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GCSEChemistryWJECRevision Notes2. Chemical Bonding, Application Of Chemical Reactions & Organic Chemistry2.5 Crude Oil, Fuels & Organic ChemistryEnergy from Fuels

Energy from Fuels (WJEC GCSE Chemistry)

Revision Note

Alexandra Brennan

Last updated

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 cm3 conical flask
  • 100 cm3 measuring cylinder
  • Thermometer
  • Electronic balance ± 0.01g
  • Spirit burners containing:
    • Methanol
    • Ethanol
    • Propanol
    • Butanol

Diagram

How to set up your apparatus 

measuring-energy-changes-apparatus

Method

  1. Use a measuring cylinder to measure 100 cm3 of water into a conical flask
  2. Clamp the conical flask to the clamp stand at a suitable height so the spirit burner can be placed below 
  3. Record the temperature of the water using the thermometer 
  4. Record the mass of the spirit burner (including lid) using the balance 
  5. Place the spirit burner under the conical flask and light it
  6. Allow the burner to heat the water until the temperature increases by approximately 40 oC
  7. Record the temperature of the water
  8. Extinguish the flame and record the mass of the spirit burner 
  9. 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 space released space per space gram space of space fuel space equals space fraction numerator mass space of space water space open parentheses straight g close parentheses cross times space temp space change space open parentheses degree straight C close parentheses space cross times space 4.2 over denominator mass space of space alcohol space open parentheses straight g close parentheses end fraction

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    

   

i)
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 space released space per space gram space of space fuel space equals space fraction numerator mass space of space water space open parentheses straight g close parentheses cross times space temp space change space open parentheses degree straight C close parentheses space cross times space 4.2 over denominator mass space of space alcohol space open parentheses straight g close parentheses end fraction

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 

Examiner 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. 

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    Alexandra Brennan

    Author: Alexandra Brennan

    Expertise: Chemistry

    Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.