Energy Released From the Complete Combustion of Different Alcohols

        “C”        is the specific heat of water        =        4.2        J/g K

        “M”        is the mass of water        =        (kg)

        “m1”        is the initial mass of burner        =        (g)

        “m2”        is the final mass of burner        =        (g)

        “t1”        is the initial temperature of water        =        (°C)

        “t2”        is the final temperature of water        =        (°C)

9. Apply the above steps for the other four fuels and record your results in a table form.

6. For fair test:

  In order to achieve a fair test, these experiments should be carried out using the same following conditions:

•   Same apparatus.
•  Same volume of water (200 cm3, using a measuring cylinder).
•  Same initial temperature (room temperature).
•  Same final temperature of water (75°C).
•  No soot should be on the can from an experiment to the other.
•   I have used 200 cm3, because I found that it was a suitable volume. Since using 100 cm3 of water, may cause the thermometer to touch the bottom of the can and using 300 cm3 of water may cause the metal can to fall; because of the heavy weight.                        

 

7. Measurements:

•  I am going to measure the initial and the final reading of the spirit burner in order to get the mass of fuel consumed.
•  I am going to measure the number of moles used from every fuel.

8. Prediction: 

 I predict that there will be a positive correlation between the heat value in Joules and the number of carbon atoms present in each fuel. Such that pentanol would have the highest heat value because it has the biggest number of carbon atoms, while methanol would have the lowest heat value because it has the smallest number of carbon atoms. In addition I predict that the mass of the pentanol fuel consumed during the reaction would be the smallest because it has the highest heat value, on the other hand the mass of the methanol consumed during the reaction would be the biggest because it has the lowest heat value. I can predict that the bigger the molecular structure in the alcohol the more energy it will take to break the bonds.

9-Precautions:

1. For personal safety:

• Wear safety goggles since alcohols are extremely flammable and to handle the hot water.
• Wear a lab coat also to prevent alcohols from reaching our skin.
• Don't touch broken equipments directly with your hand not to get hurt.
• The windows and the door of the laboratory should be opened in case of any alcohol catching fire.
•  Never enter the lab to do such an experiment without the presence of a fire extinguisher.

2. For accurate results:

• Put an insulator around the used apparatus to decrease heat lost by radiation.
• There should be a wide opening in the apparatus to allow sufficient air to enter for complete combustion of the fuel.
• Don't let water boils because in this case there would be fuel consumption against no further increase in temperature.
• Use a can of thin walls to decrease heat gained by it.
• The thermometer shouldn't touch the metal can, or else it would give a higher temperature than the true one.
• Use an accurate balance with two digit accuracy, to get accurate results.
• Clean the can from outside by water each time after utilization.

10- Resources:

●        

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●        Earl Band Wilford LDR: chemistry 2nd edition.

●        GCSE in a week: with Dan Evans and Alex Watts.

●        GCSE success: with Emma Poole.

II. (OBTAINING EVIDENCE)

1.        Results:

a)        With Ethanol:

1-        The mass of spirit burner before heating        =        233.07g

2-        The mass of empty can        =        31.78g

3-        The mass of can with water        =        230.05g

4-        The mass of water        =        194.03g

5-        The initial temperature of water        =        18.0°C

6-        The temperature of water after heating        =        75.0 °C

7-        The mass of spirit burner after heating        =        225.81g

Heat given by fuel        =        Heat absorbed by water

        Moles        x        ΔH        =        M        x        C        x        ΔΘ

                X        ΔH        =        M        x        4.18        x        (t2-t1)

ΔH Ethanol        =                =        − 292.914 kJ/mol

b)        With Propanol:

1-        The mass of spirit burner before heating        =        253.43g

2-        The mass of empty can        =        30.51g

3-        The mass of can with water        =        220.33g

4-        The mass of water        =        189.76g

5-        The initial temperature of water        =        18.0°C

6-        The temperature of water after heating        =        75.0 °C

7-        The mass of spirit burner after heating        =        249.65 g

Heat given by fuel        =        Heat absorbed by water

        Moles        x        ΔH        =        M        x        C        x        ΔΘ

                X        ΔH        =        M        x        4.18        x        (t2-t1)

ΔH Propanol        =                =        − 717.654 kJ/mol

c)        With Butanol:

1-        The mass of spirit burner before heating        =        259.91 g

2-        The mass of empty can        =        29.91g

3-        The mass of can with water        =        226.45 g

4-        The mass of water        =        196.54 g

5-        The initial temperature of water        =        18°C

6-        The temperature of water after heating        =        75.0 °C

7-        The mass of spirit burner after heating        =        256.02 g

Heat given by fuel        =        Heat absorbed by water

        Moles        x        ΔH        =        M        x        C        x        ΔΘ

                X        ΔH        =        M        x        4.18        x        (t2-t1)

ΔH Propanol        =                =        − 890.808 kJ/mol

These calculations should be applied to the two other alcohols.

2.        For Personal Safety:        

• Wear safety goggles since alcohols are extremely flammable and to handle the hot water.
• Wear a lab coat also to prevent alcohols from reaching our skin.
• Don't touch broken equipments directly with your hand not to get hurt.

III. (ANALYSING EVIDENCE)

A.        Comments On The Data:

The graph shows that, when the molecular mass of the alcohol is increased, the heat value is also increased. This direct relationship can be explained by the fact that when the carbon atoms increase in an alcohol, the number of hydrogen atoms increase and therefore more oxygen is required for the complete combustion of one mole and thus more energy is released from one mole of the fuel. The line of best fit used in the graph is began from 17, this is because the relative molecular mass (Mr) of "OH" is in each type of alcohol is 17, since the mass number of hydrogen is 1 and the mass number of oxygen is 16 therefore, 16+ 1 = 17. Burning fuels is an exothermic reaction, because there is heat released and the total energy released in bond formation is greater than that energy needed for bond breaking.

B. Estimating The Theoretical Heat Value:

The table contains average bond energies.

1-        For Ethanol ‘C2H5OH’

        C2H5OH        +        3 O2        2 CO2        +        3 H2O

                                        H        H        

                                H        C        C        O        H        +        O        O                        O        C        O        +        H        O        H

                                        H        H        

 (a)        The total amount of energy needed to break the bonds of reactants.

(b)        The total amount of energy released during forming the bonds of products.

(c)        The net theoretical energy change of the complete combustion of one mole of ethanol, in kJ mol-1, shows that the reaction is exothermic where the ΔH value is −1012 kJ mol-1.

2-        For Propanol ‘C3H7OH’

        C3H7OH        +        4½ O2        3 CO2        +        4 H2O

                                H        H        H        

                        H        C        C        C        O        H        +        O        O                        O        C        O        +        H        O        H

                                H        H        H        

(a)        The total amount of energy needed to break the bonds of reactants.

(b)        The total amount of energy released during forming the bonds of products.

(c)        The net theoretical energy change of the complete combustion of one mole of propanol, in kJ mol-1, shows that the reaction is exothermic where the ΔH value is −1498 kJ mol-1.

3-        For Butanol ‘C4H9OH’

        C4H9OH        +        6 O2        4 CO2        +        5 H2O

                        H        H        H        H        

                H        C        C        C        C        O        H        +        O        O                        O        C        O        +        H        O        H

                        H        H        H        H        

(a)        The total amount of energy needed to break the bonds of reactants.

(b)        The total amount of energy released during forming the bonds of products.

(c)        The net theoretical energy change of the complete combustion of one mole of butanol, in kJ mol-1, shows that the reaction is exothermic where the ΔH value is −1984 kJ mol-1.

IV. (Evaluating Evidence)

1- Quality of the evidence and suitability of the procedure:

I think that my work was pretty good overall. All my points on the graph seemed very close to the line of best fit, except the point at ethanol which seemed a little far from the line of best fit. I suggest that maybe the temperature of the lab had slightly changed, but I am sure that I have accidentally dropped some ethanol during out the experiment. I have found from the graph that the theoretical results are much bigger on the graph from the practical results. I suggest the following sources of error for this great difference between the practical results and the theoretical results:

• There is some alcohol evaporating from the spirit burner without complete combustion.
• There is some carbon in the form soot on the walls of the can which indicates the incomplete combustion of the fuel.
• The spirit burner itself, the metal can and the thermometer absorb some heat.
• There is heat lost by radiation and convection.
•  May the water has boiled, leading to energy consumption without no further rise in temperature.
•  The thermometer might had touched the bottom of the metal can giving a false reading.

2- Improvements:

     I suggest using a heat insulator to prevent any heat loss by radiation. May be increasing the number of used alcohols may improve to the quality of the evidence. In addition, I suggest using an air tight to prevent any alcohol from evaporating out of the spirit burner and supplying enough oxygen for the complete combustion of the fuel.

3- Further work:

I am going to repeat the experiment for the alcohol ethanol three times and get an average to get accurate and precise results.