alcohol (g)}$

Conclusion

Describe what the results show, for example:
- Which fuel releases the most energy per gram?
- Which fuel releases the least energy per gram?
Describe any patterns in the results
- Is there a relationship between the size of the molecule and the amount of energy released per gram
- Remember, the size of the alcohols beginning with the smallest is:
  - methanol < ethanol < propanol < butanol

Worked example

A student investigated how the amount of energy released per gram of fuel varies depending on the fuel being burned.

They obtained the following results.

Fuel	Initial mass of burner (g)	Final mass of burner (g)	Change in mass of burner (g)	Initial temperature (^oC)	Final temperature (^oC)	Change in temperature (^oC)	Energy released per gram (J)
Methanol	205.63	203.38		21	56
Ethanol	203.88	202.51		21	53
Propanol	202.35	201.17		20	50
Butanol	202.15	200.19		20	73

Calculate the change in mass and temperature.

ii)

Calculate the energy released per gram.

iii)

Identify the fuel which releases the most energy per gram.

iv)

Describe the relationship between the size of the alcohol and the energy released per gram.

Answers:

i) The change in mass and temperature are:

Fuel	Initial mass of burner (g)	Final mass of burner (g)	Change in mass of burner (g)	Initial temperature (^oC)	Final temperature (^oC)	Change in temperature (^oC)	Energy released per gram (J)
Methanol	205.63	203.38	2.25	21	56	35
Ethanol	203.88	202.51	1.37	21	53	32
Propanol	202.35	201.17	1.18	20	50	30
Butanol	202.15	200.19	1.96	20	73	53

ii) To determine the energy released per gram use:

$energy released per gram of fuel = \frac{mass of water (g) \times temp change (° C) \times 4.2}{mass of alcohol (g)}$

Fuel	Initial mass of burner (g)	Final mass of burner (g)	Change in mass of burner (g)	Initial temperature (^oC)	Final temperature (^oC)	Change in temperature (^oC)	Energy released per gram (J)
Methanol	205.63	203.38	2.25	21	56	35	6533
Ethanol	203.88	202.51	1.37	21	53	32	9810
Propanol	202.35	201.17	1.18	20	50	30	10 678
Butanol	202.15	200.19	1.96	20	73	53	11 357

iii) The fuel which releases the most energy per gram is butanol at 11 357 J per gram

iv) Looking at the relationship between the size of the alcohol and the energy released:

The larger the molecule, the more energy released per gram

Exam Tip

You are not expected to be able to recall the equation to calculate the energy released per gram- this would be provided to you in an exam.

Energy from Fuels (WJEC GCSE Chemistry)

Revision Note