Combustion reactions are always exothermic, due to the substances (fuels) releasing energy through combustion (temperature increases), which the change in enthalpy () is negative value. All fuels of this experiment are alcohols – ethanol, 1-pentanol and two mixtures of them with different ratios (90%:10% ; 80%:20%), which are the most common organic compounds. Alcohols contain the hydroxyl functional group (-OH) (Diagram 1) bonded to an alkyl group’s carbon atom (Chemwiki, 2014). The classification of alcohols depends on the number of carbon atoms are attached to the carbon atom that is attached to OH group. Both ethanol and 1-pentanol are primary () alcohols, which only one carbon atom is attached to the carbon atom with the OH group (Chemiwiki, 2014). Secondary () and tertiary () alcohols are when two and three carbon atoms are attached to the carbon atom with OH group, but they would not be used in the experiment.
In this experiment, 1-pentanol () (Diagram 2) has the longest carbon chain (five carbon atoms) of the fuels, which is expected to produce the highest amount of heat during combustion (), and ethanol () (Diagram 1) has the lowest number of carbon atoms (two carbon atoms), which is believed to release low energy during combustion (). During combustion, the fuel/alcohol reacts with oxygen which produces carbon dioxide and water vapour. It is believed that the greater the change in enthalpy value, the more efficiency of the fuel and more energy will be released. The energy released is calculated through bond energy and measured data, which the heat of combustion of water is divided by the number of moles of the fuel used ().
All fuels used in the experiment are hydrocarbon, which is when the carbon atoms joining together form different bonds. There are three main groups of bonding types (Diagram 3): alkanes, alkenes, and alkynes (Encyclopedia Britannica, 2015). Ethanol and 1-pentanol are both alkanes, that they only contain single bonds between carbon atoms; whereas, alkenes and alkynes group contain double and triple bonds between the carbon atoms. The calculated enthalpy change of energy is only an approximation, as the bond energy value for each bond is only an average. In addition, the bond energy value provided for each bond is measured in various states for different situations. This accounts for the difference between the calculated and the actual (Harcourt Education, 2007-2010).
By investigating the energy produced during the combustion of different fuels, a linear increasing relationship (Figure 3) was discovered that the longer the carbon chain lengths (larger molar mass) in each fuel molecule, the higher energy released during combustion. Due to two of the fuels being mixtures, the number of carbon was hard to define; hence, the molar mass was used to compare with the average enthalpy. According to Figure 2, the average enthalpy and the fuels’ molar mass had a linear relationship, that the larger molar mass of the fuel, the higher the heat of combustion. Ethanol had the lowest molar mass of 46.069g and 1-pentanol had the highest molar mass of 88.15g (Table 2), hence ethanol had the lowest average heat of combustion (668.6403kJ/mole) whereas 1-pentanol had the highest value (1588.2183kJ/mole) (Table 1). There was a large percentage increase of 137.53% from ethanol to 1-pentanol, which supported the increasing trend and the significant difference in Figure 2. Since a longer carbon chain lengths contain more C-H and C-C bonds (more bond energies) which produce more C=O and O-H bonds ( and ); more bonds would be needed to break and larger level of energy released. Figure 1 clearly illustrated that the secondary and calculated data both supported the increasing trend as the blue and orange bars continued to rise with the increase in molar mass. Thus, 1-pentanol was expected and justified to be the most efficient fuel for this experiment.
Mixtures of ethanol and 1-pentanol were used to explore whether ethanol would have a great impact on 1-pentanol and the mixture with ratio of 90% ethanol and 10% 1-pentanol was expected to be the most efficient fuel in real life context. Through calculations, both mixtures had larger values of heat of combustion than pure ethanol; hence, ethanol had an impact on 1-pentanol as the percentage change of the mixture with 10% of 1-pentanol was 22.78% and 20% of 1-pentanol mixture was 39.40% in comparison to pure ethanol. Theoretically, the percentage change of the mixture with 20% of 1-pentanol should be more efficient than the mixture with 10% 1-pentanol due to it produced more energy, which it did have a higher value of energy (933.9340 kJ/mole) released than the other mixture (820.9308 kJ/mole). However, the Australian Government regulations would have a proportion of 10% ethanol in petrol (Biofuels Association of Australia, 2014), due to most petrol used in gasoline and diesel engines (in Australia) have an expansion ratio of 10:1 for premium fuel or 9:1 for regular fuel, and some other engines have ratios of 12:1 or higher (Wikipedia, 2015). Therefore, although the mixture with 20% 1-pentanol had higher efficiency than mixture with 10% 1-pentanol, the hypothesis was justified that the mixture with a ratio of 90% ethanol and 10% 1-pentanol was a more efficient fuel for petrol because it had a more most efficient ratio in real life, whereas the mixture with ratio of 80%:20% would be not as effective and may require different design/structure of engine or cause damage to the engine.
The trials of the experiment were completed with consistency, although heat loss occurred, the increasing trend still applied through measured, secondary and calculated data. Moreover, heat loss occurred throughout the whole experiment which justified the consistency, thus the experiment was valid. From Table 3, the average percentage error showed that the mixture with ratio of 80%:20% was the most accurate fuel with lowest percentage error (44.74%) and pure 1-pentanol was the least accurate and had the highest percentage error (50.35%). All percentage errors were mainly caused by heat loss, which was probably due to incomplete combustion occurring during combustion as some black substances were on the bottom of the small cans after burning, which would have reduced the fuel efficiency so more mass of fuel was used. The insufficiency of oxygen caused incomplete combustion to produce carbon monoxide and water vapour, which could be avoided by washing off the char on the can after use each time to ensure the fuel efficiency in the next trial. Fibreglass was used as insulator wrapped around outside the big can, it could be improved by wrapping another layer of cellulose insulator (mainly made of shredded newspaper and mixed with several of chemicals to reduce its flammability) for the next experiment to increase energy efficiency. Fibreglass’ main weakness is that its incapability of blocking air from passing though, whereas cellulose insulator has higher density which would limit the air movement and prevent air-leakage better (Binford C, 2011). The non-consistency of the flame may have reduced the fuel efficiency as well, due to the flame was not stable while burning, but this could be overcome by conducting the experiment with a diminished-scale of compartment and calorimeter (over the top of the flame and cans) to investigate the maximum heat released rate and the combustion efficiency.
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The precision between all trials of pure ethanol, 1-pentanol and the mixture with 10% of 1-pentanol were good as the results of mass used of fuels were consistent. However, there was a big anomaly due to heat loss, occurring in the second trial of the mixture with 20% of 1-pentanol that the mass of fuel used was 0.65g, which was approximately 9.25% higher than the other two trials. The anomaly may be due to the air-conditioning was just starting which affect the room temperature and the temperature around the flame. This could be improved by completing the experiment at a certain room temperature with no air-conditioning, and it would keep the consistency of surrounding’s temperature. The experiment could be extended by changing the percentage of ethanol (e.g. 10% of ethanol), based upon the Australian Government that most petrol have 10% ethanol because at this ratio ethanol produces the most efficient energy (justified in Analysis). It could be extended by using mixtures ratio of 50%:50% of ethanol and 1-pentanol to explore which fuel would have a greater impact, as well as mixtures with ratios of 10%:90% and 20%:80% of ethanol and 1-pentanol to investigate whether mixture with 10% of ethanol had the most fuel efficiency. Another way to extend this experiment is to use different concentration of fuels used in the original experiment, by adding water or evaporating the fuel (heating – increase the temperature) to decrease the concentration of fuels, then burn the fuels and calculate the mass of fuel used to then find the heat of combustion to investigate whether the trend of increase in carbon chain lengths/molar mass causes increase in the fuel efficiency still holds.
In summary, the experiment was investigating the energy released of ethanol, 1-pentanol and their mixtures with ratios of 90% ethanol and 10% 1-pentanol and 80% of ethanol and 20% of 1-pentanol during combustion. Furthermore, the results of all fuels’ energy released were shown to obey the trend of increasing in carbon chain length (molar mass) lead to the increase in energy released, and the mixture with ratio of 90%:10% was discovered to be the most efficient fuel ratio for petrol as the Australian Government uses. Therefore, the hypothesis was justified that pure1-pentanol was the most efficient but when relating to real life circumstances fuel mixtures containing a ratio of 90%:10% were the most efficient for petrol.
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