What energy do fuels liberate when burnt?

• Fuel
• A source of oxygen (usually air)
• Heating to raise the fuel to a high enough temperature

Fuels are burned to provide us with energy. We often use this energy directly to keep us warm. Fuels are chemicals, which react with an oxidising agent, usually oxygen. Energy is released during the reaction and new chemicals are formed. Any substance that reacts with oxygen, or another oxidizing agent, could be used as a fuel.

The following diagram indicates what happens when a fuel burns.

I am going to plan and perform an experiment in order to find out what energy fuels liberate when burnt.

Possible variables for this experiment are:

• The temperature change
• The volume of water (cm3) being used

I will be changing two variables whilst finding out what energy fuels liberate when burnt. These are the type of fuel being burnt and the temperature change of the fuel. I have decided to change two variables because in order to be able to see how the change in temperature affects the amount of energy a fuel liberates, I will need to differ the temperature change. However, in order to compare the amount of energy different fuels liberate I will need to test a number of different fuels.

The following are the temperature changes that I will be using during this experiment:

• 5°C
• 10°C
• 15°C
• 20°C
• 25°C
• 50°C

The following are the different fuels that I will be burning during this experiment:

• Methanol (CH3OH)
• Ethanol (C2H5OH)
• Propan-1-ol (C3H7OH)
• Butan-1-ol (C4H9OH)
• Pentan-1-ol (C5H12OH)

In order to make sure that my results are precise and accurate, I will test each of the fuels at each temperature change three times.

Equipment

The equipment that I will be using for each individual experiment is listed and pictured below:

• Measuring cylinder
• Heat proof mat
• Clamp
• Pole
• Boss
• Splints
• Thermometer
• Bunsen burner
• Spirit burner (containing one of the five fuels)
• Copper pot
• 50 cm3 of water
• Balance scale

Copper pot containing 50 cm3 of water

Thermometer

Boss & clamp

                        Pole                                10cm

Balance scale                                Spirit burner

                                                                                                                                                                        Bunsen burner                                                                                Heat proof mat

Method

Before I begin the experiment I will set up the required equipment as pictured in the diagram above. Once it is all in the correct place, I will begin by weighing the spirit burner with its lid on (containing either methanol, ethanol, propan-1-ol, pentan-1-ol or butan-1-ol). Once I have recorded the weight I will then place the spirit burner under the copper pot containing 50cm3 of water and remove its lid. With a splint lit from the Bunsen burner, I will then light the wick. As soon as the temperature of the water has increased by the desired amount (either 5°C, 10°C, 15°C, 20°C, 25°C or 50°C) I will replace the lid of the spirit burner and weigh it for a second time. Whilst I weigh the burner to find the change in mass, my partner Louise will replace the water in the copper pot with another 50cm3  of water. I will repeat this procedure three times for each fuel at each of the required temperature changes.

In order for this experiment to be a fair test I will do the following:

• Use 50cm3 of water for each experiment
• Place the base of the copper pot 10cm away from the table top
• Make sure that the lid is placed on the spirit burner when it is being weighed
• Make sure that the temperature change is correct before replacing the lid of the spirit burner
• Change the water after each experiment

Prediction

I predict that as the molecular size increases, the smaller the amount of fuel it will take to heat 50cm3 of water by the required temperature change. This is because the more energy being given out to the surroundings by bonds breaking, the less energy per gram that will be required to heat the water.

• Methanol (CH3OH)

     H                                        C-H bond=413 KJ/mole (x3)= 1239 KJ/mole

                                        C-O bond= 336 KJ/mole

H-C-O-H                                O-H bond= 464KJ/mole

              

    H

Total energy change= -2039 KJ/mole (The answer is negative because it is an exothermic reaction and energy is given out to the surroundings in the from of heat).

• Ethanol (C2H5OH)

     H   H                                C-H bond=413 KJ/mole (x5) = 2065 KJ/mole

                                        C-O bond= 336 KJ/mole

H-C- C- O-H                                O-H bond= 464KJ/mole

              

    H   H

Total energy change = -2865 KJ/mole

• Propan-1-ol (C3H7OH)

     H  H  H                                C-H bond=413 KJ/mole (x7) = 2891 KJ/mole

                                        C-O bond= 336 KJ/mole

H-C- C- C- O-H                        O-H bond= 464KJ/mole

              

    H  H  H

Total energy change = -3691 KJ/mole

• Butan-1-ol (C4H9OH)

     H  H  H  H                                C-H bond=413 KJ/mole (x9) = 3717 KJ/mole

                                        C-O bond= 336 KJ/mole

H-C- C- C- C- O-H                        O-H bond= 464KJ/mole

              

    H  H  H   H

Total energy change = -4517 KJ/mole

• Pentan-1-ol (C5H11OH)

    H  H H H H                         C-H bond=413 KJ/mole (x11) = 4543 KJ/mole

                                        C-O bond= 336 KJ/mole

H-C-C-C-C-C-O-H                        O-H bond= 464KJ/mole

 

    H H H  H H

Total energy change = -5343 KJ/mole

Having illustrated the molecular sizes and calculated the bond energy change for each of the fuels I am able to predict that methanol will have the biggest mass change and that pentan-1-ol will have the smallest mass change. This is because pentan-1-ol has the biggest molecular size and releases a greater amount of energy from bonds being broken. Therefore less fuel will be required to heat 1g of water by 1°C. Methanol has the smallest molecular size and therefore little energy is released from bonds being broken. As a result, more fuel is required to heat 1g of water by 1°C.  

Therefore I predict that the mass change of the fuels will follow the order listed below (biggest mass change first):

• Methanol (CH3OH)
• Ethanol (C2H5OH)
• Propan-1-ol (C3H7OH)
• Butan-1-ol (C4H9OH)
• Pentan-1-ol (C5H11OH)

My results indicate that methanol lost the greatest mass when burnt and that pentan-1-ol lost the least mass. They also indicate that pentan-1-ol had the biggest energy change per gram and per mole, while methanol had the least energy change per gram and per mole.

The pattern that my results show is, the bigger the molecular size and bond energy changes, the lower the mass change and the bigger the energy change per gram and mole of fuel.

From analysing my results I am able to conclude that the molecular size of a fuel effects the amount of energy that it liberates when burnt. The bigger the molecular size of a fuel, the smaller the mass that it will lose when burnt. This is because, the greater the molecular size, the more bonds there are to be broken and the bigger the bond energy change. Therefore the bigger the bond energy change, the more energy the fuel gives out to the surroundings in the form of heat and the smaller the mass of fuel required to heat

1 g of water by 1°C.  

My prediction was correct, the bigger the molecular size of a fuel, the smaller the amount of fuel it will take to heat 50cm3 of water by the required temperature. This is because the bigger the bond energy changes, the more energy that is lost to the surrounding area in the form of heat.

My prediction was also correct because the mass decrease of the fuels followed the list below as predicted (biggest mass loss first):

• Methanol (CH3OH)
• Ethanol (C2H5OH)
• Propan-1-ol (C3H7OH)
• Butan-1-ol (C4H9OH)
• Pentan-1-ol (C5H11OH)