In my investigation I will measure the heat given out by different types of primary alcohols when combusting with oxygen and compare the difference in the energy out put per mole of different alcohols.

GCSE CHEMISTRY COURSEWORK

Aim:

Hypotheses

Before we can have a look at the heat content of the different alcohols, we must be able to understand the process of combustion, the changes of the energy within the system while combusting with oxygen and how to determine the difference of the energy given out per unit.

First it is crucial to know that it is the external energy that we are measuring of the system, because as the Second Law of thermodynamics states that heat cannot be completely converted into work without some part of the system undergoing change, a equation is applied to the law illustrates that H (total heat content) = G (free energy) + TS (temperature×entropy, TS is the unfree energy which is associated with the degree of disorder of the system), and H can only be equal to G when TS=0 which only takes place at the temperature of absolute zero. We therefore cannot measure the total change in enthalpy (total heat content) since the 3rd law of thermodynamics states that absolute zero cannot be reached; the entropy which measures the degree of disorder also increases spontaneously that the particles of the system become disorderer or more random. What this investigation measures is the spontaneous change in the free energy which is converted into heat as stated in the second law of thermodynamics.

Then it is necessary to know that the reaction is exothermic that is heat or in another word kinetic energy and light energy, which is the emission given out when the exited electron return to a lower energy levels, are given out to the surroundings causing the surroundings to gain more kinetic energy and rise in temperature. We can measure the different values of the bond energy of the reactants and the variation between the two values, if the value of variation is positive i.e. when energy is gained the reaction is endothermic, but if the value appears to be negative, then the reaction is exothermic. It is possible to calculate the average bond energy measured in kilo joules per mole (DH/KJ mol-1) and determine the variation. Knowing that C—H=413; C—O=336; C==O=805; H—O=464; O==O=498.3 and the following equation it is possible to calculate ΔE.

2CH3 (OH) +3O2= 2CO2 +4H2O

(413×3+336+464)×2+498.3×3-805×2×2-464×2×4=-1359.1 KJ, thus the reaction is exothermic. And since the reactions of other alcohols are similar then we can say that the other alcohol combustions are exothermic.

Another fact that we must know is that the end products of any combustion reaction are carbon dioxide and water, but before the new molecules can be formed, a certain energy level is needed to give sufficient energy for the collision by what means that if the energy is not sufficient or incorrectly orientated the reactants will not form, sufficient energy that is needed to be reached is the activation energy of the system to form the activated complex that the atoms are separated.

From the state of transition the particles in the molecules now attract to particles of opposite charge, and the orbital become over lapped that the some electrons of the orbital in the outer shell are influenced by both nuclei and therefore forms the molecular orbital (M.O.), and the electrons occupies the orbital whose energy is the lowest available to it. The filling of electrons to the M.O. i.e. the orbital in the sub-shells of the shells are filled up with electrons leads to the drop in energy level of the electrons which can also be called bond energy since it is the energy needed for the transition of energy level of the electrons. This can also be regarded as spontaneous change of the system that it tend to a minimum in potential energy, because the first law of thermodynamics states that energy can neither be created nor destroyed but is simply transferred from one form or system another, the decrease in the energy level of the electrons must mean that energy is transferred to the surroundings.

From the information above I can now decide that the energy released per mole is determine by the decrease in energy level of the electrons of the valence shell spontaneously and the number of M.O. When there are more carbon atoms with in an alcohol, there will be more hydrogen atoms within the molecule as well, so that when the new molecules are formed there will be more M.O. of the similar sort formed and more energy will be released, according to the theories I have explained above. If we know the value of the reduction in the energy level when the M.O. are formed (the bond energy), the symbol equation of the reactions and the relative atomic mass (R.A.M.), then it is possible to calculate the energy given out per mole. I will work out the energy released per mole and I would expect the result of my experiment follow a similar pattern of increase of the set of values below. As we already knew, C—H=413; C—O=336; C==O=805; H—O=464; O==O=498.3; C—C=347

Methanol: 2CH3OH+3O2= 2CO2 +4H2O

(413×3+336+464)×2+498.3×3-805×2×2-464×2×4=-1359.1 kJ; 1359.1÷2=679.55kJ mol-1

Ethanol: C2H5OH+3O2=2CO2+3H2O

347+413×5+336+464+3×498.3-(805×2)×2-(464×2)×= -1297.1kJ

1297.1÷1=1297.1 kJ mol-1

Propan-1-ol: 2C3H7OH+9O2=6CO2+8H2O

(347×2+413×7+336+464)×2+498.3×9-805×2×6-464×2×8= -3829.3kJ

3829.3÷2=1914.65 kJ mol-1

Butan-1-ol: C4H9OH+6O2=4CO2+5H2O

347×3+413×9+336+464+498.3×6-805×2×4-464×2×5= -2532.2kJ

2532.2÷1=2532.2kJ mol-1

From these set of results it is possible to predict that as the number of carbon atoms increases within an alcohol molecule, the energy out put per mole will also increase.

Safety

Alcohols are highly flammable so that the spirit burners must be dealt with caution, any alcohol spillages on any surfaces must be cleaned immediately in case of fire.
Alcohols can cause blindness and death when took into the ...

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Methanol: 2CH3OH+3O2= 2CO2 +4H2O

(413×3+336+464)×2+498.3×3-805×2×2-464×2×4=-1359.1 kJ; 1359.1÷2=679.55kJ mol-1

Ethanol: C2H5OH+3O2=2CO2+3H2O

347+413×5+336+464+3×498.3-(805×2)×2-(464×2)×= -1297.1kJ

1297.1÷1=1297.1 kJ mol-1

Propan-1-ol: 2C3H7OH+9O2=6CO2+8H2O

(347×2+413×7+336+464)×2+498.3×9-805×2×6-464×2×8= -3829.3kJ

3829.3÷2=1914.65 kJ mol-1

Butan-1-ol: C4H9OH+6O2=4CO2+5H2O

347×3+413×9+336+464+498.3×6-805×2×4-464×2×5= -2532.2kJ

2532.2÷1=2532.2kJ mol-1

From these set of results it is possible to predict that as the number of carbon atoms increases within an alcohol molecule, the energy out put per mole will also increase.

Safety

Alcohols are highly flammable so that the spirit burners must be dealt with caution, any alcohol spillages on any surfaces must be cleaned immediately in case of fire.
Alcohols can cause blindness and death when took into the body, one must seek medical attention when took in.
Avoid touching the apparatus directly after experiments, when which will be heated up.
The flame must be kept distance away from other unnecessary apparatus.
Safety goggles and lab coats must be worn, and first aid kits and other first aid equipments must be ready in case of emergencies. (See photo 4)

Apparatus

Spirit burner containing methanol, ethanol, propanol-1-ol and butanol-1-ol
Stand, boss and clamp
Thermometer
Digital balance accurate to 10-2 digits
Ruler
2×100ml2 measuring cylinder
Safety goggles, lab coats
Stop clock
Tin can
Heat proof mat

(See photo 1)

Method

In order to carry out this investigation, two measurements must be taken; they are the loss in the weight of the alcohols, and the temperature change of the water. Subsequently before we can start the experiment we must measure the weight of the spirit burner, and record the reading. Then the stand boss and the clamp should be set up to fix the tin in place. The reason why I have chosen the tin to be the calorimeter is because it is a good conductor to heat thus it has a relatively low heat capacity, and more kinetic energy can be passed to the water molecules efficiently to make accurate my results. The spirit burner should be positioned directly beneath the tin and a heat proof mat is needed to be placed under the spirit burner. The distance between the peak of the wick and the bottom of the tin must be kept constant in order to carry out a fair comparison. A draught shield is needed to be placed around the spirit burner and the tin to prevent the unnecessary heat loss that could vary the result of the experiment. (See photo 2) After the apparatus have been set up as described above, then we must measure 200ml of water of 2×100ml measuring cylinder and add which to the tin, and then place the thermometer to the water and wait until the reading is stable. It important to keep the volume of water constant since the energy required to raise 1oc is associated with the mass. This stable reading can give the actual temperature before the experiment and this also tells me the temperature of the tin since the one have higher energy tends to give out energy spontaneously and equilibrium can be reached between the two. Knowing the heat capacity of the tin it is possible to calculate how much energy the calorimeter receives and this can give me a more accurate value of the heat generated by the alcohol. Since equilibrium can be reached by energy transfer from a hotter system to a colder, I can therefor say that the temperature of the tin can is almost the same as the water. Now that knowing the heat capacity of the calorimeter system which is 0.277 kJ m3 oc and the weight of the tin can is 40.937g, then the energy that had been absorbed in each of the experiments can be calculated.

After the preparations it is time to start the experiment by lighting the spirit burner thought the hole in the draught shield using a stick. The experiment will be carried out for approximately 180 seconds, the time is set upon the preliminary work I have done that this could prevent the maximum evaporation of water while efficient energy can be gained by the water to show a positive result. The reading from the thermometer must be taking quite exactly the same time when the flame extinguishes so that an accurate value can result that further out put of energy that will not be taken into account can be avoided. The spirit burner must also be weighed immediately to give a result as accurate as possible, since the weight can be reduced from other ways such as evaporation. The experiment of each alcohol will be carried out 3 times to give an accurate result and the same experiment will be repeated for each alcohol. Between each experiment it is necessary to clean off the carbon as the result of incomplete combustion produced at the bottom of the tin, because it can act as an insulator which will have effects on the accuracy of the results. One other of the natures of combustions is that the flame is not stable and within a flame the distribution of heat is not equal. Consequently it must me made sure that the sizes of the flames are the same since the distance between the wick and the tin can is already at constant. A difference between the cotton part, which is the interior of the wick, and the exterior material can cause a difference in the size of the flame, thus I have decided to cut the wick before each experiment to maintain the size of the flames. (See Photo 3) At this point it is vital to point out that the range of alcohol used be limited within the primary alcohols, which all have similar molecular structures in terms of the arrangement, so that methanol, ethanol, propan-1-ol and butan-1-ol are used. I also felt that there is no significance in caring out the experiment further that pentan-1-ol and hexan-1-ol are necessary, since I can discover the pattern from the first four.

Fair test

In the table below I will illustrate the thing that I will keep constant and that will vary to produce results of use.

Results:

(See table 1, 2, 3 and graph 1, 2,3)

Analysis:

By looking a Graph 1, it is possible to see the correlation between the number of carbon atoms and the energy output per mole. The correlation had illustrated that both the expected values and observed values are positive, how ever the expected values increase steadily with a high gradient than the values of the experiment. The outcome was expected and the cause had already been explained in the hypothesis. First by nature it is not possible to make 100% use from the energy given out by the reaction, because the energy given is mainly in the form of heat, the movement of particles that can be lost very easily. Evidence have also shown that the combustion is not complete because carbon, one of the activated complex in the process of the reaction had not been combined with oxygen forming carbon dioxide, thus less energy is produced by the reaction due to the incomplete combustion; the amount of carbon collected at the bottom of the tin can had increased as the alcohols containing more carbon was used, showing that more of the combustion/reaction was incomplete when the time for each spirit burner to combust was roughly 180 seconds. From graph 1 along, it is difficult to identify the anomalies due to its scale, thus the observed results had been put onto a new graph. (See graph 3) From the graph it is then possible to see that there is no anomalies in the experiment since the line of best fit goes through the error bars of the plotted points on the graph, the error bars represented the possible variation of data due to the inaccuracies experiment. But when looking at graph 2, the percentage of accuracy decreases as the number of carbon increases, butan-1-ol in particular had an anomalous value. If one of the values of the data in the percentage/accuracy graph is anomalous, then this value must also be at the wrong position in the energy output/carbon atoms graph, however there is not enough evident to investigate further.

The characteristics of this calorimeter system only allows us to measure the heat transfer from one system to another, however heat was not the only product of the reaction that a relatively large part of the energy is in the forms of light, which cannot be measured by the calorimeter system. However the difference in the gradient of the two lines of best fit had shown that they increase at different rates, i.e. there are errors in the last experiments which had made the rate of increase of the observed values differ from the expected. The percentage of accuracy of the results obtain from the experiment is shown on graph 2. From which it is clear that the results became less accurate as the chain of carbons got longer, in theory the molecule will increase its activation energy as the number of carbon atoms increases because each carbon atom have 4 half full sub-shells, since the electrons follow the Hund principle which states that when filling a sub-shell, there is less electron repulsion if each of each orbital is half-filled first before any single one is completely filled. In the M shell of carbon, 2s, 2px, 2py and 2pz orbital are half filled of that the 4 unpaired electrons can attract 4 other nuclei, thus the more carbon there is the half filled orbital there are. Again, as I had stated in my prediction that the energy level, the potential energy in particular of the electrons will drop, when an orbital is full, therefore more energy is required to provide sufficient energy i.e. bring back the initial energy level of the electrons for collision to occur properly. The more carbon atoms there are, the fuller orbital thus larger amount of energy will fall. The higher activation energy made difficult the supply of which, thus a larger amount is combusted improperly, and this can then explain the decrease in percentage of accuracy.

Equally if there are more carbon atoms in the reactants, more products will form, and I had already investigated in my prediction that the reaction is exothermic, i.e. the energy given by the products is higher than the energy put in to the system due to the characteristics of different types of bonds. In this case, there are more bonds in the products, although the type of bonding may be different (α and π bonds) that they have different energy levels.

From my results it is possible of see that as the number of carbon atoms increased in a primary alcohol molecule, the energy output of it also increases. This had matched my prediction although the pattern between the expected and the observed is different.

Evaluation:

The results had shown a positive correlation between the set of results, however the results had also shown that the experiment did not accurately measure the energy output of the alcohols on graph 1. However there is potential to make more accurate the experiment by using method that could prevent as much heat loss as possible.

Before it is possible to make further improvements on the investigation, probable errors must be identified and analyzed. It would consolidated my conclusion, in witch the particular errors are unidentified, if I had more evidence about the trend in the deduction of the percentage of accuracy, thus I would be a good idea to obtain the values of pentan-1-ol and hexan-1-ol. However if the equipments such as a variety of alcohols are not available, then more accurate methods can my applied to the current investigation to reduce the probability of anomalies occurring. First the heat produced by the reaction is not efficiently used to heat up the water, which can cause potential inaccuracies; some heat is lost to the surroundings and not taking in by water, because some would be conducted away by the apparatus e.g. the tin can some would be absorbed by the cylinder (not include in the calculation) and some lost through convection or move spontaneously to where air molecules have less energy due to nature. To solve the problem black painted tin cans which is able to absorb the maximum radiation given, and the infra red waved are able to heat up the water molecules. Yet not all part of the tin should be painted black, the inside and the outside of the sides of the tin can must maintain its shiny colour to prevent radiation given out by the heated up tin can. Some sort of insulation of the tin can is also able to avoid heat lost through conduction, consequently the tin can should be wrapped with wools to preserve the heat within. The apparatus that are purposed to preserve the heat are not all ideal, the cylinder used to increase the heat given to tin can should have had a higher heat capacity, because the draught shield was heated up but the large amount of energy, due to the large change in temperature, cannot and was not calculated. It would possibly be a good idea to use a better insulated draught shield, so that it would also by wrapped with wools and coated with silver paint to reduce conduction and radiation to the minimal. An enclosed and well insulated chamber can also increase the efficiency of the usage of the energy, by bring to a close the escape of the heat.

Many other faults were caused by ineffectual apparatus. The hole that was used to light spirit burner acted as a drought, which provided oxygen which is an element in the equation of combustion, but this also had made the flame unstable due to the movement of the air. The thermometer was accurate 0.5 oC which could impact the final result, if the thermometer was accurate to 10-2 the results would be more accurate and more reliable. The shielding that are purposed to prevent heat loss can impede the process of putting off the flame, therefore the change in mass did not match the reading of the thermometer when taken, and the level of heat with in the chamber will not be consistent through out the experiment when attempting to distinguish the flame by capping the burner. This problem would be very difficult to solve that it would be difficult to have an enclosed system to prevent heat loss and to ensure that the flame can be distinguished easily.

One other cause of error of my experiment was that the alcohols are not completely combusted because the activation energy cannot be reached when only little partition of the alcohols in the spirit burner are being burnt and not producing enough energy for efficient collision between particles so that carbon atoms are formed at the bottom of the tin can. The building up of the carbon also acted as an insulator that the heat cannot be conducted to the tin can and will impact on the results of my experiment. Again this is a problem that I have to face when the experiment is repeated, although the carbon can be displace after each experiment, but the effect of which during the experiment cannot be solved.

Now that I think it is appropriate to introduce a different method which would guarantee the energy of the reaction can be accurately measured. In the current experiment heat loss to the surroundings and therefore the energy towards the water was not efficient, but if the surroundings, where heat is given to, is water then maximum percentage of energy released by the experiment would be given to water where it can be measured. The error occurred in measuring the mass was also a factor affecting the anomalous results, thus a given mass can be allowed to be completely combusted so that an accurate value can be obtained. Some of the energy given to water are lost, or transferred to the surrounding of the calorimeter which cannot be measured, the heat is transferred is because one system has more energy than the other. Knowing this if the calorimeter can be maintained at the same temperature as the water with, no heat transfer can occur through conduction, and when the calorimeter is concealed no further heat can escape by convection. In order to reduce radiation, as described above, the calorimeter can be coated in shiny colour. Rather than preceding the experiment in a spirit burner, where only a little mass of alcohol was being burnt which could not proved efficient energy output that the wrong proportion of which could be used to provide the efficiency for collision, and result in incomplete combustion. A larger mass could be burnt in a given length of time to produce more energy to secure further efficiency in collision. The appropriate apparatus which suites the description above would be a bomb calorimeter. (See diagram 1) The combustion would take place in the bomb where is cased with steel, which could with stand the impact of the explosion when alcohol being combusted and also conduct heat to the calorimeter. Within the bomb the reaction is triggered by a fuse-wire which is conducted to the mains, the fuse then burns the cotton attached to it and light the flame. The bomb is also well supplied with 25 atmosphere of oxygen, pumped in with a pipe, thus no draught is need. The bomb then will be placed in a calorimeter system filled with water, as stated above that maximum energy can be transferred in the water which can then be measured, the wall of this calorimeter will be filled with water and the temperature of which is controlled by a coil which equalizes the temperature so that heat cannot can not escape or get into the calorimeter. Because the heat cannot be transferred in or out of the calorimeter, the maximum temperature the system reaches would be almost, if not all the free energy of the reaction.

As well as to discover the pattern among the primary alcohols, secondary and tertiary alcohols can also be investigated. We already know the relationship between the numbers of carbon atoms or the length of the carbon chain and the energy output, it would be equally significant to learn the science when the carbons are not in a chain or a irregular chain, and how a hybridized carbon if any at all is able to alter the results. Investigations could be among propan-2-ol, butan-2-ol, pentan-2-ol, hexan-2-ol, cyclohexanol and 2-methyl-propan-2-ol.

Coursework completed by Lu Xiao (Adrian) in November 2002-12-2

Bibliography: “Advancing Chemistry” of Oxford University Press by Michael Lewis and Guy Waller, “Chemistry for you” by Lawrie Ryan, “Chemistry, a practical approach” by , and “www.creative-chemistry.org”

Acknowledgements from D.G.Galbreath PhD and Mr. S.D.Penthreath M.A.