The heats of combustion from the investigation can be calculated by the formula:
Energy given out (J) = specific heat capacity x mass of water x temperature change
As the specific heat capacity is 4.2 J/kg/oC, and the mass of water will be 100g and the temperature change will be 10oC. Using the formula the energy given out will always be 4200J or 4.2kJ. Using this formula:
Heat of combustion (kJ/mol) = 4.2/ mass of alcohol used x relative molecular mass
The heat of combustion per mole of the alcohol can be calculated using the formula above.
Below is a list of all the apparatus that I will use and why I will be using them:
- Copper beaker – this will be used to hold the water, copper is used because it is a very good conductor of heat and will transfer more energy from the combustion to the water than other less good conductors. The bottom of the can has been painted black to absorb more heat through radiation.
- Measuring cylinder calibrated in ml – 100ml has a mass of 100g therefore the minimum unit of accuracy needs to 100ml
- Silver shiny heat shield – the heat shield will be placed under the beaker it will reflect radiation from the combustion and would allow less energy to be lost and not measured. There is a hole in the heat shield to allow a supply of oxygen for the reaction and also to allow the flame to be blown out.
- Alcohol burner with glass lid – this will be needed for the reaction, the glass lid is needed to stop any alcohol evaporating when it is not burning, as it is a very volatile liquid. If any do the results will be unreliable.
- Thermometer graded in oC – this will be needed to measure the temperature rise in the water, it is graded in oC as it will need to show when the water has risen 10oC.
- Stand and clamp – these pieces of apparatus will be needed to hold the thermometer in place, if the thermometer is not held in place, the thermometer would rest on the copper beaker and will be measuring the temperature of the copper beaker not the water. The results will be become unreliable as copper has a different specific heat capacity.
- Electronic scales – these scales will be accurate to 0.01g, variation of 0.01g will affect the results dramatically therefore this degree of accuracy will be very useful.
- Stirrer – a stirrer will be used to ensure that the water is heated evenly throughout.
The apparatus will be assembled as illustrated on the diagram above; the copper can will be put over the heat shield. The alcohol burner is inside the heat shield, under the copper can. As hot air rises, this will heat the copper can through conduction, an in turn the copper will heat the water conduction. The copper will also absorb heat through radiation as the bottom of the can has been painted black, this heat will also be transferred to the water through conduction.
The end of the thermometer will be placed in the water; however, it would not be touching the copper. If it does, the thermometer would be measuring the temperature of the copper not the water. Copper has a different specific heat capacity to water, and therefore the energy required to raise the temperature of copper by 10oC would be different to the energy required to raise the temperature of water by 10oC. Hence the results of the investigation would be unreliable.
The alcohol burner will be used to heat up the 100g, once the temperature of the water has risen by 10oC, according to the calculations made on the previous page that energy given out would have been 4.2kJ therefore. Therefore to work out the heats of combustion (kJ/Mol), the percentage of 1 mole of the alcohol that gave out 4.2kJ of energy needs to be calculated. This is done by measuring the mass of the alcohol burnt and then dividing it by the molecular mass of the alcohol. While the water is being heated the water will be stirred to ensure that it is evenly heated throughout the entire liquid.
To do this the mass of the alcohol will be measured before and after the combustion. The difference between the two values will be mass of the alcohol burnt, this value will then divided by the molecular mass of the alcohol to work out the percentage of 1 mole used. According to the formula to work out the heat of combustion, the energy given out (4.2kJ) will be divided by the percentage of 1 mole used. The resulting value will be then be how much energy is released from burning 1 mole of that alcohol.
For this investigation the independent variable will be the alcohol used, the dependent variable will be the mass of each alcohol used to heat the water so that its temperature rises by 10oC.
To make sure that the investigation is a fair test, the same pieces of equipment will be used for all of the reactions of the 8 alcohols. This will ensure that if there is any energy loss from the equipment, the energy loss will be the same. The stirring of the water will be 1 revolution every 2 seconds, this will be the same for every reaction, so that there won’t be any difference in stirring to affect the results of the investigation. There will be a lid put on the alcohol burner when it is not burning, this is used top prevent the alcohol from evaporating. Alcohols are volatile liquids, once alcohol is evaporated it cannot be weighed, this would affect the results of this investigation, as any evaporated alcohol would be measured as alcohol burned to heat the water. Therefore when the lid is placed on the alcohol burner, an alcohol that does evaporate would not be able to escape and would still be weighed.
To make the investigation safe, goggles will be worm, alcohol can cause damage to eyes, therefore eye protection must be worn at all times. As the copper can may get hot after the burning, therefore tongs will be used to handle the cans rather than with hands to prevent hands being burned. In addition, the alcohol burners will be lit with splints, as this is the safest method of lighting them.
The results of this investigation will be recorded in a table and graphs will be plotted to present the information in a more clear way.
Below is a table displaying the theoretical values, the data book values (accepted as accurate experimental values) and the actual values from the investigation.
Also included in this investigation are graphs displaying the results from the investigation and also comparing to the theoretical values and also the data book values.
From the graphs it can be seen that there is a very distinct pattern, that the heats of combustion is increases as the sizes of the molecules of alcohol increases. This can seen by that for methanol, the heat of combustion is 154 kJ/Mol, while the heat of combustion of octanol is 1761 kJ/Mol. From table 3 and table 4 it can bee seen that as the size of the molecules rises, the heat of combustion also rises, the alcohols have been arranged on order from the smallest to the largest, with methanol the smallest at the top and octanol the largest at the bottom. The heats of combustion of each alcohol also follow this pattern with the lowest heat of combustion at the top for methanol and the highest heat of combustion at the bottom for octanol.
This pattern can also be seen in the graph drawn as well. This is because the line of best fit of Graph 1 seems almost to be a straight line, but it curves up gradually, the gradient of the graph increases gradually as the curve is very slight. Therefore it can be concluded that the heat of combustion is not directly proportional to the size of the molecule. However, it can be seen that the heat of combustion still increases as the size of the molecules of the alcohols increases. In addition, the rate of increase also increases.
Though not all of the points lie exactly on the line of best fit, the points that do not lie on the line of best fit are all very close to it, therefore there are no real anomalous results.
The reaction between an alcohol and oxygen is an exothermic reaction. (For the purpose of display in, the reaction of ethanol is used, this can be used as an example of all alcohol combustions, and all the reactions are similar) Alcohol reacts with oxygen producing carbon dioxide and water. For the reaction to take place there is an activation energy requirement. This energy was required to break the different bonds in ethanol and oxygen. This energy must be taken in from the surroundings before the reaction begins. During the reaction carbon dioxide and water is formed (Fig 4), as the new bonds between the atoms of these two new compounds are formed, energy is given out into the surroundings. Because the C=O and H-O bonds have a higher total bond energy than the total bond energy of all of the bonds in the ethanol and oxygen molecules, more energy id given out. As the energy given out from the formation of these new bonds is greater than the activation energy taken in to start the reaction, the reaction gives out more energy than it takes in; hence it is an exothermic reaction.
Bust as the molecules of alcohols bigger, more oxygen is needed to react with the alcohol, as a result more carbon dioxide and water are produced. For increase in the number of carbon atoms in the alcohol molecule, there is a proportional increase in the C=O double bonds and H-O bonds. But as the H-O and C=O bonds have higher bond energy values, the increase in the energy given out by the reaction is greater than the increase in the energy taken in by the reaction. Therefore the heat of combustion increases as the molecules of the alcohols increase in size.
In Graphs 3 & 4 and in table 4 the experimental heat of combustion values are compared to the theoretical heat of combustion values as well as data book values. (Data book values are values which are accepted as accurate experimental results) Clearly there is a very big difference between the experimental values and the theoretical values as well as the data book values, possible reasons for this will be explained in the Evaluation section. In addition the data book values, seem to be slightly more than the theoretical values. This will also be explained in the Evaluation section. However, though the there are bid difference between the theoretical values and the experimental values, the shapes of the lines of best fit of the graphs are of similar shape.
The results obtained from this investigation has supported my original hypothesis in some parts, in other parts it has undermined it. As I had correctly hypothesised, the heats of combustion will increase as the size of molecule of the alcohol increases. As the results tables and the graphs have shown, the heats of combustion does indeed increase as the alcohol molecules increase in size. However, I had also hypothesised that the graph (Graph 1) would be of similar shape to the graph showing the theoretical values. The line of best fit on that graph was a straight line. However, the line of best fit in Graph 1 is not a straight line, but a curved one. Though the curve is slight, it is still apparent, I had based my hypothesis on the line in the graph displaying the theoretical heats of combustion. This proved to be wrong as the experimental values plotted on a graph were in a gradual curve.
There was a very large difference between the theoretical values and the actual experimental values. This was because that the equipment used for the investigation lost a large amount of energy:
- There was a hole at the bottom of the heat shield to allow the oxygen in to react with the alcohol, there was also a hole at the op of the heat shield to allow the flame to be blown out once water’s temperature had risen by 10oC. Cold air could have flowed in though the bottom hole, once heated it would have risen, and could have flowed out through the top hole. This convection current would have been one of the main losses of energy. Fig 5 one the next page demonstrates this, and how the convection current would have formed
- The heat shield could have absorbed some of the energy through radiation , though painting it a shiny silver colour would have reflected back most of it. This heat was lost as it was not used to heat up the water.
- The combustion of the alcohol was not complete, there was smoke produced from the alcohol burner showing that there was not enough oxygen. The smoke carried particles of carbon, this was shown on the under side of the copper can. There was a large amount of soot (carbon) on the underside, this shows that not the combustion was not 100% efficient, and there would have been less C=O and H-O bonds formed as the combustion was not complete.
- The copper was felt hot at the end of the reaction. This shows that the copper also absorbed some of the heat from the reaction. This was also lost heat, as it was not measured, because it was not used to heat the water.
This reduced the accuracy of the results as not all the heat produced was measured, and also the combustion of the alcohol was incomplete, therefore, the reaction produced less energy than it should have done, if the combustion of the alcohol was complete. An improvement would be to use a calorimeter, this surrounds the entire reaction with water, which would absorb heat in all directions. The gases produced from the reaction would be passed through copper coils also surrounded by water, as much heat as possible from the gases would also be absorbed by the water, this could then be measured. The alcohol would also react with pure oxygen ensuring a complete combustion so that there would not be any carbon left, and all the heat possible would be released from the reaction.
There were not real anomalous results, as all of the points of Graph 1 were either on the line of best fit or very close to it. There were no points that were really far off from the line of best fit.
The reason for the small difference between the data book values and the theoretical values is because the theoretical values does not take into account the intermolecular forces. These forces ensure that the molecules of the substance stay together, there are two types:
- As the electrons orbit the molecule, most of the time, it will not be symmetrical this means, that one side of the molecule will have more electrons than the other side. This creates dipoles, where the opposite ends of the molecule has opposite charges. The opposite dipoles of the electron attract each other, and this force of attraction keeps the molecules together in the substance.
- However, most dipoles are only temporary as the electrons are constantly moving, therefore dipoles change. Another force named Van der Waal’s force, keeps the molecules of gases together. This force, unlike dipoles is permanent.
When new substance are formed, this releases energy, therefore, the formation of new substance from the reaction, releases a small amount of extra energy which accounts fro the difference between the data book values and the theoretical values.
The results are reliable enough for the investigation as the reaction was repeated to ensure that an the average was taken from the 2 sets of results. The 2 sets of results seemed to be very close to each other, showing no significant differences between them. Therefore, it can be concluded that the results were reliable enough for the investigation. To improve on the reliability of the results of the investigation, the investigation could have been repeated more times, taking the average from multiple sets of results. This would have reduced the chance of 1 incorrect result affecting the final results of the investigation. It would also give more results for comparison, to see which results were more reliable than others. If some most of the results were very close to one value, or repeated that value, it show that that particular value would be most reliable to use as a result for the investigation.
To extend the investigation, perhaps more alcohols could have been investigated. To see whether the trend shown in the first 8 alcohols also continues with in the other alcohols. This would also give a better graphs as there would be more points on the graph, and it would mean a more accurate line of best fit. For this extension, all the alcohols up to and including quindecanol could be investigated. Firstly their theoretical heats of combustion could be calculated from the bond energies, and then they too could be burnt to see how much alcohol was needed to raise 100g 10oC. However, investigating further alcohols would require more water. If the trend carries on in the alcohols, as the results of this investigation indicates, it would not take long for 100g of water to increase in temperature by 10oC as there would be more energy released from the higher alcohols. Hence more water is needed, or else the reaction would not be stopped in time, and more alcohol would be burned than needed, making the results inaccurate and unreliable.