To Investigate the Combustion of Fuels

Plan

A fuel is a substance from which energy can be obtained, usually by combustion. I have chosen to use four different alcohols as fuel – methanol, ethanol, propanol and butanol. We can measure how much energy is released by putting the burning fuel under a beaker of water, and recording how much the temperature goes up by. Then the equation E = MCΔT is used, where:

E = Energy Released

M = Mass of material heated

C = Specific heat capacity of the material. For water this is 4.2

ΔT = Temperature change

Method

For this investigation I will need:

Clamp

Stand

Metal dish

Conical flask

Bunsen burner

Thermometer

Measuring cylinder

Wooden splints

Scales

Water

Four spirit burners containing ethanol, methanol, propanol and butanol

I will set up this equipment as shown below.

First I will light the Bunsen burner, and make sure it is at least a metre away from the rest of my apparatus. Then I will measure out 100ml of water (which equals 100g) using the measuring cylinder, and pour it into the conical flask. Next I will record the temperature of the water using the thermometer, and record the total mass of the spirit burner with its lid on using the scales. I will take the lid off the spirit burner, and set the distance between the top of the wick and the bottom of the beaker to 10cm. Then, using a splint lit from the Bunsen burner, I will light the spirit burner. I then will let it burn until the temperature of the water has risen by 10o, keeping an ye on the thermometer all the time. When the desired temperature has been reached, I will put out the flame by replacing the lid. Before recording the new temperature of the water, I will watch the thermometer for another 30 seconds, and then record the highest temperature it reads. At this point I will record the new mass of the spirit burner, using the scales again. Finally, before taking repeats or readings for a different fuel, I will pour out the water in the sink, and run the conical flask under the tap so it returns to room temperature.

I will take readings for four different fuels: ethanol, methanol, propanol and butanol. For each I will repeat the reading three times, and so get four readings for each. I will take the average of these readings and plot graphs with these. Here is an example of my results table:

To make it a fair test, I will always keep the amount of water the same, at 100ml; the temperature of the water at the start will be the same; the amount the temperature must rise by the same, at 10o; the distance from the bottom of the conical flask to the top of the wick will be the same, at 10cm; the Bunsen burner will be kept at least one metre from the rest of the apparatus, so it does not heat it up.

For safety, I will wear protective goggles and an apron at all times. I will also put the spirit burner in a metal dish, so if any alcohol is spilt out, it will only go into the dish.

Prediction

The combustion of a fuel involves bond breaking which needs energy and bond formation which releases energy. If the amount of energy released exceeds the amount of energy used, the reaction is exothermic. It is possible to find out the energy released per mole of each of the alcohols I am using when they are ...

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For safety, I will wear protective goggles and an apron at all times. I will also put the spirit burner in a metal dish, so if any alcohol is spilt out, it will only go into the dish.

Prediction

The equation for the complete combustion of methanol is:

2CH3OH + 3O2 → 2CO2 + 4H2O

H O

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2 H–C–O–H + 3 O=O → 2 C + 4 H-O-H

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H O

First we must find the energy needed to break the bonds on the left half of the equation. To do this, we must know the bond energy values of all the bonds. They are:

C–H = 410 KJ mol-1

C–O = 360 KJ mol-1

O–H = 460 KJ mol-1

O=O = 496 KJ mol-1

Now we add up the bond energy values of every bond that appears in the reactants, multiplied by the number of times it appears. This is the activation energy of the reaction:

C–H x 6 = 2460 KJ mol-1

C–O x 2 = 720 KJ mol-1

O–H x 2 = 920 KJ mol-1

O=O x 3 = 1488 KJ mol-1

5588 KJ mol-1 Energy used to break bonds

Next we must find the energy needed to form the bonds on the right half of the equation. To do this, we must know the bond energy values of all the bonds. They are:

C=O = 740 KJ mol-1

O–H = 460 KJ mol-1

Then we add up the bond energy values of every bond that appears in the products, multiplied by the number of times it appears:

C=O x 4 = 2960 KJ mol-1

O–H x 8 = 3680 KJ mol-1

6640 KJ mol-1 Energy released forming bonds

Finally, we take away the energy used breaking the bonds from the energy released forming the bonds and divide that number by two (because there are two moles of methanol in the equation), to find the total energy released per mole of methanol.

(6640 – 5588) / 2 = 526 KJ mol-1

Using the same method, I calculated the energy released per mole burnt for ethanol, propanol and butanol:

Ethanol: 1012 KJ mol-1

Propanol: 1498 KJ mol-1

Butanol: 1984 KJ mol-1

Now I need to calculate how much energy is released per gram of the fuels, as that is what my results will be in. Here is an example with methanol.

First we must calculate the relative molecular mass (Mr):

C=12 H=1 O=16

CH3OH = 12 + (1x4) + 16 = 32

Now we divide the energy released by 32 to find the energy released per gram:

526 /32 = 16.44 KJ g-1

I have calculated the energy released per gram for all the fuels. Here is a table of my predicted results:

I have plotted these results on a bar graph. Although I would expect my actual results to follow the same pattern as my predicted results, I would only expect my actual results to be a small fraction of them. This is because not all of the energy will get to the water for reasons such as:

Some energy is used to heat the container.
Energy heats up the surroundings due to radiation and convection currents.
Draughts disperse energy.

Preliminary Experiment

I carried out a preliminary experiment to help me decide how to conduct my actual experiment. In it I used the same basic method found in my actual experiment.

Things I found out:

-50g of water is not enough to cover the whole thermometer bulb. 100g are sufficient.

-The height should be measured from the top of the wick to the bottom of the flask. 10cm should be used here, as if the distance is less, when using butanol a black layer of carbon is left on the bottom of the flask.

-The temperature will be allowed to rise by 10oC before the flame is put out, as this takes an appropriate amount of time to do.

Results

Below are the results I collected in my investigation.

( = anomalous result)

Methanol:

Ethanol:

Propanol:

Butanol:

Analysis

I have drawn (on the same bar graph that has my predicted results) the actual results for my investigation. By looking at it I can see that the more carbon and hydrogen atoms there are in a molecule of fuel, the more the temperature of the water rises when it is burnt and therefore the more energy that is released.

However, the amount the energy increases by with different fuels is not regular. From methanol to ethanol the change is 885.53 J g-1, but from ethanol to propanol the increase is 1207.74 J g-1, and from propanol to butanol the change is 1734.79 J g-1. That is, as the number of carbon atoms increases, the amount by which the energy released goes up also increases.

From this I can conclude that the more hydrogen and carbon atoms in an alcohol, the more energy will be released when it is burnt. This is because the difference between energy released by bonds being formed and energy used by bonds being broken is greater with more carbon and hydrogen atoms.

My results agreed with my prediction by the alcohols that had more carbon and hydrogen molecules releasing more energy than the alcohols with less, but disagreed in that in my prediction, as the number of carbon atoms increased, I predicted the amount by which the energy released goes up would decrease, but my results showed that as the number of carbon atoms increased, the amount by which the energy released goes up also increased.

Evaluation

I think that my experiment went well, and that I encountered very few problems. However, the results I obtained from each fuel were quite spread out, and I had many anomalous readings. They are marked on my results tables in PINK. I cannot identify exactly what caused the anomalies for methanol, ethanol or propanol, but it was probably either:

-Energy being used to heat the conical flask.

-Energy heating up the surroundings due to radiation and convection currents.

-Draughts dispersing energy.

However, for butanol I am fairly sure what the cause was; I changed the fuel burner I was using after the first two readings, as it was of a different design to the others, and seemed to be releasing less energy than it should have, and this could have affected the second two results.

The spread out results suggest that my evidence was not totally reliable. A lot of heat was lost between the flame and the water, and so the temperature of the water was only a fraction of what it would have been if all the heat had reached it. This fraction was made even smaller because of the long distance the flask was from the wick. This was so that the bottom of the flask was not covered in carbon when butanol was burnt.

To reduce the heat loss, a bomb calorimeter could be used. This surrounds the flame with water and has a small fan which circulates the water around, keeping it all the same temperature. Also, to measure the temperature more accurately and help to tell the correct time to put out the flame I could have used a mercury thermometer, which registers temperature changes more quickly and accurately than the alcohol ones used in my experiment. To improve the reliability I could also take more repeats of readings.

Although my results agreed partly with my predictions, by the alcohols that had more carbon and hydrogen molecules releasing more energy than the alcohols with less, it disagreed in that in my prediction, as the number of carbon atoms increased, I predicted the amount by which the energy released goes up would decrease, but my results showed that as the number of carbon atoms increased, the amount by which the energy released goes up also increased. This, combined with the large amount of anomalies and the amount that my results were spread out, gives an unreliable conclusion. In the experiment with methanol, where I only had one anomaly, the highest amount of energy that I recorded to be released was 3111.11 J g-1, which was roughly 30% bigger than the smallest. I had to include both results, though, as the third result was right in the middle of the two, so I could not say which one was anomalous and which was correct. If I had taken another reading, I might have been enough to decide, and it would make my evidence more reliable.

If I had more time to study the combustion of fuels, I would look at alcohols further along in the chain, such as pentanol. I would also look at how quickly energy is released by different fuels. For this I would set up the experiment in virtually the same way, but I would put out the flame after a minute and measure how much 100g of water rises in temperature in that time.